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6 Catalogues and Constellations
in both astronomy and the cultural identity of the civilisations that created them.
Modern astronomers use the constellations as derived from Greek mythology with
some additional constellations added at later dates that consist of the stars too far
south to have been visible from the world of the ancient Greeks. It is also common,
especially when speaking to non-astronomers, to use an anglicised version of the
Greek name; hence the Great Bear is Ursa Major and the Swan is Cygnus.
The use of constellations may seem archaic, but they simplify the naming scheme
of many astronomical objects in that they act in many respects as the county or state
does as part of a postal address. Although there is no need to be able to identify
all 88 constellations, as defined by the International Astronomical Union (IAU), by
sight, you should be aware of the brighter and more prominent ones visible from your
site, for example Ursa Major for northern observers, Crux for southern observers,
and Orion and Leo for all observers. A full list of constellations can be found in
Table 6.1, which also shows their official abbreviations and the hemisphere in which
they are located.
Asterisms, like constellations, are collections of stars that often have no association beyond their location in a particular region of the celestial sphere. Technically,
constellations are a subset of asterisms, those that they have been formally accepted
as such, whereas most asterisms are simply a few of the brightest members of a constellation, a combination of constellations, or even a small segment of a constellation
that possesses an easily identified structure. Hence in general, asterisms have far
fewer constituents than constellations. Perhaps the most well known asterisms are
the Plough (also known as the Big Dipper or the Saucepan), comprising the brightest
seven members of the constellation Ursa Major, and the Summer Triangle, which is
a northern hemisphere feature formed from the stars Deneb, Vega, and Altair, which
are respectively the brightest members of Cygnus, Lyra, and Aquila.
If you are unlucky enough to be observing from a light-polluted site, it is probable
that the asterisms formed from bright stars will be more prominent than the constellations. This will also be the case if you are setting up during twilight, when faint
stars are still invisible. Bright stars are very important in observational astronomy.
They are used to set up the alignment of GoTo telescopes,
1 as a check of alignment, a focusing target, and the first point of call when star hopping if you are not
using a GoTo. It is unlikely that you will be able to identify the first visible star of
the evening’s observing (which might even be a planet), especially if you have been
unable to observe for some time. However, as the sky darkens and more stars become
visible, the asterisms will appear first, allowing you to identify a few stars and get
set up before it is truly dark.
1 GoTo telescopes have powered mounts and either a custom-built interface or computer control
that allows the user to move rapidly between targets. Typically, the user has to point the telescope
to three guide stars to set it up.
6 Catalogues and Constellations
in both astronomy and the cultural identity of the civilisations that created them.
Modern astronomers use the constellations as derived from Greek mythology with
some additional constellations added at later dates that consist of the stars too far
south to have been visible from the world of the ancient Greeks. It is also common,
especially when speaking to non-astronomers, to use an anglicised version of the
Greek name; hence the Great Bear is Ursa Major and the Swan is Cygnus.
The use of constellations may seem archaic, but they simplify the naming scheme
of many astronomical objects in that they act in many respects as the county or state
does as part of a postal address. Although there is no need to be able to identify
all 88 constellations, as defined by the International Astronomical Union (IAU), by
sight, you should be aware of the brighter and more prominent ones visible from your
site, for example Ursa Major for northern observers, Crux for southern observers,
and Orion and Leo for all observers. A full list of constellations can be found in
Table 6.1, which also shows their official abbreviations and the hemisphere in which
they are located.
Asterisms, like constellations, are collections of stars that often have no association beyond their location in a particular region of the celestial sphere. Technically,
constellations are a subset of asterisms, those that they have been formally accepted
as such, whereas most asterisms are simply a few of the brightest members of a constellation, a combination of constellations, or even a small segment of a constellation
that possesses an easily identified structure. Hence in general, asterisms have far
fewer constituents than constellations. Perhaps the most well known asterisms are
the Plough (also known as the Big Dipper or the Saucepan), comprising the brightest
seven members of the constellation Ursa Major, and the Summer Triangle, which is
a northern hemisphere feature formed from the stars Deneb, Vega, and Altair, which
are respectively the brightest members of Cygnus, Lyra, and Aquila.
If you are unlucky enough to be observing from a light-polluted site, it is probable
that the asterisms formed from bright stars will be more prominent than the constellations. This will also be the case if you are setting up during twilight, when faint
stars are still invisible. Bright stars are very important in observational astronomy.
They are used to set up the alignment of GoTo telescopes,
1 as a check of alignment, a focusing target, and the first point of call when star hopping if you are not
using a GoTo. It is unlikely that you will be able to identify the first visible star of
the evening’s observing (which might even be a planet), especially if you have been
unable to observe for some time. However, as the sky darkens and more stars become
visible, the asterisms will appear first, allowing you to identify a few stars and get
set up before it is truly dark.
1 GoTo telescopes have powered mounts and either a custom-built interface or computer control
that allows the user to move rapidly between targets. Typically, the user has to point the telescope
to three guide stars to set it up.