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A solar day is slightly longer than a sidereal day, because the Earth must rotate through an additional angle before the Sun again crosses the meridian.
SIDEREAL TIME
We have already noted how the stars visible on any night slowly change throughout the year, new stars rising over the eastern horizon at dusk, as stars that have been visible for some months disappear beneath the western horizon. This change occurs because although our civil time is based on the Earth’s rotation, relative to the Sun, where one day equals 24 hours, relative to the stars, the Earth rotates once in 23 hours, 56 minutes, and 4 seconds. (The difference arises because of the Earth’s motion around the Sun, which means it has to rotate a little further to bring the Sun back into the same position in the sky.)
Steve Massey
Knowing the sidereal time (the RA on the meridian) can be of considerable help in locating faint objects, such as the Helix Nebula (NGC 7293) in Aquarius.
Sidereal time is time as determined by the stars. It is equal to the right ascension of objects that are currently on the meridian. If you know the sidereal time, you can immediately tell which objects are visible in the sky. Sidereal time obviously differs for every location around the planet. Handbooks often tabulate the sidereal time at the Greenwich Meridian for 00:00 UT, and you can obtain your local sidereal time by making the appropriate corrections for the time that has elapsed since 00:00 UT and for your longitude. Many amateur astronomers, however, adjust a clock to run slightly fast, and thus keep sidereal time. Once set, this gives an immediate indication of objects that are on their local meridian.
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You may occasionally come across the term ‘hour angle’ used to describe the position of an object in the sky. Hour angle is the angle, measured westwards along the equator (i.e., anticlockwise looking down on the North Pole), between the meridian and the hour circle passing through the object in question. Hour angle therefore increases with the passage of time.
SEE ALSO
The celestial sphere is divided into 88 individual constellations. Many of these hark back to antiquity, and various ancient civilizations divided the sky in different ways and had a whole range of myths that were associated with the various groups of stars. The constellation names that we use today largely derive from Greek and Roman sources. Very few bear any resemblance to the objects that they purport to represent.
Northern constellations are often ancient, but many of those for the southern hemisphere were introduced about the middle of the 18th century by the French astronomer de Lacaille. For hundreds of years the boundaries between constellations were extremely fluid, and largely depended on the whim of the person drawing up the descriptions or charts. The boundaries were more or less arbitrary lines around the figures. Stars might be allocated to one constellation by one cartographer, and to the neighbouring constellation by another person. Various map-makers introduced smaller constellations of their own, some of which have survived, but most of which have disappeared.
Royal Astronomical Society
This pictorial depiction of the constellation of Cygnus comes from the second edition of Johannes Bayer’s URANOMETRIA, originally published in 1603.
The end to this confusion came in 1920 when the International Astronomical Union (IAU) adopted a standard set of constellation names; and again in 1930, when specific constellation boundaries were laid down. The latter were defined as straight lines in right ascension and declination for epoch 1855. This date was selected because that epoch had already been used for defining many southern constellations. If you glance at a modern chart, you will see that, because of precession, the constellation boundaries have drifted away from the modern co-ordinate grid.
SEE ALSO
Although popular names for the constellations are sometimes encountered, astronomers always use the Latin forms, so these are employed throughout this book, as are the appropriate genitive forms and standardized three-letter abbreviations, all of which were formally adopted by the IAU.
ASTERISMS
An asterism is a conspicuous pair or group of stars that is given a specific name, but which does not necessarily form a whole constellation. One example is The Plough – known as the Big Dipper in North America, as well as by many other names – the seven brightest stars in Ursa Major, a large constellation that includes dozens of other stars.
Neil Bone
The most famous asterism of all: the seven stars of ‘The Plough’ or ‘Big Dipper’.
THE ZODIAC
The Zodiac was originally a specific set of twelve constellations with a particular significance for ancient civilizations. They were the constellations in which the Sun, Moon, and major planets were found. As such, they were centred on the ecliptic, the Sun’s apparent path across the sky, and each constellation was regarded as extending for 30° along the ecliptic. Because the majority of the constellations were associated with animals, the whole band was named the Zodiac.
It was once believed that stars and planets exerted a direct influence upon human affairs, so the twelve constellations assumed an astrological significance and were seen as specific ‘signs’. Rational people no longer believe in such superstitions, and precession has shifted the position of the constellations eastwards by some 30-odd degrees, so they no longer agree with their signs. Changes in constellation boundaries mean that some constellations occupy more or less than 30° of the ecliptic, and also that the Sun, Moon and planets may appear in various additional constellations, most notably Cetus, Orion, Sextans, and Ophiuchus. The Zodiac is now defined as a band, stretching 8° on either side of the ecliptic, and regarded as including parts of all these different constellations. Charts of the zodiac are useful for showing planetary positions, as seen later in this book.
ZODIACAL CONSTELLATIONS
Virgo to Aquarius: the zodiac is the strip between the two broken green lines, where the Sun, Moon and planets are found.
Pisces to Leo: the zodiac is the strip between the two broken green lines, where the Sun, Moon and planets are found.