The Big Dipper, Leo and Virgo

Seven stars in the north of the sky make the shape of a saucepan, called the Big Dipper in North America and the Plough in Britain. It is the brightest part of Ursa Major, the Great Bear, and a pattern of this kind within a constellation is called an asterism (Section O1.1). Seen from Ankara, six of its seven stars never set. They circle the north celestial pole once every sidereal day, and only Alkaid, at the end of the handle, dips just below the northern horizon for a short time each day. On 13 May the Dipper stands above the pole at 21:00 local mean time, high in the north, which makes the evenings of spring its season. Seen from Sydney the picture is very different, because Dubhe, Merak and Megrez never rise there, and the other four stars at best scrape the northern horizon, with Alkaid reaching 7°. The Dipper is the key to the northern spring sky in the way that Orion is the key to the winter sky, because hops from it lead to Polaris, to Arcturus and Spica, and to Regulus in Leo. In this section we learn the Dipper and the pole first, then the hops, and then Leo, Boötes, Virgo and the fainter constellations between them.

O1.3.1The Big Dipper and the pole

The bowl of the Dipper has four stars. Dubhe (α UMa) and Merak (β UMa) make its outer edge, the one farther from the handle, and Phecda (γ UMa) and Megrez (δ UMa) make the inner edge. The handle runs on from Megrez through Alioth (ε UMa) and Mizar (ζ UMa) to Alkaid (η UMa). Here Bayer's letters follow the order of the stars round the figure, and Alioth, which is ε, is in fact the brightest of the seven at magnitude 1.76. Mizar has a fainter companion, Alcor, 11.8′ away, and good eyes see the two as a pair on a dark night.

Merak and Dubhe are called the Pointers. The line from Merak through Dubhe, continued about five times as far as the gap between them, leads to Polaris (α UMi), which is less than a degree from the north celestial pole. Polaris has magnitude 1.97, which makes it only about the 48th brightest star of the sky, but it is easy to find because nothing else near the pole is as bright. It marks the end of the handle of the Little Dipper, Ursa Minor. The bowl of the Little Dipper ends in Kochab (β UMi) and Pherkad (γ UMi), 3.2° apart, which are called the Guardians of the Pole, and the other four stars of the Little Dipper are between magnitude 4.2 and 5.0, too faint to see from a town.

On the far side of the pole from the Dipper, at about the same distance, lies the W of Cassiopeia, which Section O1.5 teaches. When the Dipper is high above the pole, Cassiopeia is low beneath it, and half a sidereal day later the two change places.

O1.3.2From the Dipper to the spring sky

The handle of the Dipper is not straight. It curves gently from Alioth through Mizar to Alkaid, and following that curve on for about 31° beyond Alkaid brings us to Arcturus (α Boo), an orange star of magnitude −0.05 in Boötes, the Herdsman. Following the same arc about 33° beyond Arcturus brings us to Spica (α Vir), a blue-white star of magnitude 0.98 in Virgo, the Maiden. Together the two hops are remembered as "arc to Arcturus, then speed on to Spica".

The bowl gives a third hop. A line from Megrez through Phecda, along the inner edge of the bowl and away from the handle, continued about ten times the gap between them, reaches Regulus (α Leo), magnitude 1.36, the brightest star of Leo, the Lion. Two shorter hops start from Arcturus and Alkaid. About 20° north-east of Arcturus lies Alphecca (α CrB), the brightest star in the small semicircle of Corona Borealis, the Northern Crown. About 14° south-west of Alkaid, under the handle of the Dipper, lies Cor Caroli (α CVn), magnitude 2.89, the only bright star of Canes Venatici, the Hunting Dogs.

O1.3.3The Spring Triangle

Arcturus, Spica and Denebola (β Leo), the star at the tail of Leo, make a large triangle with sides of 32.8°, 35.1° and 35.3°, very nearly equilateral, which northern observers call the Spring Triangle. Like the Winter Triangle of Section O1.2 it is an asterism made of stars from several constellations, and it is a good frame for finding the fainter constellations that lie inside and round it.

O1.3.4Leo, Boötes and Virgo

Leo is easy to picture as a lion lying down. The front of the Lion is the Sickle, a curve of six stars shaped like a question mark written backwards, with Regulus as the dot at its bottom. From Regulus the Sickle runs north through η Leo to Algieba (γ Leo), magnitude 2.01, and curls round through Adhafera (ζ Leo) and Rasalas (μ Leo) to Ras Elased Australis (ε Leo). The back of the Lion is a triangle of three stars, Denebola (β Leo) at the tail, Zosma (δ Leo) and Chertan (θ Leo). Regulus lies only 0.5° from the ecliptic, so the Moon and the planets often pass close to it.

Boötes is shaped like a kite, or an ice-cream cone, with Arcturus at the narrow end. The east side of the kite runs north from Arcturus through Izar (ε Boo) to Nekkar (β Boo) at the top, and the west side comes back through Seginus (γ Boo). Muphrid (η Boo) lies just west of Arcturus. Arcturus looks clearly orange, because it is a cool giant star.

Virgo is large and faint apart from Spica. Spica lies 2° south of the ecliptic, so it too is often visited by the Moon. From Spica the main stars of Virgo spread north-west in a shape like a Y lying on its side, through Porrima (γ Vir), whose two stars a telescope can split, to Vindemiatrix (ε Vir). Between Vindemiatrix and Denebola, 18° apart, lies the Virgo Cluster, more than a thousand galaxies about 55 million light-years away. Its brightest members, such as the giant elliptical galaxy M87, are near magnitude 9 and need binoculars or a telescope.

O1.3.5Fainter neighbours

Five fainter constellations fill the rest of the spring sky. Cancer, the Crab, lies between Gemini and Leo and has no star brighter than magnitude 3.5, but in its middle is the Beehive Cluster, M44, an open cluster of magnitude 3.1 that the naked eye sees as a misty patch under a dark sky, 15° from Pollux on the way to Regulus. Hydra, the Water Snake, is the largest constellation of all and winds nearly 100° across the sky south of Cancer, Leo and Virgo. Its only bright star is Alphard (α Hya), an orange star of magnitude 1.99 whose name means the solitary one, because no star brighter than magnitude 2.5 lies within 22° of it.

Corvus, the Crow, is a small, neat quadrilateral of four stars near magnitude 3, Gienah (γ Crv), Kraz (β Crv), Algorab (δ Crv) and ε Crv, about 14° south-west of Spica. Its northern edge, from Gienah to Algorab, points towards Spica. Coma Berenices, Berenice's Hair, is a loose scatter of faint stars between Denebola and Arcturus, and Canes Venatici lies under the handle of the Dipper with Cor Caroli as its marker. The Whirlpool Galaxy, M51, is 3.6° from Alkaid, though at magnitude 8.4 it needs a telescope.

O1.3.6The spring sky from the southern hemisphere

Seen from Sydney, Santiago or Johannesburg, the constellations of this section lie in the northern half of the sky and are seen in the autumn, from March to June. Polaris never rises there, and the Big Dipper shows at most a few stars low over the northern horizon, so southern observers start from other stars. Spica is the easiest, because it stands 67° up from Sydney when it crosses the meridian, and the northern edge of Corvus points to it, though from the south it is the lower edge of the four. Arcturus is lower in the north, 37° up at best, and Leo is upside down, with the Sickle below Regulus. The hops from Spica to Arcturus and from Regulus to Denebola work as well from the south as from the north, and only the words up, down, left and right change.