Constellations

Constellations are areas of the sky within which the brighter stars have been grouped together to form distinctive patterns to make it easier for us to find our way around the sky. In the western tradition, this practice began with the Babylonians. Modern star maps divide the celestial sphere into 88 areas of differing shapes and sizes, within each of which is to be found a number of bright stars that have been linked together to make a pattern that can, with some effort of imagination on your part, suggest the shape of a fanciful figure. The boundary of each constellation has straight edges that are parallel to either celestial longitude or celestial latitude. Stars visible within the boundaries of a particular constellation, but which do not form part of its shape, can be identified by the constellation within which they happen to lie.

A group of stars that does not make up a constellation, but which is nevertheless distinctive enough to have been named, is known as an asterism. Among of the best-known asterisms are the Plough, or Big Dipper, the Square of Pegasus, the Summer Triangle (Vega, Deneb and Altair), the Pleiades, or Seven Sisters, and the Southern Cross (visible only from the southern hemisphere).

The most distinctive constellation in the sky is probably Orion. Furthermore, since Orion lies on the celestial equator everyone on Earth can see it. Most people, however, probably associate constellations with the zodiac. This is a collection of constellations that happen to lie on the ecliptic, i.e. on the Sun's apparent path through the sky during the year. They are known collectively as the zodiac because all but one of them has been named after a creature. The astrologers of ancient Mesopotamia divided the ecliptic into 12 constellations. Since that time, however, astronomers have made various adjustments to the original scheme and in modern maps of the sky the ecliptic passes through 13 constellations. The thirteenth constellation, which is not recognised in astrology, is Ophiuchus and lies between Scorpius and Sagittarius. Many of the zodiacal constellations are quite difficult to see in their entirety with the naked eye since they all contain several faint stars.

Although the constellations that make up the zodiac are of unequal size, the largest being Virgo and the smallest being Libra, astrologers divide the ecliptic into 12 equal sections. Astrologically speaking, therefore, the Sun takes approximately one month to pass through each of these sections and its position on the celestial sphere can thus be given approximately by stating the sign through which it is passing. This means that the zodiacal constellations that you see in the night sky are those through which the Sun passed six months earlier. For example, although Cancer and Leo are visible high in the midnight sky during February, the Sun actually passes through them in August. Although astronomers do not make use of the zodiac, informal skywatchers may find it convenient, if they wish to give the approximate position of a planet or the Moon, to refer to the zodiacal constellation in which the celestial body can be seen rather than using the system of celestial coordinates employed by astronomers.

The starting point of the zodiac was fixed by Hipparchus in the second century b.c. He established the convention that the spring equinox occurs as the Sun enters the constellation of Aries. However, because of the precession of the Earth's axis, the ecliptic also precesses and in the 2000 years since Hipparchus, it has moved on by some 30°. Thus the spring equinox no longer falls in Aries. It now coincides with the start of Pisces, but it is still known as the first point of Aries even though it should now be the first point of Pisces. From an astrological point of view, this means that the Sun reaches the edge of the constellation of Pisces on 21 March of each year. Astronomically, the first point of Aries is where Sun crosses the celestial equator from the southern to the northern hemisphere in spring. This is the vernal or spring equinox.

The table below lists some of the more conspicuous objects visible in the night sky.

Some interesting celestial objects

Name

Date of Culmination Remarks

Milky Way Galaxy

The Pleiades

Great Nebula in Orion

Great cluster in Perseus Great Nebula in Andromeda

Highest in summer and winter evenings

18 November

16 December

31 October

1 October

The Coal Sack in Crux

2 August

This is actually our 'home' galaxy and it forms a broad diffuse band across the sky from one horizon to the other passing through the constellations of Cassiopeia and Cygnus in the northern hemisphere and through Crux in the southern hemisphere.

A distinctive and closely grouped collection of several stars, some six of which are easily visible to the naked eye (they are also known as 'the Seven Sisters'). The Pleiades is a star cluster (i.e. they are all in more-or-less the same place and were formed at the same time). Just visible as a fuzzy star below the 'belt' of Orion. Binoculars reveal its true identity: not a star but a nebula. A closely packed group of stars that looks like a fuzzy star to the naked eye. Not really a nebula though to the naked eye it might well be one. Actually it is the only galaxy (apart from our own) that can be distinctly seen from the northern hemisphere with the naked eye. It looks like a faint patch of light. Use binoculars for a better view.

A dark gap in the band of the Milky Way that is due to the presence of dust between the Sun and it. Use binoculars for a better

Some interesting celestial objects (cont.)

Name

Date of Culmination Remarks

Mizar and Alcor (known as the Horse and Rider)

Algol

The Dipper is circumpolar and so never sets at high northern latitudes

6 November

The pair of stars in the Big Dipper's handle (see figure 12.4). The horse is Mizar and the rider is Alcor. They are separated by 11 minutes of arc and so should be visible as separate entities to the naked eye. Algol is a variable star: i.e. its brightness changes periodically. Such stars are common, but Algol is unusual in that its brightness varies visibly from 2nd magnitude to 4th magnitude for 4.5 hours once every 2.5 days.

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