ch_astronomical_phenomena.tex

%\chapterimage{chapter-t4-bg} % Chapter heading image (Now given in Master document)

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%% 2015-12 wiki->LaTeX
%% 2016-04-05 GZ fixed Grammar/spelling, a few enhancements, updated labels.


\chapter{Astronomical Phenomena}
\chapterauthor*{Barry Gerdes, with additions by Georg Zotti}
\label{ch:Phenomena}

This chapter focuses on the observational side of astronomy --- what we
see when we look at the sky.

\section{The Sun}
\label{sec:Phenomena:sun}

Without a doubt, the most prominent object in the sky is the Sun. The
Sun is so bright that when it is in the sky, its light is scattered by
the atmosphere to such an extent that almost all other objects in the
sky are rendered invisible.

The Sun is a star like many others but it is much closer to the Earth
at approximately 150 million kilometres (a distance also called
1~Astronomical Unit). The next nearest star, Proxima Centauri is
approximately 260,000 times further away from us than the Sun! The Sun
is also known by its Latin name, \emph{Sol}.

Over the course of a year, the Sun appears to move round the celestial
sphere in a great circle known as the \emph{ecliptic}. Stellarium can
draw the ecliptic on the sky. To toggle drawing of the ecliptic, press
the \key{,} key.

\emph{WARNING: Looking at the Sun can permanently damage the eye. Never
look at the Sun without using the proper filters! By far the safest way
to observe the Sun it to look at it on a computer screen, courtesy of
Stellarium!}

\section{Stars}
\label{sec:Phenomena:stars}

The Sun is just one of billions of stars. Even though many stars have a
much greater absolute magnitude than the Sun (they give out more light),
they have an enormously smaller apparent magnitude due to their large
distance. Stars have a variety of forms --- different sizes,
brightnesses, temperatures, and colours. Measuring the position,
distance and attributes of the stars is known as \emph{astrometry}, and
is a major part of observational astronomy.

\subsection{Multiple Star Systems}
\label{sec:Phenomena:multipleStars}

Many stars have stellar companions. As many as six stars can be found
orbiting one-another in close associations known as
\emph{multiple star systems} --- \emph{binary systems} being the most
common with two stars. Multiple star systems are more common than
solitary stars, putting our Sun in the minority group.

Sometimes multiple stars orbit each other in a way that means one will
periodically eclipse the other. These are \emph{eclipsing binaries} or
\emph{Algol variables}.

\subsubsection{Optical Doubles \& Optical Multiples}
\label{sec:Phenomena:multipleStars:optical}

Sometimes two or more stars appear to be very close to one another in
the sky, but in fact have great separation, being aligned from the point
of view of the observer but of different distances. Such pairings are
known as \emph{optical doubles} and \emph{optical multiples}.

\subsection{Constellations}
\label{sec:Phenomena:Constellations}

The constellations are groupings of stars that are visually close to one
another in the sky. The actual groupings are fairly arbitrary ---
different cultures have grouped stars together into different
constellations. In many cultures, the various constellations have been
associated with mythological entities. As such people have often
projected pictures into the skies as can be seen in figure~\ref{fig:ursamajor} which shows the constellation of Ursa Major. On the
left is a picture with the image of the mythical Great Bear, on the
right only a line-art version (or \emph{stick figure}) is shown. The seven bright stars of Ursa
Major are widely recognised, known variously as ``the plough'', the
``pan-handle'', and the ``big dipper''. This sub-grouping is known as an
\emph{asterism} --- a distinct grouping of stars. On the right, the
picture of the bear has been removed and only a constellation diagram
remains.

\begin{figure}[hb]
\centering\includegraphics[scale=0.8]{uma.png}
\caption{Ursa Major}
\label{fig:ursamajor}
\end{figure}


Stellarium can draw both constellation diagrams and artistic
representations of the constellations. Multiple sky cultures are
supported: Western, Polynesian, Egyptian, Chinese, and several other sky cultures are
available, although at time of writing the non-Western constellations
are not complete, and as yet there are no artistic representations of
these sky-cultures.

Aside from historical and mythological value, to the modern astronomer
the constellations provide a way to segment the sky for the purposes
of describing locations of objects, indeed one of the first tasks for
an amateur observer is \emph{learning the constellations} --- the
process of becoming familiar with the relative positions of the
constellations, at what time of year a constellation is visible, and
in which constellations observationally interesting objects reside.
The International Astronomical Union has adopted 88 ``Western''
constellations as a common system for segmenting the sky. They are
based on Greek/Roman mythology, but with several additions from
Renaissance and later centuries.  As such some formalisation has been
adopted, each constellation having a \emph{proper name}, which is in
Latin, and a three letter abbreviation of that name.  For example,
Ursa Major has the abbreviation UMa. Also, the ``Western''
constellation have clearly defined boundaries \citep{Delporte:1930}, which you can draw in
Stellarium when you press the \key{B} key\footnote{These boundaries or
  borders have been drawn using star maps from 1875. Due to the effect
  of \emph{precession}, these borders are no longer parallel to
  today's coordinates.}. The IAU\index{IAU} constellation a selected object is placed in 
is also available in the object information data \citep{1987PASP...99..695R}, 
regardless of the currently active skyculture.  On the other hand, the shapes 
of mythological figures, and also stick figures, have not been canonized, so 
you will find deviations between Stellarium and printed atlases.

\subsection{Star Names}
\label{sec:Phenomena:StarNames}

Stars can have many names. The brighter stars often have \emph{common
names} relating to mythical characters from the various traditions. For
example the brightest star in the sky, Sirius is also known as The Dog
Star (the name Canis Major --- the constellation Sirius is found in ---
is Latin for ``The Great Dog''). 

Most bright names have been given names in antiquity. \name{Ptolemy}'s most
influential book, the \emph{Syntaxis}, was translated to the Arab
language in the age of early Muslim scientists. When, centuries later,
the translation, called \emph{Almagest}, was re-introduced to the
re-awakening European science, those names, which often only designated
the position of the star within the figure, were taken from the books,
often misspelled, and used henceforth as proper names.

A few more proper names have been added later, sometimes dedicatory
names added by court astronomers into their maps. There are also 3
stars named after the victims of the Apollo~1 disaster in 1967.
Today, the International Astronomical Union (IAU)\index{IAU} is the only
scientifically accepted authority which can give proper names to
stars. Some companies offer a paid name service for commemoration or
dedication of a star for deceased relatives or such, but all you get
here is a piece of paper with coordinates of (usually) an unremarkably
dim star only visible in a telescope, and a name to remember, stored
(at best) in the company's database.

There are several more formal naming conventions that are in common use.

\subsubsection{Bayer Designation}
\label{sec:Phenomena:StarNames:Bayer}

German astronomer \name[Johann]{Bayer} (1572--1625) devised one such system for his
atlas, the \emph{Uranographia}, first published in 1603. His scheme
names the stars according to the constellation in which they lie
prefixed by a lower case Greek letter, starting at $\alpha$ for
(usually) the brightest star in the constellation and proceeding with
$\beta, \gamma, \ldots$ in descending order of apparent magnitude. For
example, such a \emph{Bayer Designation} for Sirius is ``$\alpha$
Canis Majoris'' (note that the genitive form of the constellation name
is used; today also the short form $\alpha$ CMa is in use). There are
some exceptions to the descending magnitude ordering, and some
multiple stars (both real and optical) are named with a numerical
superscript after the Greek letter, e.g.\ $\pi^1$...  $\pi^6$ Orionis.

\subsubsection{Flamsteed Designation}
\label{sec:Phenomena:StarNames:Flamsteed}

English astronomer \name[John]{Flamsteed} (1646--1719) numbered stars in each
constellation in order of increasing right ascension followed by the genitive
form of the constellation name, for example ``61 Cygni'' (or short: ``61 Cyg'').

\subsubsection{Hipparcos}
\label{sec:Phenomena:StarNames:Hipparcos}

Hipparcos (for High Precision Parallax Collecting Satellite) was an
astrometry mission of the European Space Agency (ESA) dedicated to the
measurement of stellar parallax and the proper motions of stars. The
project was named in honour of the Greek astronomer \name{Hipparchus}.

Ideas for such a mission dated from 1967, with the mission accepted by
ESA in 1980. The satellite was launched by an Ariane 4 on 8 August 1989.
The original goal was to place the satellite in a geostationary orbit
above the earth, however a booster rocket failure resulted in a highly
elliptical orbit from 315 to 22,300 miles altitude. Despite this
difficulty, all of the scientific goals were accomplished.
Communications were terminated on 15 August 1993.

The program was divided in two parts: the \emph{Hipparcos experiment}
whose goal was to measure the five astrometric parameters of some
120,000 stars to a precision of some 2 to 4 milli arc-seconds and the
\emph{Tycho experiment}, whose goal was the measurement of the
astrometric and two-colour photometric properties of some 400,000
additional stars to a somewhat lower precision.

The final Hipparcos Catalogue (120,000 stars with 1 milli arc-second
level astrometry) and the final Tycho Catalogue (more than one million
stars with 20-30 milli arc-second astrometry and two-colour photometry)
were completed in August 1996. The catalogues were published by ESA in
June 1997. The Hipparcos and Tycho data have been used to create the
Millennium Star Atlas: an all-sky atlas of one million stars to visual
magnitude 11, from the Hipparcos and Tycho Catalogues and 10,000
non-stellar objects included to complement the catalogue data.

There were questions over whether Hipparcos has a systematic error of
about 1 milli arc-second in at least some parts of the sky. The value
determined by Hipparcos for the distance to the Pleiades is about 10\%
less than the value obtained by some other methods. By early 2004, the
controversy remained unresolved.

Stellarium uses the Hipparcos Catalogue for star data, as well as having
traditional names for many of the brighter stars. The stars tab of the
search window allows for searching based on a Hipparcos Catalogue number
(as well as traditional names), e.g. the star Sadalmelik in the
constellation of Aquarius can be found by searching for the name, or
its Hipparcos number, 109074.

%
%\subsubsection{Catalogues}
%\label{sec:Phenomena:StarNames:Catalogues}
%
%As described in section~\ref{ch:Catalogues}, various star
%catalogues assign numbers to stars, which are often used in addition
%to other names. Stellarium gets its star data from the Hipparcos
%catalogue, and as such stars in Stellarium are generally referred to
%with their Hipparcos number,
%e.g.\ ``HIP~62223''. 

Figure~\ref{fig:starnames} shows the information
Stellarium displays when a star is selected. At the top, the common
name, Bayer/Flamsteed designations and Hipparcos number are shown,
followed by the RA/Dec coordinates, apparent magnitude, distance and
other data.

\begin{figure}[h]
\centering\includegraphics{names.png}
\caption{Star Names and Data}
\label{fig:starnames}
\end{figure}

\subsection{Spectral Type \& Luminosity Class}
\label{sec:Phenomena:SpectralTypeLuminosityClass}

Stars have many different colours. Seen with the naked eye most appear
to be white, but this is due to the response of the eye --- at low
light levels the eye is not sensitive to colour. Typically the unaided
eye can start to see differences in colour only for stars that have
apparent magnitude brighter than 1. Betelgeuse, for example has a
distinctly red tinge to it, and Sirius appears to be blue, while Vega
is the prototype ``white'' star.

By splitting the light from a star using a prism attached to a telescope
and measuring the relative intensities of the colours of light the star
emits --- the \indexterm{spectrum} --- a great deal of interesting information
can be discovered about a star including its surface temperature, and
the presence of various elements in its atmosphere.

\begin{table}[tb]
  \centering\small
  \begin{tabular}{lll}
\toprule
\emph{Spectral Type} & \emph{Surface Temperature (K)} & \emph{Star Colour}\\\midrule
O & 28,000---50,000 & Blue\\
B & 10,000---28,000 & Blue-white\\
A & 7,500---10,000 & White-blue\\
F & 6,000---7,500 & Yellow-white\\
G & 4,900---6,000 & Yellow\\
K & 3,500---4,900 & Orange\\
M & 2,000---3,500 & Red\\
\bottomrule    
  \end{tabular}
  \caption{Spectral Types}
  \label{tab:spectraltype}
\end{table}

Astronomers groups stars with similar spectra into \indexterm{spectral
  types}, denoted by one of the following letters: O, B, A, F, G, K
and M.\footnote{The classic mnemonic for students of astrophysics
  says: ``Oh, Be A Fine Girl, Kiss Me''.}  Type O stars have a high
surface temperature (up to around 50,000~K) while the at other end of
the scale, the M stars are red and have a much cooler surface
temperature, typically 3000~K. The Sun is a type G star with a surface
temperature of around 5,500~K. Spectral types may be further
sub-divided using a numerical suffix ranging from 0-9 where 0 is the
hottest and 9 is the coolest. Table~\ref{tab:spectraltype} shows the
details of the various spectral types.

For about 90\% of stars, the absolute magnitude increases as the
spectral type tends to the O (hot) end of the scale. Thus the whiter,
hotter stars tend to have a greater luminosity. These stars are called
\indexterm{main sequence} stars. There are however a number of stars that
have spectral type at the M end of the scale, and yet they have a high
absolute magnitude. These stars are close to the ends of their lives
and have a very large size, and consequently are known as
\indexterm{giants}, the largest of these known as \indexterm{super-giants}.

There are also stars whose absolute magnitude is very low regardless
of the spectral class. These are known as \indexterm{dwarf stars}, among
them \indexterm{white dwarfs} (dying stars) and \indexterm{brown dwarfs}
(``failed stars'').

The \indexterm{luminosity class} is an indication of the type of star ---
whether it is main sequence, a giant or a dwarf. Luminosity classes are
denoted by a number in roman numerals, as described in table~\ref{tab:luminosityclass}.

\begin{table}[tb]
  \centering\small
  \begin{tabular}{lll}
\toprule
\emph{Luminosity class} & \emph{Description}\\\midrule
Ia, Ib & Super-giants\\
II     & Bright giants\\
III    & Normal giants\\
IV     & Sub-giants\\
V      & Main sequence\\
VI     & Sub-dwarfs\\
VII    & White-dwarfs\\
\bottomrule
\end{tabular}
\caption{Luminosity Classification}
\label{tab:luminosityclass}
\end{table}

Plotting the luminosity of stars against their spectral type/surface
temperature gives a diagram called a Hertzsprung-Russell diagram
(after the two astronomers \name[Ejnar]{Hertzsprung} (1873--1967) and 
\name[Henry Norris]{Russell} (1877--1957) who devised it). A slight variation of this is shown
in figure~\ref{fig:colourmag} (which is technically a colour/magnitude
in figure~\ref{fig:colourmag} (which is technically a colour/magnitude
plot).

\begin{figure}[tp]
\centering\includegraphics[width=\textwidth]{colour_magnitude_graph.png}
\caption{Hertzsprung-Russell Diagram}
\label{fig:colourmag}
\end{figure}

\subsection{Variable Stars}
\label{sec:Phenomena:variableStars}

Most stars are of nearly constant luminosity. The Sun is a good example
of one which goes through relatively little variation in brightness
(usually about 0.1\% over an 11 year solar cycle). Many stars, however,
undergo significant variations in luminosity, and these are known as
\emph{variable stars}. There are many types of variable stars falling
into two categories, \emph{intrinsic} and \emph{extrinsic}.

Intrinsic variables are stars which have intrinsic variations in
brightness, that is, the star itself gets brighter and dimmer. There
are several types of intrinsic variables, probably the best-known and
most important of which is the Cepheid variable whose luminosity is
related to the period with which its brightness varies. Since the
luminosity (and therefore absolute magnitude) can be calculated,
Cepheid variables may be used to determine the distance of the star
when the annual parallax is too small to be a reliable guide. This is
especially welcome because they are giant stars, and so they are even
visible in neighboring galaxies.

Extrinsic variables are stars of constant brightness that show changes
in brightness as seen from the Earth. These include rotating variables,
stars whose apparent brightness change due to rotation, and eclipsing
binaries.

Stellarium's catalogs include many kinds variable stars. 
See section~\ref{sec:StarCatalogues:VariableStars} for more details. 

\section{Our Moon}
\label{sec:Moon}

The Moon is the large satellite which orbits the Earth approximately
every 28 days. It is seen as a large bright disc in the early night sky
that rises later each day and changes shape into a crescent until it
disappears near the Sun. After this it rises during the day then gets
larger until it again becomes a large bright disc again.

\subsection{Phases of the Moon}
\label{sec:Moon:Phases}

As the moon moves round its orbit, the amount that is illuminated by the
sun as seen from a vantage point on Earth changes. The result of this is
that approximately once per orbit, the moon's face gradually changes
from being totally in shadow to being fully illuminated and back to
being in shadow again. This process is divided up into various phases as
described in table~\ref{tab:moonphases}.

\begin{table}[b]
\small
\begin{tabularx}{\textwidth}{l|X}
\toprule
New Moon       & The moon's disc is fully in shadow, or there is just a slither of illuminated surface on the edge.\\
Waxing Crescent& Less than half the disc is illuminated, but more is illuminated each night.\\
First Quarter  & Approximately half the disc is illuminated, and increasing each night.\\
Waxing Gibbous & More than half of the disc is illuminated, and still increasing each night.\\
Full Moon      & The whole disc of the moon is illuminated.\\
Waning Gibbous & More than half of the disc is illuminated, but the amount gets smaller each night.\\
Last Quarter   & Approximately half the disc is illuminated, but this gets less each night.\\
Waning Crescent& Less than half the disc of the moon is illuminated, and this gets less each night.\\
\bottomrule
\end{tabularx}
\caption{Lunar Phases}
\label{tab:moonphases}
\end{table}

\newpage
\section{The Major Planets}
\label{sec:Planets}

Unlike the stars whose relative positions remain more or less constant,
the planets seem to move across the sky over time (the word ``planet''
comes from the Greek for ``wanderer''). The planets are siblings of the Earth,
massive bodies that are in orbit around the Sun. Until 2006 there was no
formal definition of a planet, leading to some confusion about the
classification for some bodies traditionally regarded as being planets, but
which didn't seem to fit with the others.

In 2006 the International Astronomical Union (IAU)\index{IAU} defined a planet as a
celestial body that, within the Solar System:

\begin{enumerate}
\item
  is in orbit around the Sun
\item
  has sufficient mass for its self-gravity to overcome rigid body forces
  so that it assumes a hydrostatic equilibrium (nearly round) shape; and
\item
  has cleared the neighbourhood around its orbit
\end{enumerate}

or within another system:

\begin{enumerate}
\item
  is in orbit around a star or stellar remnants
\item
  has a mass below the limiting mass for thermonuclear fusion of
  deuterium; and
\item
  is above the minimum mass/size requirement for planetary status in the
  Solar System.
\end{enumerate}

Moving from the Sun outwards, the 8 major planets are: Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Since the formal
definition of a planet in 2006 Pluto has been relegated to having the
status of \emph{dwarf planet}, along with bodies such as Ceres and Eris.
See figure~\ref{fig:planets}.

\begin{figure}[t]
  \centering
  \includegraphics[width=0.9\linewidth]{pictures/the_planets.jpg}
  \caption{The Planets}
  \label{fig:planets}
\end{figure}


\subsection{Terrestrial Planets}%\label{terrestrial-planets}

The planets closest to the sun are called collectively the
\emph{terrestrial planets}. The terrestrial planets are: Mercury, Venus,
Earth and Mars.

The terrestrial planets are relatively small, comparatively dense, and
have solid rocky surface. Most of their mass is made from solid matter,
which is mostly rocky and/or metallic in nature.

\subsection{Jovian Planets}%\label{jovian-planets}

Jupiter, Saturn, Uranus and Neptune make up the \emph{Jovian planets},
also called \emph{gas giants}. They are much more massive than the
terrestrial planets, and do not have a solid surface. Jupiter is the
largest of all the planets with a diameter of about 12, and mass over
300 times that of the Earth!

The Jovian planets do not have a solid surface -- the vast majority of
their mass being in gaseous form (although they may have rocky or
metallic cores). Because of this, they have an average density which is
much less than the terrestrial planets. Saturn's mean density is only
about $0.7 \g/\cm^3$ -- it would float in water!

\section{The Minor Bodies}%\label{the-minor-planets}

As well as the Major Planets, the solar system also contains
innumerable smaller bodies in orbit around the Sun. These are
generally the \emph{dwarf planets} (Ceres, Pluto, Eris), the other
\emph{minor planets}, also known as \emph{planetoids} or
\emph{asteroids}, and comets.

\subsection{Asteroids}
\label{sec:Phenomena:Asteroids}

Asteroids are celestial bodies orbiting the Sun in more or less regular
orbits mostly between Mars and Jupiter. They are generally rocky bodies
like the inner (terrestrial) planets, but of much smaller size. They
are countless in number ranging in size from about ten meters to
hundreds of kilometres.

\subsection{Comets}
\label{sec:Phenomena:Comets}

A comet is a small body in the solar system that orbits the Sun and (at
least occasionally) exhibits a coma (or atmosphere) and/or a tail.

Most comets have a very eccentric orbit (featuring a highly flattened
ellipse, or even a parabolic track), and as such spend most of their
time a very long way from the Sun. Comets are composed of rock, dust
and ices. When they come close to the Sun, the heat evaporates the
ices, causing a gaseous release. This gas and loose material which
comes away from the body of the comet is swept away from the Sun by
the Solar wind, forming the tail.

Most larger comets exhibit two kinds of tail: a straight gas tail
(often blue-green in photographs), and a wider, occasionally curved
dust tail (reflecting whitish sunlight).

Comets whose orbit brings them close to the Sun more frequently than
every 200 years are considered to be \emph{short period} comets, the
most famous of which is probably Comet Halley, named after the British
astronomer \name[Edmund]{Halley} (1656--1741/42\footnote{Halley lived 
in a time when Great Britain still used the Julian calendar and started 
the years in March. He died January 14th, 1741 (Julian), 
which was called January 25th 1742 (Gregorian) in most other European countries.}), 
which has an orbital period of roughly 76~years.


\section{Meteoroids}
\label{sec:Phenomena:Meteoroids}

These objects are small pieces of space debris left over from the early
days of the solar system or which crumbled off a comet when it came close to the sun. 
These particles orbit the Sun and come in a variety of shapes, 
sizes an compositions, ranging from microscopic dust particles
up to about ten meters across.

Sometimes these objects collide with the Earth. The closing speed of
these collisions is generally extremely high (tens of kilometres per
second). When such an object ploughs through the Earth's atmosphere, a
large amount of kinetic energy is converted into heat and light, and a
visible flash or streak can often be seen with the naked eye. Even the
smallest particles can cause these events which are commonly known as
\emph{shooting stars}.

While smaller objects tend to burn up in the atmosphere, larger, denser
objects can penetrate the atmosphere and strike the surface of the
planet, sometimes leaving meteor craters.

Sometimes the angle of the collision means that larger objects pass
through the atmosphere but do not strike the Earth. When this happens,
spectacular fireballs are sometimes seen.

To clarify some terminology:
\begin{description}
\item[Meteoroids]  are the objects when they are floating in space.
\item[Meteor] is the name given to the visible atmospheric phenomenon.
\begin{description}
 \item[Shooting Star] colloquial term for a small meteor
 \item[Fireball, Bolide] term for a very bright meteor. These illuminate the landscape, 
  sometimes for several seconds, and occasionally even cause sounds. These are also candidates for 
\end{description}
\item[Meteorites], the objects that penetrate the
atmosphere and land (or \emph{impact}) on the surface.
\end{description}

In some nights over the year you can observe increased meteorite
activity. Those meteors seem to come from a certain point in the sky,
the \emph{Radiant}. But what we see is similar to driving through a
mosquito swarm which all seem to come head-on. Earth itself moves
through space, and sweeps up a dense cloud of particles which
originates from a comet's tail. Stellarium's Meteor Shower plugin (see
section~\ref{sec:plugins:MeteorShowers}) can help you planning your next
meteor observing night.

\section{Zodiacal Light and \emph{Gegenschein}}
\label{sec:Phenomena:ZodiacalLight}

In very clear nights on the best observing sites, far away from the
light pollution of our cities, you can observe a feeble glow also
known as ``false twilight'' after evening twilight in the west, or
before dawn in the east. The glow looks like a wedge of light along
the ecliptic. Exactly opposite the sun, there is another dim glow that
can be observed with dark-adapted eyes in perfect skies: the
\emph{Gegenschein} (counterglow). 

This is sunlight reflected off the same dust and meteoroids in the
plane of our solar system which is the source of meteors. Stellarium's
sky can show the Zodiacal light \citep{Kwon:2004:ZodiacalLight}, but
observe how quickly light pollution kills its visibility!


\section{The Milky Way}
\label{sec:Phenomena:MilkyWay}

There is a band of very dense stars running right round the sky in huge
irregular stripe. Most of these stars are very dim, but the overall
effect is that on very dark clear nights we can see a large, beautiful
area of diffuse light in the sky. It is this for which we name our
galaxy the \indexterm{Milky Way}.

The reason for this effect is that our galaxy is somewhat like a disc,
and we are off to one side. Thus when we look towards the centre of the
disc, we see more a great concentration of stars (there are more star in
that direction). As we look out away from the centre of the disc we see
fewer stars - we are staring out into the void between galaxies!

It's a little hard to work out what our galaxy would look like from far
away, because when we look up at the night sky, we are seeing it from
the inside. All the stars we can see are part of the Milky Way, and we
can see them in every direction. However, there is some structure. There
is a higher density of stars in particular places.

\section{Nebulae}
\label{sec:Phenomena:Nebulae}

Seen with the naked eye, binoculars or a small telescope, a
\emph{nebula} (plural \emph{nebulae}) is a fuzzy patch on the sky.
Historically, the term referred to any extended object, but the modern
definition excludes some types of object such as galaxies.

Observationally, nebulae are popular objects for amateur astronomers
-- they exhibit complex structure, spectacular colours (in most cases
only visible in color photography) and a wide variety of forms. Many
nebulae are bright enough to be seen using good binoculars or small to
medium sized telescopes, and are a very photogenic subject for
astro-photographers.

Nebulae are associated with a variety of phenomena, some being clouds of
interstellar dust and gas in the process of collapsing under gravity,
some being envelopes of gas thrown off during a supernova event (so
called \emph{supernova remnants}), yet others being the remnants of
dumped outer layers around dying stars (\emph{planetary nebulae}).

Examples of nebulae for which Stellarium has images include the Crab
Nebula (M1), which is a supernova remnant, and the Dumbbell Nebula
(M27) and the Ring Nebula (M57) which are planetary nebulae.

\subsection{The Messier Objects}
\label{sec:Phenomena:Messier}

The \emph{Messier} objects are a set of astronomical objects catalogued
by \name[Charles]{Messier} (1730--1817) in his catalogue of \emph{Nebulae and Star Clusters}
first published in 1774. The original motivation behind the catalogue
was that Messier was a comet hunter, and he was frustrated by objects which
resembled but were not comets. He therefore compiled a list of these annoying 
objects.

The first edition covered 45 objects numbered M1 to M45. The total list
consists of 110 objects, ranging from M1 to M110. The final catalogue
was published in 1781 and printed in the \emph{Connaissance des Temps}
in 1784. Many of these objects are still known by their Messier number.

Because the Messier list was compiled by astronomers in the Northern
Hemisphere, it contains only objects from the north celestial pole to a
celestial latitude of about $-35\degree$. Many impressive Southern objects, such
as the Large \index{Magellanic Cloud!Large} and Small \index{Magellanic Cloud!Small} 
Magellanic Clouds are excluded from the list.
Because all of the Messier objects are visible with binoculars or small
telescopes (under favourable conditions), they are popular viewing
objects for amateur astronomers. In early spring, astronomers sometimes
gather for ``Messier Marathons'', when all of the objects can be viewed
over a single night.

Stellarium includes images of many Messier objects.


\section{Galaxies}
\label{sec:Phenomena:Galaxies}

Stars, it seems, are gregarious -- they like to live together in groups.
These groups are called galaxies. The number of stars in a typical
galaxy is literally astronomical -- many \emph{billions} -- sometimes over
\emph{hundreds of billions} of stars!

Our own star, the sun, is part of a galaxy. When we look up at the
night sky, all the stars we can see are in the same galaxy. We call
our own galaxy the Milky Way (or sometimes simply ``the
Galaxy''\footnote{Which means closely the same thing, the word
  deriving from Greek \emph{gala}=milk.}).

Other galaxies appear in the sky as dim fuzzy blobs. Only four are
normally visible to the naked eye. The Andromeda galaxy (M31) visible in
the Northern hemisphere, the two Magellanic clouds, visible in the
Southern hemisphere, and the home galaxy Milky Way, visible in parts
from north and south under dark skies.

There are thought to be billions of galaxies in the universe comprised
of an unimaginably large number of stars.

The vast majority of galaxies are so far away that they are very dim,
and cannot be seen without large telescopes, but there are dozens of
galaxies which may be observed in medium to large sized amateur
instruments. Stellarium includes images of many galaxies, including the
Andromeda galaxy (M31)\index{M31}, the Pinwheel Galaxy (M101), the Sombrero Galaxy
(M104) and many others.

Astronomers classify galaxies according to their appearance. Some
classifications include \emph{spiral galaxies}, \emph{elliptical
galaxies}, \emph{lenticular galaxies} and \emph{irregular galaxies}.



\section{Eclipses}
\label{sec:Eclipses}

Eclipses occur when an apparently large celestial body (planet, moon
etc.) moves between the observer (that's you!) and a more distant object
-- the more distant object being eclipsed by the nearer one.

\subsection{Solar Eclipses}
\label{sec:Eclipses:solar}

Solar eclipses occur when our Moon moves between the Earth and the Sun.
This happens when the inclined orbit of the Moon causes its path to
cross our line of sight to the Sun. In essence it is the observer
falling under the shadow of the moon.

There are three types of solar eclipses:

\textbf{Partial} The Moon only covers part of the Sun's surface.

\textbf{Total} The Moon completely obscures the Sun's surface.

\textbf{Annular} The Moon is at apogee (furthest from Earth in its
elliptic orbit) and its disc is too small to completely cover the Sun.
In this case most of the Sun's disc is obscured -- all except a thin ring
around the edge.

\subsection{Lunar Eclipses}
\label{sec:Eclipses:lunar}

Lunar eclipses occur when the Earth moves between the Sun and the Moon,
and the Moon is in the Earth's shadow. They occur under the same basic
conditions as the solar eclipse but can occur more often because the
Earth's shadow is so much larger than the Moon's.

Total lunar eclipses are more noticeable than partial eclipses because
the Moon moves fully into the Earth's shadow and there is very
noticeable darkening. However, the Earth's atmosphere refracts light
(bends it) in such a way that some sunlight can still fall on the Moon's
surface even during total eclipses. The blue parts of sunlight are scattered and filtered away, 
so that there is a marked reddening of the light as it passes through the atmosphere, and this 
makes the Moon appear in a deep red colour. The darkness and colors are notably influenced by 
atmospheric clarity and clouds. After big volcanic eruptions eclipses are often reported darker.



\section{Observing Hints}
\label{sec:observing_hints}

When stargazing, there's a few little things which make a lot of
difference, and are worth taking into account.

\begin{description}
\item[Dark skies] For many people getting away from light pollution
isn't an easy thing. At best it means a drive away from the towns, and
for many the only chance to see a sky without significant glow from
street lighting is on vacation. If you can't get away from the cities
easily, make the most of it when you are away.

\item[Wrap up warm] The best observing conditions are the same
conditions that make for cold nights, even in the summer time. Observing
is not a strenuous physical activity, so you will feel the cold a lot
more than if you were walking around. Wear a lot of warm clothing, don't
sit/lie on the floor (at least use a camping mat, consider taking a
deck-chair), and take a flask of hot drink.

\item[Dark adaptation] The true majesty of the night sky only
becomes apparent when the eye has had time to become accustomed to the
dark.  This process, known as dark adaptation, can take up to half an
hour, and as soon as the observer sees a bright light they must start
the process over. Red light doesn't compromise dark adaptation as much
as white light, so use a red torch if possible (and one that is as dim
as you can manage with). A dim single red LED light is ideal, also to
have enough light to take notes.

\item[The Moon] Unless you're particularly interested in observing the
Moon on a given night, it can be a nuisance---it can be so bright as
to make observation of dimmer objects such as nebulae impossible. When
planning what you want to observe, take the phase and position of the
Moon into account. Of course Stellarium is the ideal tool for finding
this out!

\item[Averted vision] A curious fact about the eye is that it is more
sensitive to dim light towards the edge of the field of view. If an
object is slightly too dim to see directly, looking slightly off to the
side but concentrating on the object's location can often reveal it.

\item[Angular distance] Learn how to estimate angular distances. Learn
  the angular distances described in
  section~\ref{sec:Concepts:Angles:HandyAngles}. If you have a pair of
  binoculars, find out the angular distance across the field of view
  and use this as a standard measure.
\end{description}


%% The following 3 were grouped to reduce the part TOC sections. 
\section{Atmospheric effects}
\label{sec:phenomena:Atmosphere}

%% Refraction section by GZotti 2016-04-12
\subsection{Atmospheric Extinction}
\label{sec:phenomena:Extinction}

\begin{figure}[tb]
\centering
\ifpdf
\includegraphics[width=\textwidth]{gz_extinctionmag}
\else
\includegraphics[width=\textwidth]{gz_extinctionmag.png}
\fi
\caption{Airmass and Extinction.  The figure shows Airmass (blue)
  along the line of sight in the altitude labeled on the left side.
  The green curves show how many magnitudes an object is dimmed down,
  depending on extinction factor $k$ (called $k_v$ in the figure). The
  red curves indicate at which altitude a star of given magnitude can
  be seen with good eyesight, again depending on $k$. The black dots
  are observed values found in the literature.}
\label{fig:Extinction}
\end{figure}

\indexterm{Atmospheric Extinction} is the attenuation of light of a
celestial body by Earth's atmosphere. In the last split-second of its
travel into our eyes or detectors, light from outer space has to pass
our atmosphere, through layers of mixed gas, water vapour and dust. If
a star is in the zenith, its light must pass one \indexterm{air mass}
and is reduced by whatever amount of water and dust is above you. When
the star is on the horizon, it has to pass about 40 times longer
through the atmosphere: 40 air masses (Fig.~\ref{fig:Extinction}). The
number of air masses increases fast in low altitudes, this is why we
see so few stars along the horizon. Usually blue light is extinguished
more, this is why the sun and moon (and brigher stars) appear reddish
on the horizon.

Stellarium can simulate extinction, and you can set the opacity of
your atmosphere with a global factor $k$, the \emph{magnitude loss per
  airmass} (see section~\ref{sec:gui:view:sky:atmosphere}). The best
mountaintop sites may have $k=0.15$, while $k=0.25$ seems a value
usable for good locations in lower altitudes.

%% Refraction section by GZotti 2016-04-12
\subsection{Atmospheric Refraction}
\label{sec:phenomena:Refraction}

%\begin{figure}[p]
%\centering\includegraphics[angle=90,width=.8\textwidth]{gz_refraction}
%\caption{Refraction. The figure shows corrective values (degrees)
%  which are subtracted from observed altitudes (left side) to reach
%  geometric altitudes, or values to be added to computed values (right
%  side). The models used are not directly inverse operations.}
%\label{fig:Refraction}
%\end{figure}

\begin{sidewaysfigure}%[p]
\centering
\ifpdf
\includegraphics[width=\textwidth]{gz_refraction}
\else
\includegraphics[width=\textwidth]{gz_refraction.png}
\fi
\caption{Refraction. The figure shows corrective values (degrees)
  which are subtracted from observed altitudes (left side) to reach
  geometric altitudes, or values to be added to computed values (right
  side). The models used are not directly inverse operations.}
\label{fig:Refraction}
\end{sidewaysfigure}


\indexterm{Atmospheric Refraction} is a lifting effect of our
atmosphere which can be observed by the fact that objects close to the
horizon appear higher than they should be if computed only with
spherical trigonometry. Stellarium simulates refraction for
terrestrial locations when the atmosphere is switched on. Refraction
depends on air pressure and temperature. Figure~\ref{fig:Refraction}
has been created from the same formulae that are employed in
Stellarium. You can see how fast refraction grows very close to the
mathematical horizon.

Note that these models can only give approximate conditions. There are
many weird effects in the real atmosphere, when temperature inversion
layers can create light ducts, double sunsets etc.

Also note that the models give meaningful results only for altitudes
above approximately $-2\degree$. Below that, in nature, there is
always ground which blocks our view. In Stellarium you can switch off
the ground, and you can observe a sunset with a strange egg-shaped sun
below the horizon. This is of course nonsense. Stellarium is also not
able to properly recreate the atmospheric distortions as seen from a
stratosphere balloon, where the height of earth's surface is several
degrees below the mathematical horizon.

\subsubsection{Scintillation}
\label{sec:phenomena:Scintillation}
\indexterm[scintillation]{Scintillation} is the scientific name of the twinkling which stars show
in turbulent atmosphere. The twinkling is caused by moving pockets of air with different temperature. 
Observe a star low on the horizon with a telescope, and you will note it does not stand still 
but dances around a bit, often also changing color to show red or blue hues.
While the twinkling of stars may look fine and comforting to the naked eye on a nice warm summer evening, 
telescopic observers and astrophotographers don't like it at all. They rather complain about ``bad seeing'', 
because it deteriorates the optical resolution of their instruments and photographs. 

Just like extinction and refraction are stronger along the horizon, 
stars in low altitude in general show more twinkling than stars higher up in the sky. 

Note that planets seem to be affected less by scintillation: they are not point sources, but little disks, 
and so the effect of the turbulent pockets of air distorting different parts of the disks cancels out a bit 
when observing with the naked eye: planets appear to be more stable.
Of course, the view in a telescope will still be deteriorated by the turbulent atmosphere.

While bad seeing was one key motivation of sending telescopes into earth orbit since the 1970s, 
the latest generation of ground-based giant telescopes is able to compensate for the turbulent 
motion by rapidly deforming their secondary mirror. 



\subsection{Light Pollution}
\label{sec:phenomena:LightPollution}

An ugly side effect of civilisation is a steady increase in outdoor
illumination. Many people think it increases safety, but while this
statement can be questioned, one definite result, aside from
environmental issues like dangers for the nocturnal fauna, are ever
worsening conditions for astronomical observations or just enjoyment
of the night sky. 

Stellarium can simulate the effect of light pollution on the visibility 
of celestial objects. This is controlled from the
light pollution section of the \emph{Sky} tab of the \emph{View}
window (see section~\ref{fig:gui:view:sky}).  Light pollution levels are set using a numerical value
between 1 and 9 which corresponds to the \emph{Bortle Dark Sky Scale}
(see Appendix~\ref{ch:BortleScale}). Given that public lighting became 
bright enough to cause adverse effects on observability only during 
the 19th century, Stellarium presents an unpolluted sky for dates before 1825. 
Note that of course light pollution was not switched on over night, 
but became gradually noticeable over decades, but you have to estimate the appropriate level. 

In addition, local variations of
the amount of light pollution can be included in a light pollution
layer in the landscapes, see
section~\ref{ch:landscapes} for details. This foreground layer may include 
all kinds of light, e.g.\  campfires, so this is also available for earlier 
dates.


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