1%\chapterimage{chapter-t4-bg} % Chapter heading image (Now given in Master document) 2 3%% Part of Stellarium User Guide 4%% 2015-12 wiki->LaTeX 5%% 2016-04-05 GZ fixed Grammar/spelling, a few enhancements, updated labels. 6 7 8\chapter{Astronomical Phenomena} 9\chapterauthor*{Barry Gerdes, with additions by Georg Zotti} 10\label{ch:Phenomena} 11 12This chapter focuses on the observational side of astronomy --- what we 13see when we look at the sky. 14 15\section{The Sun} 16\label{sec:Phenomena:sun} 17 18Without a doubt, the most prominent object in the sky is the Sun. The 19Sun is so bright that when it is in the sky, its light is scattered by 20the atmosphere to such an extent that almost all other objects in the 21sky are rendered invisible. 22 23The Sun is a star like many others but it is much closer to the Earth 24at approximately 150 million kilometres (a distance also called 251~Astronomical Unit). The next nearest star, Proxima Centauri is 26approximately 260,000 times further away from us than the Sun! The Sun 27is also known by its Latin name, \emph{Sol}. 28 29Over the course of a year, the Sun appears to move round the celestial 30sphere in a great circle known as the \emph{ecliptic}. Stellarium can 31draw the ecliptic on the sky. To toggle drawing of the ecliptic, press 32the \key{,} key. 33 34\emph{WARNING: Looking at the Sun with even the smallest telescope or 35 binoculars can permanently damage the eye. Never look at the Sun 36 without using the proper filters attached to the front end 37 (objective lens) of your instrument! Never use screw-in ocular 38 filters, they may break in the heat and expose your eye to damaging 39 amounts of energy. By far the safest way to observe the Sun is to 40 look at it on a computer screen, courtesy of Stellarium!} 41 42\subsection{Twilight} 43\label{sec:Phenomena:sun:twilight} 44 45The sunlight is scattered in Earth's atmosphere even if the Sun is 46below the horizon. The period after sunset when the Sun is higher than 47$-6°$ is called \indexterm[twilight, civil]{civil twilight}. The sky 48is generally bright enough for outdoor activity or reading the 49newspaper. 50 51As time progresses, the first stars will appear. The phase where the 52Sun is between $-6°$ and $-12°$ below the (mathematical) horizon is 53called \indexterm[twilight, nautical]{nautical twilight}: before the 54invention of satellite navigation, celestial navigation was used to 55find a ship's position on the oceans. This required that it was still 56bright enough so that the horizon line was visible, but already dark 57enough to see enough stars to clearly identify them and measure the 58altitude of a few of them. (See also section \ref{sec:plugins:NavigationalStars}.) 59 60The twilight phase when the Sun is between $-12°$ and $-18°$ is called 61\indexterm[twilight, astronomical]{astronomical twilight}. Only the 62western horizon shows some brightening, but else the natural sky is 63``almost'' as dark as it can get. 64 65In the morning the game is reversed. Dawn starts with astronomical 66twilight, progresses to nautical and civil twilight, and sunrise 67ends the night. 68 69In Stellarium, you can \newFeature{0.21.2} set the time when the Sun 70reaches a configured altitude. See 71section~\ref{sec:gui:help:hotkeys:example} about how to configure the 72program. 73 74 75\section{Stars} 76\label{sec:Phenomena:stars} 77 78The Sun is just one of billions of stars. Even though many stars have a 79much greater absolute magnitude than the Sun (they give out more light), 80they have an enormously smaller apparent magnitude due to their large 81distance. Stars have a variety of forms --- different sizes, 82brightnesses, temperatures, and colours. Measuring the position, 83distance and attributes of the stars is known as \emph{astrometry}, and 84is a major part of observational astronomy. 85 86\subsection{Multiple Star Systems} 87\label{sec:Phenomena:multipleStars} 88 89Many stars have stellar companions. As many as six stars can be found 90orbiting one-another in close associations known as 91\emph{multiple star systems} --- \emph{binary systems} being the most 92common with two stars. Multiple star systems are more common than 93solitary stars, putting our Sun in the minority group. 94 95Sometimes multiple stars orbit each other in a way that means one will 96periodically eclipse the other. These are \emph{eclipsing binaries} or 97\emph{Algol variables}. 98 99\subsubsection{Optical Doubles \& Optical Multiples} 100\label{sec:Phenomena:multipleStars:optical} 101 102Sometimes two or more stars appear to be very close to one another in 103the sky, but in fact have great separation, being aligned from the point 104of view of the observer but of different distances. Such pairings are 105known as \emph{optical doubles} and \emph{optical multiples}. 106 107\subsection{Constellations} 108\label{sec:Phenomena:Constellations} 109 110The constellations are groupings of stars that are visually close to one 111another in the sky. The actual groupings are fairly arbitrary --- 112different cultures have grouped stars together into different 113constellations. In many cultures, the various constellations have been 114associated with mythological entities. As such people have often 115projected pictures into the skies as can be seen in figure~\ref{fig:ursamajor} which shows the constellation of Ursa Major. On the 116left is a picture with the image of the mythical Great Bear, on the 117right only a line-art version (or \emph{stick figure}) is shown. The seven bright stars of Ursa 118Major are widely recognised, known variously as ``the plough'', the 119``pan-handle'', and the ``big dipper''. This sub-grouping is known as an 120\emph{asterism} --- a distinct grouping of stars. On the right, the 121picture of the bear has been removed and only a constellation diagram 122remains. 123 124\begin{figure}[hb] 125\centering\includegraphics[scale=0.8]{uma.png} 126\caption{Ursa Major} 127\label{fig:ursamajor} 128\end{figure} 129 130 131Stellarium can draw both constellation diagrams and artistic 132representations of the constellations. Multiple sky cultures are 133supported: Western, Polynesian, Egyptian, Chinese, and several other sky cultures are 134available, although at time of writing the non-Western constellations 135are not complete, and as yet there are no artistic representations of 136these sky-cultures. 137 138Aside from historical and mythological value, to the modern astronomer 139the constellations provide a way to segment the sky for the purposes 140of describing locations of objects, indeed one of the first tasks for 141an amateur observer is \emph{learning the constellations} --- the 142process of becoming familiar with the relative positions of the 143constellations, at what time of year a constellation is visible, and 144in which constellations observationally interesting objects reside. 145The International Astronomical Union\index{IAU} has adopted 88 ``Western'' 146constellations as a common system for segmenting the sky (Table~\ref{tab:IAUConstellations}). They are 147based on Greek/Roman mythology, but with several additions from 148Renaissance and later centuries. As such some formalisation has been 149adopted, each constellation having a \emph{proper name}, which is in 150Latin, and a three letter abbreviation of that name. For example, 151Ursa Major has the abbreviation UMa. Also, the ``Western'' 152constellation have clearly defined boundaries \citep{Delporte:1930}, which you can draw in 153Stellarium when you press the \key{B} key\footnote{These boundaries or 154 borders have been drawn using star maps from 1875. Due to the effect 155 of \emph{precession}, these borders are no longer parallel to 156 today's coordinates.}. The IAU constellation a selected object is placed in 157is also available in the object information data \citep{1987PASP...99..695R}, 158regardless of the currently active skyculture. On the other hand, the shapes 159of mythological figures, and also stick figures, have not been canonized, so 160you will find deviations between Stellarium and printed atlases. 161 162 163\begin{table}[p] 164 \footnotesize\centering 165 \begin{tabular}{lll||lll} 166 \toprule 167\emph{Abbr.} & \emph{Name} & \emph{Genitive} & \emph{Abbr.} & \emph{Name} & \emph{Genitive} \\\midrule 168And & Andromeda & Andromedae & Lac & Lacerta & Lacertae \\ 169Ant & Antlia & Antliae & Leo & Leo & Leonis \\ 170Aps & Apus & Apodis & LMi & Leo Minor & Leonis Minoris \\ 171Aqr & Aquarius & Aquarii & Lep & Lepus & Leporis \\ 172Aql & Aquila & Aquilae & Lib & Libra & Librae \\ 173% & % \\ 174Ara & Ara & Arae & Lup & Lupus & Lupi \\ 175Ari & Aries & Arietis & Lyn & Lynx & Lyncis \\ 176Aur & Auriga & Aurigae & Lyr & Lyra & Lyrae \\ 177Boo & Bootes & Bootis & Men & Mensa & Mensae \\ 178Cae & Caelum & Caeli & Mic & Microscopium & Microscopii \\ 179% & % \\ 180Cam & Camelopardalis & Camelopardalis & Mon & Monoceros & Monocerotis \\ 181Cnc & Cancer & Cancri & Mus & Musca & Muscae \\ 182Cvn & Canes Venatici & Canum Venaticorum & Nor & Norma & Normae \\ 183CMa & Canis Maior & Canis Maioris & Oct & Octans & Octantis \\ 184CMi & Canis Minor & Canis Minoris & Oph & Ophiuchus & Ophiuchi \\ 185% & % \\ 186Cap & Capricornus & Capricorni & Ori & Orion & Orionis \\ 187Car & Carina & Carinae & Pav & Pavo & Pavonis \\ 188Cas & Cassiopeia & Cassiopeiae & Peg & Pegasus & Pegasi \\ 189Cen & Centaurus & Centauri & Per & Perseus & Persei \\ 190Cep & Cepheus & Cephei & Phe & Phoenix & Phoenicis \\ 191% & % \\ 192Cet & Cetus & Ceti & Pic & Pictor & Pictoris \\ 193Cha & Chamaeleon & Chamaeleonis & Psc & Pisces & Piscium \\ 194Cir & Circinus & Circini & PsA & Piscis Austrinus & Piscis Austrini \\ 195Col & Columba & Columbae & Pup & Puppis & Puppis \\ 196Com & Coma Berenices & Comae Berenicis & Pyx & Pyxis & Pyxidis \\ 197% & % \\ 198CrA & Corona Australis & Coronae Australis & Ret & Reticulum & Reticuli \\ 199CrB & Corona Borealis & Coronae Borealis & Sge & Sagitta & Sagittae \\ 200Crt & Crater & Crateris & Sgr & Sagittarius & Sagittarii \\ 201Crv & Corvus & Corvi & Sco & Scorpius & Scorpii \\ 202Cru & Crux & Crucis & Scl & Sculptor & Sculptoris \\ 203% & % \\ 204Cyg & Cygnus & Cygni & Sct & Scutum & Scuti \\ 205Del & Delphinus & Delphini & Ser & Serpens & Serpentis \\ 206Dor & Dorado & Doradus & Sex & Sextans & Sextantis \\ 207Dra & Draco & Draconis & Tau & Taurus & Tauri \\ 208Equ & Equuleus & Equulei & Tel & Telescopium & Telescopii \\ 209% & % \\ 210Eri & Eridanus & Eridani & Tri & Triangulum & Trianguli \\ 211For & Fornax & Fornacis & TrA & Triangulum Australe & Trianguli Australis \\ 212Gem & Gemini & Geminorum & Tuc & Tucana & Tucanae \\ 213Gru & Grus & Gruis & UMa & Ursa Major & Ursae Majoris \\ 214Her & Hercules & Herculis & UMi & Ursa Minor & Ursae Minoris \\ 215% & % \\ 216Hor & Horologium & Horologii & Vel & Vela & Velae \\ 217Hya & Hydra & Hydrae & Vir & Virgo & Virginis \\ 218Hyi & Hydrus & Hydri & Vol & Volans & Volantis \\ 219Ind & Indus & Indi & Vul & Vulpecula & Vulpeculae \\ 220\bottomrule 221 \end{tabular} 222 \caption{The official 88 IAU constellation names and abbreviations} 223 \label{tab:IAUConstellations} 224\end{table} 225 226 227 228\subsection{Star Names} 229\label{sec:Phenomena:StarNames} 230 231The brighter stars often have one or more \emph{common 232names} relating to mythical characters from the various traditions. For 233example the brightest star in the sky, Sirius is also known as The Dog 234Star (the name Canis Major --- the constellation Sirius is found in --- 235is Latin for ``The Great Dog''). 236 237Most bright names have been given names in antiquity. \name{Ptolemy}'s most 238influential book, the \emph{Syntaxis}, was translated to the Arab 239language in the age of early Muslim scientists. When, centuries later, 240the translation, called \emph{Almagest}, was re-introduced to the 241re-awakening European science, those names, which often only designated 242the position of the star within the figure, were taken from the books, 243often misspelled, and used henceforth as proper names. 244 245A few more proper names have been added later, sometimes dedicatory 246names added by court astronomers into their maps. There are also 3 247stars named after the victims of the Apollo~1 disaster in 1967. 248Today, the International Astronomical Union (IAU)\index{IAU} is the only 249scientifically accepted authority which can give proper names to 250stars. Some companies offer a paid name service for commemoration or 251dedication of a star for deceased relatives or such, but all you get 252here is a piece of paper with coordinates of (usually) an unremarkably 253dim star only visible in a telescope, and a name to remember, stored 254(at best) in the company's database. 255 256There are several more formal naming conventions that are in common use. 257 258\subsubsection{Bayer Designation} 259\label{sec:Phenomena:StarNames:Bayer} 260 261The German astronomer \name[Johann]{Bayer} (1572--1625) devised one 262such system for his atlas, the \emph{Uranographia}, first published in 2631603. His scheme names the stars according to the constellation in 264which they lie prefixed by a lower case Greek letter (see 265Tab.~\ref{tab:Phenomena:StarNames:BayerLetters}), starting at $\alpha$ 266for (usually) the brightest star in the constellation and proceeding 267with $\beta, \gamma, \ldots$ in descending order of apparent 268magnitude. For example, such a \emph{Bayer Designation} for Sirius is 269``$\alpha$ Canis Majoris'' (note that the genitive form of the 270constellation name is used, refer to Table~\ref{tab:IAUConstellations}; 271today also the short form $\alpha$ CMa is in use). 272There are some exceptions to the descending magnitude 273ordering, and some multiple stars (both real and optical) are named 274with a numerical superscript after the Greek letter, e.g.\ $\pi^1$... 275$\pi^6$ Orionis. 276 277 278\begin{table}[htb] 279 \centering 280 \begin{tabular}{>{$}c<{$}l>{$}c<{$}l>{$}c<{$}l>{$}c<{$}l} 281 \toprule 282 \alpha & alpha & \eta & eta & \nu & nu & \tau & tau \\ 283 \beta & beta & \theta & theta & \xi & xi & \upsilon & upsilon \\ 284 \gamma & gamma & \iota & iota & \omicron & omicron & \varphi & phi \\ 285 \delta & delta & \kappa & kappa & \pi & pi & \chi & chi \\ 286 \epsilon & epsilon & \lambda & lambda & \rho & rho & \psi & psi \\ 287 \zeta & zeta & \mu & mu & \sigma & sigma & \omega & omega \\ 288 \bottomrule 289 \end{tabular} 290 \caption{The Greek alphabet used by Bayer} 291 \label{tab:Phenomena:StarNames:BayerLetters} 292\end{table} 293 294\subsubsection{Flamsteed Designation} 295\label{sec:Phenomena:StarNames:Flamsteed} 296 297English astronomer \name[John]{Flamsteed} (1646--1719) numbered stars in each 298constellation in order of increasing right ascension followed by the genitive 299form of the constellation name, for example ``61 Cygni'' (or short: ``61 Cyg''). 300 301\subsubsection{Hipparcos} 302\label{sec:Phenomena:StarNames:Hipparcos} 303 304Hipparcos (for High Precision Parallax Collecting Satellite) was an 305astrometry mission of the European Space Agency (ESA) dedicated to the 306measurement of stellar parallax and the proper motions of stars. The 307project was named in honour of the Greek astronomer \name{Hipparchus}. 308 309Ideas for such a mission dated from 1967, with the mission accepted by 310ESA in 1980. The satellite was launched by an Ariane 4 on 8 August 1989. 311The original goal was to place the satellite in a geostationary orbit 312above the earth, however a booster rocket failure resulted in a highly 313elliptical orbit from 500 to 35,800~km (310 to 22,240 miles) altitude. Despite this 314difficulty, all of the scientific goals were accomplished. 315Communications were terminated on 15 August 1993. 316 317The program was divided in two parts: the \emph{Hipparcos experiment} 318whose goal was to measure the five astrometric parameters of some 319120,000 stars to a precision of some 2 to 4 milli arc-seconds and the 320\emph{Tycho experiment}, whose goal was the measurement of the 321astrometric and two-colour photometric properties of some 400,000 322additional stars to a somewhat lower precision. 323 324The final Hipparcos Catalogue (120,000 stars with 1 milli arc-second 325level astrometry) and the final Tycho Catalogue (more than one million 326stars with 20-30 milli arc-second astrometry and two-colour photometry) 327were completed in August 1996. The catalogues were published by ESA in 328June 1997. The Hipparcos and Tycho data have been used to create the 329Millennium Star Atlas: an all-sky atlas of one million stars to visual 330magnitude 11, from the Hipparcos and Tycho Catalogues and 10,000 331non-stellar objects included to complement the catalogue data. 332 333\begin{figure}[htb] 334\centering\includegraphics[width=0.75\textwidth]{names.png} 335\caption{Star Names and Data} 336\label{fig:starnames} 337\end{figure} 338 339There were questions over whether Hipparcos has a systematic error of 340about 1 milli arc-second in at least some parts of the sky. The value 341determined by Hipparcos for the distance to the Pleiades is about 10\% 342less than the value obtained by some other methods. By early 2004, the 343controversy remained unresolved. 344 345Stellarium uses the Hipparcos Catalogue for star data, as well as having 346traditional names for many of the brighter stars. The stars tab of the 347search window allows for searching based on a Hipparcos Catalogue number 348(as well as traditional names), e.g. the star Sadalmelik in the 349constellation of Aquarius can be found by searching for the name, or 350its Hipparcos number, 109074. 351 352% 353%\subsubsection{Catalogues} 354%\label{sec:Phenomena:StarNames:Catalogues} 355% 356%As described in section~\ref{ch:Catalogues}, various star 357%catalogues assign numbers to stars, which are often used in addition 358%to other names. Stellarium gets its star data from the Hipparcos 359%catalogue, and as such stars in Stellarium are generally referred to 360%with their Hipparcos number, 361%e.g.\ ``HIP~62223''. 362 363Figure~\ref{fig:starnames} shows the information 364Stellarium displays when a star is selected. At the top, the common 365name, Bayer/Flamsteed designations and Hipparcos number are shown, 366followed by the RA/Dec coordinates, apparent magnitude, distance and 367other data. 368 369\subsection{Spectral Type \& Luminosity Class} 370\label{sec:Phenomena:SpectralTypeLuminosityClass} 371 372Stars have many different colours. Seen with the naked eye most appear 373to be white, but this is due to the response of the eye --- at low 374light levels the eye is not sensitive to colour. Typically the unaided 375eye can start to see differences in colour only for stars that have 376apparent magnitude brighter than 1. Betelgeuse, for example has a 377distinctly red tinge to it, and Sirius appears to be blue, while Vega 378is the prototype ``white'' star. 379 380By splitting the light from a star using a prism attached to a telescope 381and measuring the relative intensities of the colours of light the star 382emits --- the \indexterm{spectrum} --- a great deal of interesting information 383can be discovered about a star including its surface temperature, and 384the presence of various elements in its atmosphere. 385 386\begin{table}[tb] 387 \centering\small 388 \begin{tabular}{lll} 389\toprule 390\emph{Spectral Type} & \emph{Surface Temperature (K)} & \emph{Star Colour}\\\midrule 391O & 28,000---50,000 & Blue\\ 392B & 10,000---28,000 & Blue-white\\ 393A & 7,500---10,000 & White-blue\\ 394F & 6,000---7,500 & Yellow-white\\ 395G & 4,900---6,000 & Yellow\\ 396K & 3,500---4,900 & Orange\\ 397M & 2,000---3,500 & Red\\ 398\bottomrule 399 \end{tabular} 400 \caption{Spectral Types} 401 \label{tab:spectraltype} 402\end{table} 403 404Astronomers groups stars with similar spectra into \indexterm{spectral 405 types}, denoted by one of the following letters: O, B, A, F, G, K 406and M.\footnote{The classic mnemonic for students of astrophysics 407 says: ``Oh, Be A Fine Girl, Kiss Me''.} Type O stars have a high 408surface temperature (up to around 50,000~K) while the at other end of 409the scale, the M stars are red and have a much cooler surface 410temperature, typically 3000~K. The Sun is a type G star with a surface 411temperature of around 5,500~K. Spectral types may be further 412sub-divided using a numerical suffix ranging from 0-9 where 0 is the 413hottest and 9 is the coolest. Table~\ref{tab:spectraltype} shows the 414details of the various spectral types. 415 416For about 90\% of stars, the absolute magnitude increases as the 417spectral type tends to the O (hot) end of the scale. Thus the whiter, 418hotter stars tend to have a greater luminosity. These stars are called 419\indexterm{main sequence} stars. There are however a number of stars that 420have spectral type at the M end of the scale, and yet they have a high 421absolute magnitude. These stars are close to the ends of their lives 422and have a very large size, and consequently are known as 423\indexterm{giants}, the largest of these known as \indexterm{super-giants}. 424 425There are also stars whose absolute magnitude is very low regardless 426of the spectral class. These are known as \indexterm{dwarf stars}, among 427them \indexterm{white dwarfs} (dying stars) and \indexterm{brown dwarfs} 428(``failed stars''). 429 430The \indexterm{luminosity class} is an indication of the type of star --- 431whether it is main sequence, a giant or a dwarf. Luminosity classes are 432denoted by a number in roman numerals, as described in table~\ref{tab:luminosityclass}. 433 434\begin{table}[tb] 435 \centering\small 436 \begin{tabular}{lll} 437\toprule 438\emph{Luminosity class} & \emph{Description}\\\midrule 439Ia, Ib & Super-giants\\ 440II & Bright giants\\ 441III & Normal giants\\ 442IV & Sub-giants\\ 443V & Main sequence\\ 444VI & Sub-dwarfs\\ 445VII & White-dwarfs\\ 446\bottomrule 447\end{tabular} 448\caption{Luminosity Classification} 449\label{tab:luminosityclass} 450\end{table} 451 452Plotting the luminosity of stars against their spectral type/surface 453temperature gives a diagram called a Hertzsprung-Russell diagram 454(after the two astronomers \name[Ejnar]{Hertzsprung} (1873--1967) and 455\name[Henry Norris]{Russell} (1877--1957) who devised it). A slight variation of this is shown 456in figure~\ref{fig:colourmag} (which is technically a colour/magnitude 457plot). 458 459\begin{figure}[tp] 460\centering\includegraphics[width=\textwidth]{colour_magnitude_graph.png} 461\caption{Hertzsprung-Russell Diagram} 462\label{fig:colourmag} 463\end{figure} 464 465\subsection{Variable Stars} 466\label{sec:Phenomena:variableStars} 467 468Most stars are of nearly constant luminosity. The Sun is a good example 469of one which goes through relatively little variation in brightness 470(usually about 0.1\% over an 11 year solar cycle). Many stars, however, 471undergo significant variations in luminosity, and these are known as 472\emph{variable stars}. There are many types of variable stars falling 473into two categories, \emph{intrinsic} and \emph{extrinsic}. 474 475Intrinsic variables are stars which have intrinsic variations in 476brightness, that is, the star itself gets brighter and dimmer. There 477are several types of intrinsic variables, probably the best-known and 478most important of which is the Cepheid variable whose luminosity is 479related to the period with which its brightness varies. Since the 480luminosity (and therefore absolute magnitude) can be calculated, 481Cepheid variables may be used to determine the distance of the star 482when the annual parallax is too small to be a reliable guide. This is 483especially welcome because they are giant stars, and so they are even 484visible in neighboring galaxies. 485 486Extrinsic variables are stars of constant brightness that show changes 487in brightness as seen from the Earth. These include rotating variables, 488stars whose apparent brightness change due to rotation, and eclipsing 489binaries. 490 491Stellarium's catalogs include many kinds variable stars. 492See section~\ref{sec:StarCatalogues:VariableStars} for more details. 493 494\section{Our Moon} 495\label{sec:Moon} 496 497The Moon is the large satellite which orbits the Earth approximately 498every 28 days. It is seen as a large bright disc in the early night sky 499that rises later each day and changes shape into a crescent until it 500disappears near the Sun. After this it rises during the day then gets 501larger until it again becomes a large bright disc again. 502 503\subsection{Phases of the Moon} 504\label{sec:Moon:Phases} 505 506As the moon moves round its orbit, the amount that is illuminated by the 507sun as seen from a vantage point on Earth changes. The result of this is 508that approximately once per orbit, the moon's face gradually changes 509from being totally in shadow to being fully illuminated and back to 510being in shadow again. This process is divided up into various phases as 511described in table~\ref{tab:moonphases}. 512 513\begin{table}[tb] 514\small 515\begin{tabularx}{\textwidth}{l|X} 516\toprule 517New Moon & The moon's disc is fully in shadow, or there is just a slither of illuminated surface on the edge.\\ 518Waxing Crescent& Less than half the disc is illuminated, but more is illuminated each night.\\ 519First Quarter & Approximately half the disc is illuminated, and increasing each night.\\ 520Waxing Gibbous & More than half of the disc is illuminated, and still increasing each night.\\ 521Full Moon & The whole disc of the moon is illuminated.\\ 522Waning Gibbous & More than half of the disc is illuminated, but the amount gets smaller each night.\\ 523Last Quarter & Approximately half the disc is illuminated, but this gets less each night.\\ 524Waning Crescent& Less than half the disc of the moon is illuminated, and this gets less each night.\\ 525\bottomrule 526\end{tabularx} 527\caption{Lunar Phases} 528\label{tab:moonphases} 529\end{table} 530 531\subsection{The Lunar Magnitude} 532\label{sec:Moon:magnitude} 533 534The eastern half of the Moon has more dark \emph{maria}, therefore 535careful studies \citep{Russell:1916} have shown that in the waxing 536phases the Moon is slightly brighter than in the waning phase of the 537same degree of illumination. 538 539Around Full Moon, the Moon shows an \indexterm{opposition surge}, a 540strong increase in brightness which \citet{Krisciunas-Schaefer:1991} 541describe as a 35\% increase in luminous flux over the curve 542extrapolated from measurements when the phase angle $|\alpha|<7^\circ$. 543 544Of course, just at Full Moon when the opposition is near-perfect, the 545Moon will also move through Earth's shadow during a Lunar eclipse. 546 547The Moon is a large reflective body which reflects not only sunlight 548which directly hits it, but also sunlight reflected from Earth to the 549Moon \citep{Agrawal:2016}. This \indexterm{Ashen Light} or 550\indexterm{Earthshine} is brightest when Earth is most strongly 551illuminated for the Moon, i.e., when the Moon as seen from Earth shows 552only a thin crescent. Therefore in these days around the New Moon, the 553thin crescent can often be seen complemented into a softly glowing 554full disk. \newFeature{0.21.0} Stellarium's estimate for visual 555magnitude is based on the mentioned references and a new standard 556value for the solar illumination constant of 133.1~klx \citep{Ashdown:2019}. 557 558 559\section{The Major Planets} 560\label{sec:Planets} 561 562Unlike the stars whose relative positions remain more or less constant, 563the planets seem to move across the sky over time (the word ``planet'' 564comes from the Greek for ``wanderer''). The planets are siblings of the Earth, 565massive bodies that are in orbit around the Sun. Until 2006 there was no 566formal definition of a planet, leading to some confusion about the 567classification for some bodies traditionally regarded as being planets, but 568which didn't seem to fit with the others. 569 570In 2006 the International Astronomical Union (IAU)\index{IAU} defined a planet as a 571celestial body that, within the Solar System: 572 573\begin{enumerate} 574\item 575 is in orbit around the Sun 576\item 577 has sufficient mass for its self-gravity to overcome rigid body forces 578 so that it assumes a hydrostatic equilibrium (nearly round) shape; and 579\item 580 has cleared the neighbourhood around its orbit 581\end{enumerate} 582 583or within another system: 584 585\begin{enumerate} 586\item 587 is in orbit around a star or stellar remnants 588\item 589 has a mass below the limiting mass for thermonuclear fusion of 590 deuterium; and 591\item 592 is above the minimum mass/size requirement for planetary status in the 593 Solar System. 594\end{enumerate} 595 596Moving from the Sun outwards, the 8 major planets are: Mercury, Venus, 597Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Since the formal 598definition of a planet in 2006 Pluto has been relegated to having the 599status of \emph{dwarf planet}, along with bodies such as Ceres and Eris. 600See figure~\ref{fig:planets}. 601 602\begin{figure}[t] 603 \centering 604 \includegraphics[width=0.9\linewidth]{pictures/the_planets.jpg} 605 \caption{The Planets} 606 \label{fig:planets} 607\end{figure} 608 609 610\subsection{Terrestrial Planets}%\label{terrestrial-planets} 611 612The planets closest to the sun are called collectively the 613\emph{terrestrial planets}. The terrestrial planets are: Mercury, Venus, 614Earth and Mars. 615 616The terrestrial planets are relatively small, comparatively dense, and 617have solid rocky surface. Most of their mass is made from solid matter, 618which is mostly rocky and/or metallic in nature. 619 620\subsection{Jovian Planets}%\label{jovian-planets} 621 622Jupiter, Saturn, Uranus and Neptune make up the \emph{Jovian planets}, 623also called \emph{gas giants}. They are much more massive than the 624terrestrial planets, and do not have a solid surface. Jupiter is the 625largest of all the planets with a diameter of about 12, and mass over 626300 times that of the Earth! 627 628The Jovian planets do not have a solid surface -- the vast majority of 629their mass being in gaseous form (although they may have rocky or 630metallic cores). Because of this, they have an average density which is 631much less than the terrestrial planets. Saturn's mean density is only 632about $0.7 \g/\cm^3$ -- it would float in water! 633 634\subsection{Apparent Magnitudes of the Planets} 635The apparent magnitude of the planets can be found in astronomical 636almanachs. Over time, several scientific studies have refined these 637models. A few of them have been implemented in Stellarium: 638\begin{description} 639\item[M\"uller 1893] G. M\"uller formulated 640 \indexterm[magnitudes, visual]{visual magnitudes} for the 641 planets from visual observations of 1877--1891. They can be found in 642 \citet{AstronomicalAlgorithms:1998} and \citet{ESAE:1961}. They give 643 notably lower brightness estimates than the later models, which however 644 provide \indexterm[magnitudes, instrumental]{instrumental magnitudes} 645 in the Johnson V photometric system. Several important 646 historical studies from the early 20th century are based on these estimates. 647\item[Astronomical Almanac 1984] This model was used in the 648 Astronomical Almanac starting in 1984. The expressions are allegedly 649 ``due to D. L. Harris'', but J. 650 \citet[p.286]{AstronomicalAlgorithms:1998} denies this origin. 651\item[Explanatory Supplement 1992] Expressions from \citet{ESAA:1992}. 652\item[Explanatory Supplement 2013] Expressions from \citet{ESAA:2013}. 653\item[Mallama \& Hilton 2018] The currently most modern expressions 654 include e.g. a model for Mars which takes the brightening and 655 dimming of albedo features into account \citep{Mallama:2018}. 656\item[Generic] A simple model based on albedo, size and distance. 657\end{description} 658 659\section{The Minor Bodies}%\label{the-minor-planets} 660 661As well as the Major Planets, the solar system also contains 662innumerable smaller bodies in orbit around the Sun. These are 663generally the \emph{dwarf planets} (Ceres, Pluto, Eris), the other 664\emph{minor planets}, also known as \emph{planetoids} or 665\emph{asteroids}, and comets. 666 667While the positions of the major planets can meanwhile be computed for 668many millennia in the past and future, the minor bodies have only been 669systematically observed since the beginning of the 19th 670century.\footnote{The first discovered asteroid, (1) Ceres, was in 671 fact discovered on January 1, 1801.} The small masses occasionally 672pass by the larger planets which exert gravitational forces, so that 673the orbits of the minor bodies are not stable and cannot be computed 674for long periods. The positions are computed by using 675\indexterm{osculating orbital elements}, which describe the motions on 676instantaneous \indexterm{Kepler orbits}\footnote{The mathematician and 677 astronomer \name[Johannes]{Kepler} (1571--1630) discovered that 678 planets do not move on circles (which had been postulated since 679 antiquity), but on ``conical sections'', i.e., ellipses, parabolae 680 or even hyperbolae. The latter two can be observed for far-out 681 comets and recently even interstellar objects passing the Sun.}. If 682you want to compute positions of these objects with Stellarium, you 683need to have valid (current) \indexterm{orbital elements}. See 684Appendix~\ref{sec:ssystem.ini:minor} for more details. 685 686\subsection{Asteroids} 687\label{sec:Phenomena:Asteroids} 688 689Asteroids are celestial bodies orbiting the Sun in more or less regular 690orbits mostly between Mars and Jupiter. They are generally rocky bodies 691like the inner (terrestrial) planets, but of much smaller size. They 692are countless in number ranging in size from about ten meters to 693hundreds of kilometres. 694 695\subsection{Comets} 696\label{sec:Phenomena:Comets} 697 698A comet is a small body in the solar system that orbits the Sun and (at 699least occasionally) exhibits a coma (or atmosphere) and/or a tail. 700 701Most comets have a very eccentric orbit (featuring a highly flattened 702ellipse, or even a parabolic track), and as such spend most of their 703time a very long way from the Sun. Comets are composed of rock, dust 704and ices. When they come close to the Sun, the heat evaporates the 705ices, causing a gaseous release. This gas and loose material which 706comes away from the body of the comet is swept away from the Sun by 707the Solar wind, forming the tail. The outgassing may also change the 708orbit of the comet, so that its orbital elements should be used only 709for a few months around their \indexterm{epoch}. 710 711Most larger comets exhibit two kinds of tail: a straight gas tail 712(often blue-green in photographs), and a wider, occasionally curved 713dust tail (reflecting whitish sunlight). 714 715Comets whose orbit brings them close to the Sun more frequently than 716every 200 years are considered to be \emph{short period} comets, the 717most famous of which is probably Comet Halley, named after the British 718astronomer \name[Edmund]{Halley} (1656--1741/42\footnote{\name{Halley} lived 719in a time when Great Britain still used the Julian calendar and started 720the years in March. He died on January 14th, 1741 (British Julian), 721which was called January 25th 1742 (Gregorian) in most other European countries.}), 722which has an orbital period of roughly 76~years. 723 724 725\section{Meteoroids} 726\label{sec:Phenomena:Meteoroids} 727 728These objects are small pieces of space debris left over from the early 729days of the solar system or which crumbled off a comet when it came close to the sun. 730These particles orbit the Sun and come in a variety of shapes, 731sizes an compositions, ranging from microscopic dust particles 732up to about ten meters across. 733 734Sometimes these objects collide with the Earth. The closing speed of 735these collisions is generally extremely high (tens of kilometres per 736second). When such an object ploughs through the Earth's atmosphere, a 737large amount of kinetic energy is converted into heat and light, and a 738visible flash or streak can often be seen with the naked eye. Even the 739smallest particles can cause these events which are commonly known as 740\emph{shooting stars}. 741 742While smaller objects tend to burn up in the atmosphere, larger, denser 743objects can penetrate the atmosphere and strike the surface of the 744planet, sometimes leaving meteor craters. 745 746Sometimes the angle of the collision means that larger objects pass 747through the atmosphere but do not strike the Earth. When this happens, 748spectacular fireballs are sometimes seen. 749 750To clarify some terminology: 751\begin{description} 752\item[Meteoroids] are the objects when they are floating in space. 753\item[Meteor] is the name given to the visible atmospheric phenomenon. 754\begin{description} 755 \item[Shooting Star] colloquial term for a small meteor 756 \item[Fireball, Bolide] term for a very bright meteor. These illuminate the landscape, 757 sometimes for several seconds, and occasionally even cause sounds. These are also candidates for 758\end{description} 759\item[Meteorites], the objects that penetrate the 760atmosphere and land (or \emph{impact}) on the surface. 761\end{description} 762 763In some nights over the year you can observe increased meteorite 764activity. Those meteors seem to come from a certain point in the sky, 765the \emph{Radiant}. But what we see is similar to driving through a 766mosquito swarm which all seem to come head-on. Earth itself moves 767through space, and sweeps up a dense cloud of particles which 768originates from a comet's tail. Stellarium's Meteor Shower plugin (see 769section~\ref{sec:plugins:MeteorShowers}) can help you planning your next 770meteor observing night. 771 772\section{Zodiacal Light and \emph{Gegenschein}} 773\label{sec:Phenomena:ZodiacalLight} 774 775In very clear nights on the best observing sites, far away from the 776light pollution of our cities, you can observe a feeble glow also 777known as ``false twilight'' after evening twilight in the west, or 778before dawn in the east. The glow looks like a wedge of light along 779the ecliptic. Exactly opposite the sun, there is another dim glow that 780can be observed with dark-adapted eyes in perfect skies: the 781\emph{Gegenschein} (counterglow). 782 783This is sunlight reflected off the same dust and meteoroids in the 784plane of our solar system which is the source of meteors. Stellarium's 785sky can show the Zodiacal light \citep{Kwon:2004:ZodiacalLight}, but 786observe how quickly light pollution kills its visibility! 787 788 789\section{The Milky Way} 790\label{sec:Phenomena:MilkyWay} 791 792There is a band of very dense stars running right round the sky in huge 793irregular stripe. Most of these stars are very dim, but the overall 794effect is that on very dark clear nights we can see a large, beautiful 795area of diffuse light in the sky. It is this for which we name our 796galaxy the \indexterm{Milky Way}. 797 798The reason for this effect is that our galaxy is somewhat like a disc, 799and we are off to one side. Thus when we look towards the centre of the 800disc, we see more a great concentration of stars (there are more star in 801that direction). As we look out away from the centre of the disc we see 802fewer stars - we are staring out into the void between galaxies! 803 804It's a little hard to work out what our galaxy would look like from far 805away, because when we look up at the night sky, we are seeing it from 806the inside. All the stars we can see are part of the Milky Way, and we 807can see them in every direction. However, there is some structure. There 808is a higher density of stars in particular places. 809 810\section{Nebulae} 811\label{sec:Phenomena:Nebulae} 812 813Seen with the naked eye, binoculars or a small telescope, a 814\emph{nebula} (plural \emph{nebulae}) is a fuzzy patch on the sky. 815Historically, the term referred to any extended object, but the modern 816definition excludes some types of object such as galaxies. 817 818Observationally, nebulae are popular objects for amateur astronomers 819-- they exhibit complex structure, spectacular colours (in most cases 820only visible in color photography) and a wide variety of forms. Many 821nebulae are bright enough to be seen using good binoculars or small to 822medium sized telescopes, and are a very photogenic subject for 823astro-photographers. 824 825Nebulae are associated with a variety of phenomena, some being clouds of 826interstellar dust and gas in the process of collapsing under gravity, 827some being envelopes of gas thrown off during a supernova event (so 828called \emph{supernova remnants}), yet others being the remnants of 829dumped outer layers around dying stars (\emph{planetary nebulae}). 830 831Examples of nebulae for which Stellarium has images include the Crab 832Nebula (M1), which is a supernova remnant, and the Dumbbell Nebula 833(M27) and the Ring Nebula (M57) which are planetary nebulae. 834 835\subsection{The Messier Objects} 836\label{sec:Phenomena:Messier} 837 838The \emph{Messier} objects are a set of astronomical objects catalogued 839by \name[Charles]{Messier} (1730--1817) in his catalogue of \emph{Nebulae and Star Clusters} 840first published in 1774. The original motivation behind the catalogue 841was that Messier was a comet hunter, and he was frustrated by objects which 842resembled but were not comets. He therefore compiled a list of these annoying 843objects. 844 845The first edition covered 45 objects numbered M1 to M45. The total list 846consists of 110 objects, ranging from M1 to M110. The final catalogue 847was published in 1781 and printed in the \emph{Connaissance des Temps} 848in 1784. Many of these objects are still known by their Messier number. 849 850Because the Messier list was compiled by astronomers in the Northern 851Hemisphere, it contains only objects from the north celestial pole to a 852celestial latitude of about $-35\degree$. Many impressive Southern objects, such 853as the Large \index{Magellanic Cloud!Large} and Small \index{Magellanic Cloud!Small} 854Magellanic Clouds are excluded from the list. 855Because all of the Messier objects are visible with binoculars or small 856telescopes (under favourable conditions), they are popular viewing 857objects for amateur astronomers. In early spring, astronomers sometimes 858gather for ``Messier Marathons'', when all of the objects can be viewed 859over a single night. 860 861Stellarium includes images of many Messier objects. 862 863% 864% Source: https://en.wikipedia.org/wiki/Caldwell_catalogue 865% 866\subsection{The Caldwell catalogue} 867\label{sec:Phenomena:Caldwell} 868 869The \emph{Caldwell} catalogue is an astronomical catalogue of 109 star 870clusters, nebulae, and galaxies for observation by amateur 871astronomers. The list was compiled by \name[Patrick]{Moore} 872(1923--2012) as a complement to the Messier catalogue 873\citep{SJOMeara:2003}. 874 875While the Messier catalogue is used by amateur astronomers as a list 876of deep-sky objects for observation, Moore noted that Messier's list 877was not compiled for that purpose and excluded many of the sky's 878brightest deep-sky objects \citep{SJOMeara:2003}, such as the Hyades, 879the Double Cluster (NGC 869 and NGC 884), and the Sculptor Galaxy (NGC 880253). The Messier catalogue was actually compiled as a list of known 881objects that might be confused with comets. Moore also observed that 882since Messier compiled his list from observations in Paris, it did not 883include bright deep-sky objects visible in the Southern Hemisphere, 884such as Omega Centauri, Centaurus A, the Jewel Box, and 47 Tucanae 885\citep{SJOMeara:2003}. Moore compiled a list of 109 objects to match 886the commonly accepted number of Messier objects (he excluded M110 887\citep{Moore:1995}), and the list was published in \emph{Sky \& 888 Telescope} in December 1995 \citep{Moore:1995}. 889 890Moore used his other surname -- Caldwell -- to name the list, since 891the initial of ``Moore'' is already used for the Messier catalogue 892\citep{SJOMeara:2003}. Entries in the catalogue are designated with a 893``C'' and the catalogue number (1 to 109). 894 895Unlike objects in the Messier catalogue, which are listed roughly in 896the order of discovery by Messier and his peers, the Caldwell 897catalogue is ordered by declination, with C1 being the most northerly 898and C109 being the most southerly \citep{SJOMeara:2003}, although two 899objects (C~26=NGC~4244 and C~41, the Hyades) are listed out of 900sequence \citep{SJOMeara:2003}. Other errors in the original list have 901since been corrected: it incorrectly identified the S Norma Cluster 902(C~89=NGC~6087) as NGC 6067 and incorrectly labelled the Lambda 903Centauri Cluster (C~100=IC~2944) as the Gamma Centauri Cluster 904\citep{SJOMeara:2003}. 905 906\section{Galaxies} 907\label{sec:Phenomena:Galaxies} 908 909Stars, it seems, are gregarious -- they like to live together in groups. 910These groups are called galaxies. The number of stars in a typical 911galaxy is literally astronomical -- many \emph{billions} -- sometimes over 912\emph{hundreds of billions} of stars! 913 914Our own star, the sun, is part of a galaxy. When we look up at the 915night sky, all the stars we can see are in the same galaxy. We call 916our own galaxy the Milky Way (or sometimes simply ``the 917Galaxy''\footnote{Which means closely the same thing, the word 918 deriving from Greek \emph{gala}=milk.}). 919 920Other galaxies appear in the sky as dim fuzzy blobs. Only four are 921normally visible to the naked eye. The Andromeda galaxy (M31) visible in 922the Northern hemisphere, the two Magellanic clouds, visible in the 923Southern hemisphere, and the home galaxy Milky Way, visible in parts 924from north and south under dark skies. 925 926There are thought to be billions of galaxies in the universe comprised 927of an unimaginably large number of stars. 928 929The vast majority of galaxies are so far away that they are very dim, 930and cannot be seen without large telescopes, but there are dozens of 931galaxies which may be observed in medium to large sized amateur 932instruments. Stellarium includes images of many galaxies, including the 933Andromeda galaxy (M31)\index{M31}, the Pinwheel Galaxy (M101), the Sombrero Galaxy 934(M104) and many others. 935 936Astronomers classify galaxies according to their appearance. Some 937classifications include \emph{spiral galaxies}, \emph{elliptical 938galaxies}, \emph{lenticular galaxies} and \emph{irregular galaxies}. 939 940 941 942\section{Eclipses} 943\label{sec:Eclipses} 944 945Eclipses occur when an apparently large celestial body (planet, moon 946etc.) moves between the observer and a more distant object 947-- the more distant object being eclipsed by the nearer one. 948 949\subsection{Solar Eclipses} 950\label{sec:Eclipses:solar} 951 952Solar eclipses occur when our Moon moves between the Earth and the Sun. 953This happens when the inclined orbit of the Moon causes its path to 954cross the ecliptic and our line of sight to the Sun. In essence it is the observer 955falling under the shadow of the Moon. 956 957By a wonderful coincidence, Sun and Moon appear of almost identical 958size in Earth's sky, but the Moon's elliptical orbit sometimes causes 959it to appear just a bit smaller than the Sun. 960 961There are therefore three types of solar eclipses: 962\begin{description} 963\item[Partial] The Moon only covers part of the Sun's surface. 964\item[Total] The Moon completely obscures the Sun's surface. 965\item[Annular] The Moon is at or close to apogee (furthest from Earth in its 966elliptic orbit) and its disc is too small to completely cover the Sun. 967In this case most of the Sun's disc is obscured -- all except a thin ring 968around the edge. 969\end{description} 970Sometimes the eclipse starts annular, but when the shadow reaches the 971part of the globe closest to the Moon, it becomes just large enough to 972cover the sun totally, and when receding, it may go back to the 973annular characteristic. This is called \emph{annular-total} in the 974classical literature, or \emph{hybrid} in more recent works. 975 976\subsection{Lunar Eclipses} 977\label{sec:Eclipses:lunar} 978 979Lunar eclipses occur when the Earth moves between the Sun and the Moon, 980and the Moon is in the Earth's shadow. They occur under the same basic 981conditions as the solar eclipse but can occur more often because the 982Earth's shadow is so much larger than the Moon's. 983 984Total lunar eclipses are more noticeable than partial eclipses because 985the Moon moves fully into the Earth's shadow and there is very 986noticeable darkening. However, the Earth's atmosphere refracts light 987(bends it) in such a way that some sunlight can still fall on the Moon's 988surface even during total eclipses. The blue parts of sunlight are scattered and filtered away, 989so that there is a marked reddening of the light as it passes through the atmosphere, and this 990makes the Moon appear in a deep red colour. The darkness and colors are notably influenced by 991atmospheric clarity and clouds. After big volcanic eruptions eclipses are often reported darker. 992\name[Andr\'e]{Danjon} (1890--1967) has introduced a scale for the apparent brightness of the 993fully eclipsed Moon, where 0 is given for extremely dark eclipses and 4 for very bright ones. 994These unpredictable brightness differences make accurate simulation of the exact appearance impossible, 995but Stellarium's display gives a pretty good impression for eclipses given as 2-3 on this \indexterm{Danjon scale}. 996 997Earth's atmosphere also makes the shadow slightly larger than what can 998be derived by the geometrical sizes and distances of the involved 999objects. The Astronomical Almanac uses a traditional enlargement by 2\% after 1000\name[William]{Chauvenet} (1820--1870) while recent eclipse experts prefer another 1001formulation which leads to a slightly smaller enlargement after \name[Andr\'e]{Danjon} \citep{Espenak-Meeus:2009}. You 1002can select which formulation you prefer (see section~\ref{sec:gui:view:sso}). 1003 1004\section{Observing Hints} 1005\label{sec:observing_hints} 1006 1007When stargazing, there's a few little things which make a lot of 1008difference, and are worth taking into account. 1009 1010\begin{description} 1011\item[Dark skies] For many people getting away from light pollution 1012isn't an easy thing. At best it means a drive away from the towns, and 1013for many the only chance to see a sky without significant glow from 1014street lighting is on vacation. If you can't get away from the cities 1015easily, make the most of it when you are away. 1016 1017\item[Wrap up warm] The best observing conditions are the same 1018conditions that make for cold nights, even in the summer time. Observing 1019is not a strenuous physical activity, so you will feel the cold %a lot 1020more than if you were walking around. Wear a lot of warm clothing, don't 1021sit/lie on the floor (at least use a camping mat, better take a 1022deck-chair), and take a flask of hot drink. 1023 1024\item[Dark adaptation] The true majesty of the night sky only 1025becomes apparent when the eye has had time to become accustomed to the 1026dark. This process, known as dark adaptation, can take up to half an 1027hour, and as soon as the observer sees a bright light they must start 1028the process over. Red light doesn't compromise dark adaptation as much 1029as white light, so use a red torch if possible (and one that is as dim 1030as you can manage with). A dim single red LED light is ideal, also to 1031have enough light to take notes. 1032 1033\item[The Moon] Unless you're particularly interested in observing the 1034Moon on a given night, it can be a nuisance---it can be so bright as 1035to make observation of dimmer objects such as nebulae impossible. When 1036planning what you want to observe, take the phase and position of the 1037Moon into account. Of course Stellarium is the ideal tool for finding 1038this out! 1039 1040\item[Averted vision] A curious fact about the eye is that it is more 1041sensitive to dim light towards the edge of the field of view. If an 1042object is slightly too dim to see directly, looking slightly off to the 1043side but concentrating on the object's location can often reveal it. 1044 1045\item[Angular distance] Learn how to estimate angular distances. Learn 1046 the angular distances described in 1047 section~\ref{sec:Concepts:Angles:HandyAngles}. If you have a pair of 1048 binoculars, find out the angular distance across the field of view 1049 and use this as a standard measure. 1050\end{description} 1051 1052 1053%% The following 3 were grouped to reduce the part TOC sections. 1054\section{Atmospheric effects} 1055\label{sec:phenomena:Atmosphere} 1056 1057%% Refraction section by GZotti 2016-04-12 1058\subsection{Atmospheric Extinction} 1059\label{sec:phenomena:Extinction} 1060 1061\begin{figure}[tb] 1062\centering 1063\ifpdf 1064\includegraphics[width=\textwidth]{gz_extinctionmag} 1065\else 1066\includegraphics[width=\textwidth]{gz_extinctionmag.png} 1067\fi 1068\caption{Airmass and Extinction. The figure shows Airmass (blue) 1069 along the line of sight in the altitude labeled on the left side. 1070 The green curves show how many magnitudes an object is dimmed down, 1071 depending on extinction factor $k$ (called $k_v$ in the figure). The 1072 red curves indicate at which altitude a star of given magnitude can 1073 be seen with good eyesight, again depending on $k$. The black dots 1074 are observed values found in the literature.} 1075\label{fig:Extinction} 1076\end{figure} 1077 1078\indexterm{Atmospheric Extinction} is the attenuation of light of a 1079celestial body by Earth's atmosphere. In the last split-second of its 1080travel into our eyes or detectors, light from outer space has to pass 1081our atmosphere, through layers of mixed gas, water vapour and dust. If 1082a star is in the zenith, its light must pass one \indexterm{air mass} 1083and is reduced by whatever amount of water and dust is above you. When 1084the star is on the horizon, it has to pass about 40 times longer 1085through the atmosphere: 40 air masses (Fig.~\ref{fig:Extinction}). The 1086number of air masses increases fast in low altitudes, this is why we 1087see so few stars along the horizon. Usually blue light is extinguished 1088more, this is why the sun and moon (and brigher stars) appear reddish 1089on the horizon. 1090 1091Stellarium can simulate extinction, and you can set the opacity of 1092your atmosphere with a global factor $k$, the \emph{magnitude loss per 1093 airmass} (see section~\ref{sec:gui:view:sky:atmosphere}). The best 1094mountaintop sites may have $k=0.15$, while $k=0.25$ seems a value 1095usable for good locations in lower altitudes. 1096 1097%% Refraction section by GZotti 2016-04-12 1098\subsection{Atmospheric Refraction} 1099\label{sec:phenomena:Refraction} 1100 1101%\begin{figure}[p] 1102%\centering\includegraphics[angle=90,width=.8\textwidth]{gz_refraction} 1103%\caption{Refraction. The figure shows corrective values (degrees) 1104% which are subtracted from observed altitudes (left side) to reach 1105% geometric altitudes, or values to be added to computed values (right 1106% side). The models used are not directly inverse operations.} 1107%\label{fig:Refraction} 1108%\end{figure} 1109 1110\begin{sidewaysfigure}%[p] 1111\centering 1112\ifpdf 1113\includegraphics[width=\textwidth]{gz_refraction} 1114\else 1115\includegraphics[width=\textwidth]{gz_refraction.png} 1116\fi 1117\caption{Refraction. The figure shows corrective values (degrees) 1118 which are subtracted from observed altitudes (left side) to reach 1119 geometric altitudes, or values to be added to computed values (right 1120 side). The models used are not directly inverse operations.} 1121\label{fig:Refraction} 1122\end{sidewaysfigure} 1123 1124 1125\indexterm{Atmospheric Refraction} is a lifting effect of our 1126atmosphere which can be observed by the fact that objects close to the 1127horizon appear higher than they should be if computed only with 1128spherical trigonometry. Stellarium simulates refraction for 1129terrestrial locations when the atmosphere is switched on. Refraction 1130depends on air pressure and temperature. Figure~\ref{fig:Refraction} 1131has been created from the same formulae that are employed in 1132Stellarium. You can see how fast refraction grows very close to the 1133mathematical horizon. 1134 1135Note that these models can only give approximate conditions. There are 1136many weird effects in the real atmosphere, when temperature inversion 1137layers can create light ducts, double sunsets etc. 1138 1139Also note that the models give meaningful results only for altitudes 1140above approximately $-2\degree$. Below that, in nature, there is 1141always ground which blocks our view. In Stellarium you can switch off 1142the ground, and you can observe a sunset with a strange egg-shaped sun 1143below the horizon. This is of course nonsense. Stellarium is also not 1144able to properly recreate the atmospheric distortions as seen from a 1145stratosphere balloon, where the height of earth's surface is several 1146degrees below the mathematical horizon. 1147 1148\subsubsection{Scintillation} 1149\label{sec:phenomena:Scintillation} 1150\indexterm[scintillation]{Scintillation} is the scientific name of the twinkling which stars show 1151in turbulent atmosphere. The twinkling is caused by moving pockets of air with different temperature. 1152Observe a star low on the horizon with a telescope, and you will note it does not stand still 1153but dances around a bit, often also changing color to show red or blue hues. 1154While the twinkling of stars may look fine and comforting to the naked eye on a nice warm summer evening, 1155telescopic observers and astrophotographers don't like it at all. They rather complain about ``bad seeing'', 1156because it deteriorates the optical resolution of their instruments and photographs. 1157 1158Just like extinction and refraction are stronger along the horizon, 1159stars in low altitude in general show more twinkling than stars higher up in the sky. 1160 1161Note that planets seem to be affected less by scintillation: they are not point sources, but little disks, 1162and so the effect of the turbulent pockets of air distorting different parts of the disks cancels out a bit 1163when observing with the naked eye: planets appear to be more stable. 1164Of course, the view in a telescope will still be deteriorated by the turbulent atmosphere. 1165 1166While bad seeing was one key motivation of sending telescopes into earth orbit since the 1970s, 1167the latest generation of ground-based giant telescopes is able to compensate for the turbulent 1168motion by rapidly deforming their secondary mirror. 1169 1170 1171 1172\subsection{Light Pollution} 1173\label{sec:phenomena:LightPollution} 1174 1175An ugly side effect of civilisation is a steady increase in outdoor 1176illumination. Many people think it increases safety, but while this 1177statement can be questioned, one definite result, aside from 1178environmental issues like dangers for the nocturnal fauna, are ever 1179worsening conditions for astronomical observations or just enjoyment 1180of the night sky. 1181 1182Stellarium can simulate the effect of light pollution on the visibility 1183of celestial objects. This is controlled from the 1184light pollution section of the \emph{Sky} tab of the \emph{View} 1185window (see section~\ref{fig:gui:view:sky}). Light pollution levels are set using a numerical value 1186between 1 and 9 which corresponds to the \emph{Bortle Dark Sky Scale} 1187(see Appendix~\ref{ch:BortleScale}). Given that public lighting became 1188bright enough to cause adverse effects on observability only during 1189the 19th century, Stellarium presents an unpolluted sky for dates before 1825. 1190Note that of course light pollution was not switched on over night, 1191but became gradually noticeable over decades, but you have to estimate the appropriate level. 1192 1193In addition, local variations of 1194the amount of light pollution can be included in a light pollution 1195layer in the landscapes, see 1196section~\ref{ch:landscapes} for details. This foreground layer may include 1197all kinds of light, e.g.\ campfires, so this is also available for earlier 1198dates. 1199 1200 1201%%% Local Variables: 1202%%% mode: latex 1203%%% TeX-master: "guide" 1204%%% End: 1205