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ОглавлениеThe circumpolar stars are the key to starting to identify the constellations. Not only are they visible at any time of the year, but nearly everyone living in the northern hemisphere is familiar with the seven stars of the Plough – known as the Big Dipper in North America – an asterism that forms part of the large constellation of Ursa Major. This is where we start. There are five main constellations to identify.
Because of the movement of the stars caused by the passage of the seasons, Ursa Major lies in different parts of the evening sky at different periods of the year. The diagram shows its position for the four main seasons to give you a guide as to where to look. The seven stars remain visible throughout the year anywhere north of latitude 40°N. At that latitude, however, they may be very low on the northern horizon, especially in autumn. Much of the remainder of the constellation is then hidden below the horizon for part of the night.
Ursa Major is a large constellation, but initially few people are familiar with the extended groups of stars that form part of it and lie well to the south and east. In the centre of the curve of stars that form the ‘tail’ of the Bear (or the ‘handle’ of the Dipper) lies the small constellation of Canes Venatici, which consists of two moderately bright stars and a scattering of fainter ones.
Ursa Major at 22:00 in winter (right), spring, summer and autumn.
Constellations in the northern circumpolar region.
POLARIS AND URSA MINOR
Once you have identified the seven bright stars, locate the two stars, α and β UMa farthest away from the ‘tail’. These two, named Dubhe and Merak, respectively, are known as the ‘Pointers’. A line from Merak to Dubhe, extended to about five times their separation, leads to a fairly isolated bright star. This is the Pole Star, Polaris, or α Ursae Minoris. All the stars in the northern sky appear to rotate around it. In fact, it lies slightly less than one degree away from the true pole, and a trailed photograph of the northern sky shows that it traces a tiny circle (about 1.5 degrees across) around the pole.
Apart from this small variation, the elevation of Polaris above the northern horizon is always equal to the observer’s latitude. This was an extremely useful property, used as a guide to navigation by generations of sailors from the time of the Greeks to comparatively modern times.
Finding UMa and Polaris.
In contrast to Ursa Major, Ursa Minor is a small constellation, consisting of little more than the relatively faint, stars that form the ‘Little Dipper’ (as it is known in North America). Strangely. the asterism has no common name in Britain, other than the Little Bear, despite being superficially similar to the seven stars of the Plough. Five stars – including a moderately close pair – form the body of the constellation, with another three forming the ‘tail’ at the tip of which lies Polaris – the end of the ‘handle’ of the Little Dipper. The two stars farthest from the pole, β and γ UMi (Kochab and Pherkad, respectively) are known in English-speaking countries as ‘The Guards’.
CASSIOPEIA
On the opposite of the North Pole from Ursa Major lies Cassiopeia. It has a highly distinctive shape, appearing as five stars that form a letter ‘W’ or ‘M’ depending on its orientation. Provided the sky is reasonably clear of clouds, you will nearly always be able to see either Ursa Major or Cassiopeia, and thus be able to orientate yourself on the sky.
Finding Cassiopeia from the Plough and Polaris.
To find Cassiopeia from Ursa Major, start with UMa (Alioth), the first star in the tail of the Bear. A line from this star extended through Polaris points directly towards γ Cas (Cih), the central star of the five. Cassiopeia lies right in the centre of the band of the Milky Way, so on a clear night a large number of fainter stars surround the five main stars, and this may sometimes make identification slightly more difficult for beginners. Even when the Milky Way is not readily visible because of moonlight or light pollution, some people are still occasionally confused by the presence of the moderately bright stars, η and κ Cas. With just a little practice, however, Cassiopeia becomes easy to recognize.
CEPHEUS
Although the constellation of Cepheus is circumpolar, it is not nearly as well known as Ursa Major, Ursa Minor or Cassiopeia. This is partly because it does not form a highly distinct pattern on the sky, and also because some of its stars are faint. Its shape is often likened to (and does actually resemble) the gable end of a house, with the ‘ground’ lying in the Milky Way not far from Cassiopeia, and the tip of the gable pointing towards the pole.
Locating the constellation of Cepheus.
Its brightest star, α Cephei (Alderamin), lies in the Milky-Way region, at the ‘bottom right-hand corner’ of the figure. A line from α to β Cassiopeiae (Shedir to Caph), extended about three times the separation, passes just north of Alderamin. The star at the tip of the ‘gable’, γ Cephei (Errai) lies close to the line from α UMa (Polaris) to β Cas (Caph). The line from the Pointers to Polaris, if extended, also passes just ‘below’ the tip of the ‘gable’.
DRACO
The last of the circumpolar constellations that is fairly easy to identify is Draco, although it is so long that it takes a bit of practice to recognize it easily. It consists of a quadrilateral of stars, known, logically enough, as the Head of Draco (and also the ‘Lozenge’), and a long chain of stars forming the neck and body of the dragon.
Finding the Head of Draco is not particularly easy using just circumpolar stars. Locate the two stars (α and β UMa) at the opposite end of the bowl of the Plough from the Pointers. Extend a line from γ UMa (Phad) through δ UMa (Megrez) by about eight times their separation. This takes you right across the sky below the Guards in Ursa Minor, and close to ξ Dra (Grumium) at one corner of the quadrilateral. The brightest star, γ Dra (Etamin) lies farther to the south. From the head of Draco, the constellation first runs north-east to δ and Dra, then doubles back southwards, before winding its way round between Ursa Minor and Ursa Major, through α Dra (Thuban), before ending at λ Dra (Giansar) almost on the line between the Pointers and Polaris.
Finding the head of Draco.
The circumpolar chart shows that there is a large area of sky between Ursa Major and Cassiopeia that has relatively few bright stars. There is one constellation here, Camelopardalis, which is always circumpolar, but which is so faint that it is easier to learn to find it after you have gained some experience, and are familiar with some of the constellations that lie farther south. Large parts of the constellations of Auriga, Cygnus, Lynx, and Perseus are also often circumpolar, depending on your exact latitude. These will be described shortly.
SEASONAL CONSTELLATIONS
Although it is generally best to start with the circumpolar constellations when learning the night sky, the visibility of areas farther south depends greatly on the season. Luckily, there are conspicuous, easily recognized constellations visible at every season: Orion, Taurus, Gemini, and Auriga in winter; Leo, Hydra, and Boötes in spring; Cygnus, Lyra, and Aquila in summer; Pegasus and Andromeda in autumn. The limitations posed by summer twilight may cause problems in learning that area of the sky, but the brilliant stars of the Summer Triangle (Deneb, Vega, and Altair) are readily seen and act as guides even in twilight.
For northern-hemisphere observers, the winter sky is always dominated by the magnificent constellation of Orion. It is readily recognizable, with orange-red Betelgeuse (α Orionis) in the Northeast, brilliant, blue-white Rigel (β Orionis) in the south-west, and the three stars forming the ‘Belt’ in the centre of the constellation.
The stars of the Belt point down towards Sirius (α Canis Majoris), the brightest star in the sky. The southern part of Canis Major, with δ (Wezen) and (Adhara), is low from most northern latitudes and requires clear skies to be readily seen. West of Canis Major, and beneath Orion, is the small, but distinctive constellation of Lepus. South of that, low on the horizon, Columba may be seen when skies are clear. Farther west, the long, straggling constellation of Eridanus begins close to Rigel and sweeps in a wide arc to the west before plunging below the southern horizon.
Following the Belt stars in the opposite direction, they point north-west, approximately in the direction of orange Aldebaran (α Tauri) and the nearby ‘V’-shaped cluster of the Hyades. Still farther north-west lies the conspicuous cluster of blue-white stars known as the Pleiades. The rest of the constellation of Taurus is relatively inconspicuous, and the second-brightest star β (Alnath) was once regarded as part of Auriga. The brightest star in Auriga itself, yellowish-white Capella (α Aurigae), is high overhead, near the zenith. The constellation forms a rough pentagon with β Tauri, with a distinctive triangle of stars (known as ‘the Kids’) west of Capella.
Northwest of Auriga, above the Pleiades, lies Perseus, with no distinct shape, but which essentially consists of three lines of stars, meeting at α Persei (Mirfak). One line runs down towards the south, the second, containing the famous variable Algol towards the south-west, and the third extends away from Taurus and Orion towards Cassiopeia. Southeast of Auriga lies Gemini, with the bright stars Castor and Pollux. Castor (α Geminorum), the northernmost, is the fainter of the two, and may also be found by extending a line from Rigel, at the foot of Orion, through Betelgeuse. Pollux (β Geminorum), lies to the south-east. The rest of the constellation largely consists of two lines of stars that run back from Castor and Pollux towards Orion.
Forming an approximate equilateral triangle with Betelgeuse and Sirius is Procyon (α Canis Minoris), the single bright star (magnitude 0.4) in an otherwise inconspicuous constellation. More or less centred in the same equilateral triangle is the sprawling, but very faint, constellation of Monoceros.
The Milky Way runs right across this region of the sky, from Cassiopeia, through Perseus, Auriga, Gemini, Orion, and Monoceros. Although less conspicuous than the dense star clouds that stretch from Cygnus to Sagittarius, it is nevertheless readily seen in dark, clear skies. The swarms of faint stars sometimes make it difficult to pick out the pattern of Monoceros, and beginners occasionally encounter problems in identifying the fainter stars in Perseus for the same reason.
The northernmost star of Orion’s Belt, Mintaka (δ Orionis) is just slightly to the south of the celestial equator. Trailed photographs of Orion rising in the east show Mintaka’s trail as an almost perfectly straight line, while the trails of stars farther north and south show increasing curvature.
The Sun reaches its northernmost declination (at the northern summer solstice) just inside the eastern border of Taurus, precession having carried it out of Gemini. If the line of latitude at which the Sun is overhead at the summer solstice were named nowadays, instead of being known as the Tropic of Cancer, it would be the Tropic of Taurus.
Winter: the bright winter constellations are easy to recognize.
The most distinctive constellation in the spring sky is Leo, whose brightest star, Regulus (α Leonis) lies extremely close to the ecliptic. When Leo is low in the sky, it may be located by extending the line of the Pointers (α and β Ursae Majoris) in the opposite direction to normal, away from Polaris. Regulus may also be found by extending a line through δ and γ Ursae Majoris (Megrez and Phad) towards the south-west. Above Regulus is the characteristic asterism known as the Sickle, looking like a ‘backwards question-mark’ or a shepherd’s crook in the sky. Two lines of stars run eastwards from Regulus and the centre of the Sickle to form the body of Leo, converging at Denebola (β Leonis) in the east. Regulus, being so close to the ecliptic, is occasionally occulted by the Moon.
Between Ursa Major and Leo is the tiny, faint constellation of Leo Minor, only three stars of which are readily visible to the naked eye. Also in this area, beginning above the Sickle, is the straggling line of faint stars that forms Lynx, another unremarkable constellation. Cancer, the faintest of the zodiacal constellations, lies west of Leo. It consists of three lines of stars that meet at the central star, δ Cancri, on the ecliptic.
Following the curve of the ‘tail’ of Ursa Major is the standard way of locating yellowish-orange Arcturus, α Boötes, which is the brightest star in the northern hemisphere of the sky. (Sirius is in the southern hemisphere.) The constellation of Boötes forms a sort of ‘kite’ or ‘P’ shape north of Arcturus. Continuing the same arc from Ursa Major takes one down to another bright blue-white star, Spica, α Virginis, which is slightly south of the ecliptic. The shape of Virgo, which is the second largest constellation, is difficult to define, but consists of a roughly rectangular arrangement of stars, with lines running from each corner towards west and east. Because Virgo is another zodiacal constellation, Spica (like Regulus) may sometimes be occulted by the Moon.
In the area between Ursa Major, Boötes, Virgo, and Leo lie two faint constellations: Canes Venatici, within the arc of the ‘tail’ of Ursa Major and, farther south, Coma Berenices, the brightest three stars of which form a right-angle in the sky.
Spring is the best time in the year for seeing the largest constellation of all, Hydra, which starts at the distinctive asterism (the ‘Head of Hydra’) south of Cancer in the west, and straggles eastwards below both Leo and Virgo. Although its brightest star, Alphard (α Hydrae), is often readily visible well below Regulus, the second brightest, γ Hydrae, is frequently lost in the horizon haze at most mid-northern latitudes.
Spring: Leo is the dominant constellation in spring skies.
The small, unremarkable constellation of Sextans lies above Hydra, and immediately south of Regulus. Farther east is the slightly more conspicuous constellation of Crater. Its stars, although faint, do actually form a shape reminiscent of the ancient drinking vessel. Still farther east, below the western end of Virgo, is the marginally brighter constellation of Corvus. Its third-magnitude stars form a rough quadrilateral that is relatively distinct.
The light summer nights and the persistence of twilight mean that some of the constellations in this part of the sky are poorly known. For the time when the centre of this chart is due south (22:00 LMT) many of the fainter stars are difficult to detect. Two hours later, at local midnight, when it is darkest, the view is dominated by the famous Summer Triangle: the three bright stars Deneb, Vega, and Altair (α Cygni, α Lyrae, and α Aquilae, respectively). On a clear night, summer and early autumn are the best times of year to appreciate the full glory of the Milky Way that runs right across this region.
Lyra is a small constellation, which, to the naked eye, basically consists of blue-white Vega and a quadrilateral of fainter stars to its east. Cygnus forms a giant cross in the sky, and is easily envisaged as a swan, with Deneb at the tail – which is what the name means in Arabic – flying ‘down’ the Milky Way. It head is marked by Albireo, β Cygni, in the south. Beginning in Cygnus and running down the centre of the Milky Way towards the south is dark area apparently devoid of many stars. This is the Great Rift, and is actually caused by interstellar dust blocking the light from vast clouds of more distant stars.
Aquila, farther south, has a somewhat similar shape, with a rhombus of stars forming the ‘wings’ of the eagle and a ‘head’ and ‘neck’ also pointing down the Milky Way. Again, Altair (α Aquilae) marks the tail. Between the two celestial birds lies the inconspicuous constellation of Vulpecula, consisting of three faint stars, and the slightly more distinctive shape of Sagitta, pointing across the Milky Way. To the west are the five stars that form the tiny, but highly distinctive, constellation of Delphinus.
Summer: summer brings a view of the densest part of the Milky Way.
The broad, constellation of Ophiuchus – sometimes described as looking like a tent with Rasalhague (α Ophiuchi) at the apex – lies to the west of the main band of the Milky Way. It represents the Serpent Bearer, and is located between the two halves of Serpens, the only divided constellation. The brighter, more distinct portion, Serpens Caput (the Head) lies in the west, while Serpens Cauda (the Tail) is a chain of faint stars lying along the centre of the Milky Way.
Below Serpens Caput, close to the horizon, and often difficult to recognize, is Libra, basically a triangle of stars lying across the ecliptic. It once formed part of the next zodiacal constellation to the east, Scorpius. Although many of the stars that form the body of the scorpion and its sting are often invisible below the horizon from mid-northern latitudes, bright red Antares (α Scorpii) and the arc of stars between it and Libra are usually visible. Still farther to the east is Sagittarius, where the ecliptic reaches its lowest declination and where the winter solstice is located. The southernmost stars of Sagittarius are often obscured, but the distinctive asterism of the ‘Teapot’, consisting of moderately bright stars is relatively easy to see.
Two more constellations lies between Boötes and Lyra. Immediately east of Boötes is the distinctive arc of stars that forms the small constellation of Corona Borealis. Farther towards Lyra lies Hercules. Here the central four stars (forming the asterism known as the ‘Keystone’) are readily recognized. From each corner, chains of stars form the ‘arms’ and ‘legs’ of Hercules (who is actually upside-down in the sky). One ‘leg’ ends close to the zenith, beyond which is the distinctive lozenge that is the head of Draco.
Steve Edberg
The constellation of Virgo, with Spica to the left of centre and towards the bottom of the picture.
As summer begins to turn to autumn and the Summer Triangle begins to dip towards the western horizon, the view to the south is dominated by the Great Square of Pegasus. Chains of stars run westwards from Markab and Scheat (α and β Pegasi) towards Delphinus and the Milky Way in Sagitta and Cygnus. One line ends at Enif ( Pegasi), and between this star and Delphinus is the extremely faint, small constellation of Equuleus.
There are very few stars in this area of the sky, and even under extremely good conditions few people are able to see more than about 12 or 13 stars within the Great Square with the naked eye.
Alpheratz (α Andromedae) is the star marking the north-eastern corner of the Great Square, and Andromeda itself consists of two lines of stars running up towards Perseus, in the Milky Way, which arches over this region of the sky, passing through Cygnus and Cassiopeia. Below Perseus are the three stars that make up the constellation of Triangulum, and below that, the small zodiacal constellation of Aries.
Another zodiacal constellation, that of Pisces, lies to the south and east of the Great Square of Pegasus. Although consisting of faint stars, the Circlet, which represents the western fish, is fairly distinct below the Great Square. A line of stars connects it to Alresha (α Piscium) in the far west, from which another chain of stars extends north-west towards the centre of Andromeda. In the opposite direction, that line of stars points towards the famous variable star, Mira, in the constellation of Cetus. However, Mira sometimes drops below naked-eye visibility, when the constellation appears as two separate portions: a loop of stars to the North-east, and an irregular pattern of six moderately bright stars, including Deneb Kaitos (β Ceti), the brightest in the constellation, low in the south-west.
There are two more zodiacal constellations in this area of the sky. The first, Aquarius, is a rather straggling constellation in which the eye has difficulty in picking out any particular patterns. There are two moderately bright stars, Sadalmelik and Sadalsud (α and β Aquarii) and, to the west of Sadalmelik, four stars arranged in the distinctive ‘Water Jar’, also known as the ‘Y of Aquarius’. To the west is the roughly triangular constellation of Capricornus, whose brightest stars, Deneb Algedi and Dabih (δ and β Capricorni) are at the eastern and western apex, respectively.
Autumn: Pegasus and Andromeda are guides to the autumn skies.
A line from Scheat (β Pegasi) through Markab (α Pegasi) points down to a solitary bright star, just above the southern horizon. This is Fomalhaut (α Piscis Austrini) in the otherwise faint constellation of Piscis Austrinus. High above, between Scheat and the bottom of Cepheus, lies a small zig-zag of faint stars that forms the constellation of Lacerta.
