25 THE CONSTELLATIONS AND THE NAVIGATIONAL STARS

90. THE CONSTELLATIONS

The grouping of the bright stars into asterisms or constellations originated in the mists of antiquity. The division of the stars into constellations is arbitrary. Most of the present-day constellations were named by the Ancient Greeks after mythological characters, gods and demi-gods, as well as ever day objects and creatures.

These names give but little assistance to the student of the night sky, because the constellations seldom bear any resemblance to their classical descriptions. Ptolemy divided the stars into forty-eight constellations in around 150 A.D., but since this time others have been added, there now being a total of 88.

The boundaries of the constellations were chosen arbitrarily, and they appeared differently in different star atlases. In recent times a rectification of the boundaries has been made by the International Astronomical Union, the boundaries now consisting of arcs of great circles perpendicular to the equinoctial and arcs of small circles of declination.

Many of the brighter stars have particular names, but in order to distinguish every star it became necessary to adopt a simple and effective system other than by giving particular names. In his famous star atlas “Uranometria” of 1603, the astronomer Beyer introduced a system in which each star in a constellation is assigned a letter of the Greek alphabet (see fig. 25-1).

In general, the brightest star in a constellation is designated αꭀ, the next brightest β, and so on. Sirius, the brightest star in the constellation Canis Major (The Great Dog), may be referred to as ɑꭀ (Canis Majoris; and the Rigel, the second brightest star in the constellation Orion (The Hunter), may be referred to as β Orionis. This general rule, however, does not always apply. In particular it does not apply to the stars forming the constellation of Ursa Major (The Great Bear, or Plough), which are lettered with the first seven stars of the Greek alphabet in order of position, not brightness, starting with Dubhe, which is ɑꭀ Ursae Majoris. When the Greek letters are exhausted, recourse is made to Roman or Italic letters in Bayer’s system. Constellation numbers, instead of letters, were first suggested by the first British Astronomer Royal, Flamsteed, and this system is now used almost universally in all the great star catalogues.

The time and effort expended in becoming acquainted with the star groups leads to a form of pleasure and enjoyment which never loses its fascination. This is reason enough for anyone who wishes to become familiar with the patterns of the constellations.

The navigator, however, has a more pressing reason for learning to recognise constellations so that he may readily identify the stars of navigation. The navigational stars are those for which astronomical data are provided in “The Nautical Almanac” totalling some 173 stars in all and including all stars with a magnitude of 3.0 or brighter.

Of this number, 57 are selected on the basis of brightness and distribution in the sky so as to give adequate coverage for normal navigational purposes, and these ‘selected stars’ are listed together with their S.H.A.’s and declinations on the daily pages of “The Nautical Almanac”. The selected stars have proper names in addition to their constellation. The proper names used on the daily pages are usually those by which they were known to the Arab or Greek astronomers of antiquity, but in the full list of 173 stars near the end of “The Nautical Almanac” both the proper names and their constellation designations are given.

To learn the constellations requires frequent observation of the night sky.

Most navigators are able to identify a particular star by recognising that it lies on a straight line or arc joining or passing through other known stars, or that it forms, with other known stars, a simple geometric Shape such as a triangle or a square. It is useful to be able to estimate the values of angles on the celestial sphere and it is therefore worth remembering that the spread of a hand at arm’s length corresponds roughly to about 20° of arc.

The easiest way to learn the constellations is to begin by finding two or three immediately recognisable constellations and to use these as signposts to the other conspicuous ones. There are four constellations which most of us have no difficulty in recognising and which serve as a useful guide to the fifteen very bright stars which are of North declination or less than 40°S. Declination and of the greatest navigational value. These constellations are Ursa Major (The Great Bear, also known as the Plough or the Dipper), Pegasus (The Flying Horse), Cassiopeia (The Lady in Her Chair) – all of which can be seen from anywhere which lies north of latitude 40°S. – and Orion (The Hunter), which straddles the celestial equator and can thus be seen from anywhere in the world, although in home waters, is only up during winter nights.

These four constellations are shown in fig. 25-2 (which is not to – scale but serves to show the relationship of these constellations to one another). In the northern hemisphere these constellations appear to revolve around Polaris (the Pole star) in an anti-clockwise direction. As previously explained in this Course, because of the rotation of the Earth, the whole celestial sphere appears to rotate once a day, and so the constellations move across the sky.

Furthermore, as the Earth moves around the Sun in its orbit, different constellations become visible in the night sky. In fact, the constellations seem to rise four minutes earlier each day until, after a full year, they have returned to their original positions. Because the Earth is a sphere, not all the constellations which are visible from one point on the Earth’s surface can be seen from another point, and vice versa. At the North Pole, for example, the stars follow paths parallel to the horizon (since the celestial pole is directly overhead and the celestial  equator runs parallel to the horizon),  and these are constantly visible when the Sun has set. At the equator, however, the celestial equator runs directly overhead and the celestial poles lie on the horizon.

– Thus all the constellations can be seen at some time, but none are visible all the time. Stars which can always be seen are (as explained in § 4) said to be circumpolar; at the poles, all the visible constellations are circumpolar, while none is so at the equator. In between these latitudes some of the constellations will be circumpolar, some will not. To find out which stars or constellations are circumpolar in your own latitude, simply subtract your own latitude from 90°, when constellations with greater declination and same name than your answer will be circumpolar. For example, in latitude 5O°N. all stars with a declination of 4O°N. or more will be circumpolar. Furthermore, you will never see stars with a declination more than 4O°S

91. THE CONSTELLATION OF URSA MAJOR AND ITS ENVIRONS

The constellation of Ursa Major (The Great Bear) is so well known to northern observers that the seven bright stars which form that part known as the Plough or Big Dipper provide a useful starting point for a survey of the constellations and the navigational stars (see fig. 25-3). The whole constellation is circumpolar north of Lat. 40°N., and when it lies below the Pole it will appear as in fig. 25-3 (a), whereas when it lies above the pole it appears as in fig.25-3 (b).

A line through the stars Merak and Dubhe (β and ɑ Ursa Majoris) known as ‘The Pointers’, leads the eye to Polaris, the pole star, which is a 2nd magnitude star lying some 30° distant, pretty well on its Own. Polaris is perhaps the most important navigational star because, being so close to the north celestial pole, its altitude is approximately equal to the northern latitude of an observer, and its bearing is approximately due north. 

The relationship of Ursa Major to nearby constellations is shown in fig.-25-3, from which it will be seen that Polaris lies at the tail end of the faint constellation Ursa Minor (The Little Bear).

The seven principal stars of this small constellation form a similar shape to the Plough except that the tail curves in the opposite direction, and in the position corresponding to Dubhe in Ursa Major lies the navigational star Kochab.

The last star on the handle of the Plough is Alkaid. Along an arc through the handle of the Plough and at an angular distance of about 30° from Alkaid, the brightest star in the constellation Bootes (The Herdsman) will be found, this being a 1st magnitude orange coloured star called Arcturus. Continuing along the same arc through Alkaid and Arcturus, and again at about 30° from Arcturus, will be found the bright star Spica in the Y-Shaped constellation Virgo (The Virgin). 

Spica is a blue-white star of exactly magnitude 1, and lies near the four stars of the constellation Corvus (The Crow) which seamen call Spike’s Spanker (not shown in Fig: 25-3b). Continuing along the same arc through Alkaid, Arcturus and Spica, and again at about 30° from Spica, will be found Alphard, the ‘lonely’ star, so named because it lies in a region in which bright stars are scarce.

On the concave side of the arc through Alkaid, Arcturus, Spica and Alphard lies the beautiful constellation Leo (The Lion), the most notable feature of which is the curved sickle of stars at the base of which lies the blue-white 1st  magnitude star, Regulus.

92. THE CONSTELLATIONS OF CASSIOPEIA AND PEGASUS AND THEIR ENVIRONS.

At the end of a great-circle arc through Polaris, at about the central position and the other end located in the vicinity of the Plough, will be found the constellation of Cassiopeia (The Lady in the Chair). It does not contain any stars of 1st magnitude but is fairly prominent in the sky and is useful in helping to identify the constellation – Pegasus.

The five brightest stars in Cassiopeia form the Shape of a letter W, and are imagined to depict a queen sitting on her throne, with Schedar standing at the right foot below Caph β Cassiopeia) which lies at the top of the right-hand side of the W.

The Milky Way runs through Cassiopeia and in 1572 a supernova explosion occurred within the constellation, the star concerned outshining even Venus ♀for a time before fading from view. A straight line from Caph through Schedar fetches up at the star Algol (the ‘eye of the demon’) in the constellation Perseus, which varies in magnitude from 2.3 to 3.5 in a period of or just under three days, and is the prototype eclipsing binary star. (An eclipsing binary star consists of two close stars revolving round each other so that one will pass in front of the other at certain times and so reduce the total amount of light coming our way).

A line from Polaris through Cassiopeia extended for some 30° leads to the constellation of Pegasus (The Flying Horse). The three stars Markab (ꭀœ  Pegasi), Scheat (β Pegasi) and Algenib (ᴕ Pegasi), together with Alpheratz (œAndromedae), the brightest star of the constellation Andromeda (The Daughter of Cassiopeia) are located respectively at the corners of a conspicuous rectangle of side about 55°, sometimes known as The Square (although the actual Shape would hardly have satisfied Euclid). The side formed by Alperatz and Algenib lies almost on the meridian through the First Point of Aries and is therefore useful for judging sidereal time.

On a straight line extending to the northwest of Alpheratz are located Mirach (β Andromedae) and Almach (ᴕ Andromedae). This line continues to Mirfak (œPersei), the brightest star in the constellation Perseus. The adjacent stars in this set of four are about 10° apart. Mirfak lies about midway between Cassiopeia and the bright yellow 1st magnitude star Capella in the constellation Auriga (The Charioteer) – shown in fig. 25-6.

To the northeast of Scheat (β Pegasi) at a distance of about 20°, is the striking constellation Cygnus (The Swan), often known as the Northern Cross because of its Shape. The star at the top of this cross is a white 1st-magnitude star, Deneb, while at the bottom of the cross lies Albireo, a beautiful double star, one gold and the other green, whose angular separation half a minute of arc (see. fig. 25-4.)’

Lying between Alpheratz and Algenib, at a distance of about 20°, are the two stars Hamal and Sheratan, the two brightest stars of the constellation Aries (The Ram). The slowly accumulating result of the movement of the stars (mentioned in § 1-5) is seen in the distance that the constellation Aries now lies from the meridian through the First Point of Aries.

Following the constellation Cygnus up the Milky way lies the constellation Aquila (The Eagle), the principal star in which is Altair, a white 1st magnitude which, together with Deneb forms a very fine triangle with Vega, the brightest star in the northern skies (magnitude 0.04) and blue-white in colour. This triangle is often known as the ‘Summer Triangle’. Vega lies is the constellation Lyra (The Harp).

93. THE CONSTELLATION OF ORION AND ITS ENVIRONS.

The constellation of Orion (The Hunter) is often regarded as being the most splendid of all the star groups. His outline is easy to see, with his ‘sword’ hanging from the three stars which make up his ‘belt’. To ancient Arab astronomers, the constellation Orion was called the Central One, because it stands astride the celestial equator.

The four principal stars of Orion form the shoulders and feet of the Hunter. The brightest star, Betelgeuse, located in the north-east shoulder of Orion, is a brilliant red variable supergiant with an average magnitude of 0.8, while the second-brightest star, Rigel, which forms his south-east foot, is a blue-white giant of magnitude 0.3 and is, in fact, a double star being easily separated in a 3-in. refractor.

The three stars forming Orion’s belt stand at the mid position of a straight line, which terminates in the constellation Canis Major (Great Dog), one of Orion’s two dogs, containing the brightest star in the sky Sirius (magnitude – 1.6) which lies about 20° to the south-west of the belt, and the constellation Taurus (The Bull) containing the red 1st magnitude star Aldebaran (the Bull’s Eye), which lies about 2O° to the north-east of the belt. Around Aldebaran lies the fine V-Shaped cluster of the Hyades and, beyond, the famed Pleiades cluster. Also in Taurus lies the Crab Nebula, the remnants of a supernova.

Due north of the three stars in Orion’s belt, and passing through a tiny group of faint stars forming Hunter’s head, lies the star Elnath also in Taurus), located about 20° north of Orion’s shoulders. further 20° along this line lies the brilliant pale yellow Capella in Auriga (The Chorister, already mentioned). Capella is readily recognised by the three small stars, known as the Kids, which form a small isosceles triangle nearby.

To the northwest of Betelgeuse at a distance of about 25°, are the two bright stars Castor and Pollux of the constellation Gemini (The Twins). Castor ꭀ** Geminorum), is the more northerly and brighter of the pair and is a brilliant white star compared with Pollux (β Geminorum) which is yellow.

Due south of Gemini and due west of Betelgeuse lies the constellation Canis Minor (Little Dog), the lesser of Orion’s two dogs containing the 1st-magnitude star, Procyon, another beautiful yellow star of magnitude -0.5.

Procyon lies at the eastern end of a straight line through Regulus, Denebola and Arcturus. Due east of Orion’s right shoulder, at which the star Bellatrix is located, at an angular distance of about 30°, lies Menkar, the brightest star in the constellation Cetus (The Whale).

94. THE SOUTHERN CONSTELLATIONS AND STARS.

Perhaps the most notable constellation of the southern skies is Crux (The Southern Cross) which lies about 30° due south of Spica’s Spanker, although this constellation only resembles a cross if the observer imagines diagonal lines joining the four stars in it (fig. 25-8). The brightest stars of the Southern Cross are Acrux (Ɣ) and Mimosa (β) both of 1st magnitude, while Gacrux (ᴕ) is of magnitude 1.6.

A line through Gacrux and Acrux points roughly to the south celestial pole but the constellation is too far removed from the Pole to be of any use in finding the observer’s latitude directly, as may be done with Polaris in the Northern Hemisphere.

About 10° to the west of the Southern Cross lie the two brightest stars of the constellation Centaurus (the Centaur), Rigel Kentaurus (Ɣ Centuari) and Hadar (β Centauri), both of the 1st magnitude. Rigil Kent is a superb double star and also contains within its system a faint red dwarf star (Proxima Centauri), the nearest star to the solar system at a distance of 4¼ light years.

About 35° to the south of Sirius and about 30° to the east of the Southern Cross lies Canopus, the brightest star in the constellation Carina (The hull of the ship Argo) and the second brightest star in the heavens.

Canopus is a yellow star of magnitude -0.9. and lies 650 light years away, shining 80,000 times brighter than the Sun. At about 30° to the east of Canopus lies another 1st magnitude star AchernarƔ Eridani) standing on one end of a long twisting constellation called Eridanus (The Heavenly River), the other end of which is located near Rigel in Orion. If a line through Canopus and Achernar is produced for a further 30° it will lead to Fomalhaut (Ɣ Piscis Australis), a magnificent 1st magnitude star in the small but otherwise faint constellation Piscis Australia (The Southern Fish).

Fomalhaut can also be located by producing a line through Scheat and Markab in the constellation Pegasus for about 45° due south.

About 40° south-west of the star Spica, and 35° north-west of Rigil Kent, lies the constellation Scorpius (The Scorpion which bit Orion) in which the principal star is Antares a deep red super giant of magnitude 1.0 with a diameter nearly 300 times greater than the Sun.