Orion and its neighbours

The belt of Orion lies almost on the celestial equator. Mintaka, the star at its western end, is only 0.3° south of it, so everywhere outside the polar circles Mintaka rises within a degree of due east and sets within a degree of due west, and Orion can be seen from almost everywhere on Earth. On 29 January the belt crosses the meridian at 21:00 local mean time, which makes the evenings of January and February Orion's season. Seen from Ankara at that moment it stands in the south, 49° above the horizon, with the orange star Betelgeuse at the upper left. Seen from Sydney it stands in the north, 57° up, upside down, with Betelgeuse at the lower right. Orion is the best place to start learning the sky, because its seven bright stars make a figure that is hard to mistake, and because the brightest stars of the January sky can all be found from it. In this section we learn Orion first, then the hops that lead from it to its neighbours, and then each neighbouring constellation in turn.

O1.2.1Orion

Seen from the northern hemisphere, Orion is a hunter facing us, and four bright stars mark the corners of his body. Betelgeuse (α Ori) is the shoulder at the upper left and Bellatrix (γ Ori) the shoulder at the upper right. Saiph (κ Ori) is the knee at the lower left and Rigel (β Ori) the foot at the lower right. The diagonal from Betelgeuse to Rigel is 18.6° long, almost two fists at arm's length. Betelgeuse looks orange and Rigel blue-white, and the difference is easy to see with the naked eye. The colour of a star shows its temperature, as Chapter 4 explains, and Betelgeuse is a cool red supergiant while Rigel is a hot blue supergiant.

Across the middle runs the belt, three stars close to magnitude 2 in a straight row 2.7° long, with the middle one almost exactly halfway between the other two. From east to west, which seen from the north is from lower left to upper right, they are Alnitak (ζ Ori), Alnilam (ε Ori) and Mintaka (δ Ori). A simple way to keep them apart is that Alnitak, at the eastern end, is on the same side of Orion as Betelgeuse and Saiph, and Mintaka, at the western end, is on the same side as Bellatrix and Rigel. Above the belt a small triangle of fainter stars around Meissa (λ Ori) makes Orion's head.

Below the belt hangs the sword, a short line of fainter stars. In the middle of the sword is the Orion Nebula, M42, a cloud of glowing gas in which new stars are forming. Under a dark sky the naked eye sees it as a slightly fuzzy star about 4° south of the belt, and binoculars show the glow round it.

The drill below asks for the seven bright stars of Orion one at a time, in a random order, on a chart that shows the figure but no names. A star missed in the drill comes back in the daily round on the Sky page the same day.

O1.2.2From Orion to its neighbours

Five hops lead from Orion to the brightest stars around it, and Figure O1.3 draws each of them.

Following the belt down to the south-east, about 23° from Alnilam, brings us to Sirius (α CMa), the brightest star of the night sky at magnitude −1.44, which marks the head of Canis Major, the Great Dog. Following the belt the other way, up to the north-west by about the same distance, brings us to Aldebaran (α Tau), an orange star of magnitude 0.87, which is the eye of Taurus, the Bull. Continuing roughly the same way for about 14° beyond Aldebaran leads to the Pleiades, a small, tight cluster of stars. Six of its stars are brighter than magnitude 4.5 and easy to see with the naked eye, and they are packed so closely that they look like a tiny dipper.

The other three hops start from the corners of Orion. A line from Rigel through Betelgeuse, continued nearly twice as far again, reaches Castor and Pollux, the twins of Gemini, which are 4.5° apart. A line from Bellatrix through Betelgeuse, continued about three and a half times as far, reaches Procyon (α CMi), the brightest star of Canis Minor, the Little Dog. A line from Alnitak through Meissa, at Orion's head, continued about three times as far, reaches Capella (α Aur), a yellowish star of magnitude 0.08 in Auriga, the Charioteer.

O1.2.3The Winter Triangle and the Winter Hexagon

Betelgeuse, Procyon and Sirius make a large triangle with sides of 26.0°, 25.7° and 27.1°, very nearly equilateral. Observers in the northern hemisphere call it the Winter Triangle, because it is high on winter evenings there. A larger ring of six bright stars, the Winter Hexagon, runs through Sirius, Procyon, Pollux, Capella, Aldebaran and Rigel, with Betelgeuse inside it. Both are asterisms, made of stars from several constellations, and both are easy to find once Orion is known.

O1.2.4The neighbours one by one

Taurus, the Bull, lies to the north-west of Orion. Aldebaran sits at one tip of a V of stars that makes the Bull's face. The other stars of the V belong to the Hyades, a real cluster of stars about 150 light-years away, but Aldebaran is only 67 light-years away and lies in front of the cluster by chance. The Bull's two long horns run from the V to Elnath (β Tau) at the tip of the northern horn and Tianguan (ζ Tau) at the tip of the southern one. Many charts join Elnath into the figure of Auriga as well, but it belongs to Taurus. About 1° from Tianguan lies the Crab Nebula, M1, the remains of a star whose explosion was seen in the year 1054, though it is too faint for the naked eye. The Pleiades, which we met above, ride on the Bull's shoulder.

Gemini, the Twins, lies to the north-east of Orion. Castor (α Gem) and Pollux (β Gem) mark the heads of the twins, and the bodies run in two roughly parallel lines of fainter stars back towards Orion. The line from Pollux ends at Alhena (γ Gem), and the line from Castor ends near the open cluster M35, a hazy patch that the naked eye can just see under a dark sky.

Auriga, the Charioteer, lies high to the north of Orion and Taurus. Its stars make a large pentagon with Capella at one corner, and the other corners are Menkalinan (β Aur), Mahasim (θ Aur), Elnath in Taurus, and Hassaleh (ι Aur). Close to Capella is a small, narrow triangle of fainter stars called the Kids. Three open clusters, M36, M37 and M38, lie in a short row in the southern part of Auriga, and binoculars show them as small hazy patches.

Canis Major, the Great Dog, lies to the south-east of Orion, with Sirius at its head and Mirzam (β CMa) 5.5° west of Sirius. Its body runs south from Sirius to a triangle of bright stars, Adhara (ε CMa), Wezen (δ CMa) and Aludra (η CMa). About 4° south of Sirius lies the open cluster M41, which the naked eye can just see under a dark sky. Canis Minor, the Little Dog, has only two bright stars, Procyon and Gomeisa (β CMi), 4.3° apart.

Three fainter constellations fill the gaps. Lepus, the Hare, lies just south of Orion's feet, and its brightest star, Arneb (α Lep), has magnitude 2.58. Eridanus, the River, starts at Cursa (β Eri), only 3.5° from Rigel, and winds a long way west and then south, ending at Achernar (α Eri), which is so far south that it never rises north of about 33° N. Monoceros, the Unicorn, fills the Winter Triangle with stars no brighter than magnitude 3.76, too faint to see from a town.

O1.2.5Orion from the southern hemisphere

Seen from Sydney, Santiago or Johannesburg, Orion is in the northern half of the sky. An observer there faces north to see it, so the figure is upside down, with Rigel and Saiph at the top, Betelgeuse and Bellatrix at the bottom, and the sword above the belt instead of below it. Many observers in Australia and New Zealand ignore the hunter and see the belt and the sword as a saucepan. The hops of this section work just as well from the south, and only the words up, down, left and right change.

Sirius is then almost overhead, 73° up from Sydney, and the southern sky adds a second very bright star, Canopus (α Car), at magnitude −0.62 the second brightest star of the night sky. It lies 36° south of Sirius, at nearly the same right ascension. Canopus has declination −52.7°, so it never rises anywhere north of latitude 37.9° N, or 37.3° N if we ignore the bending of light by the atmosphere (Section 1.4). It can be seen from Hanoi, where it rises at most 16° above the southern horizon, but never from Ankara or Beijing.