Orion: The Hunter

Seven stars, every one of them brighter than magnitude 2.3. Two shoulders — Betelgeuse and Bellatrix — two feet, Rigel and Saiph, and three belt stars ruled across the middle. No other constellation packs this many bright stars into one figure, which is why almost everyone learns this one first.

Opposite corners, opposite colours. Betelgeuse is an M1 supergiant at about 3,600 K; Rigel is a B8 supergiant at about 12,100 K. Colour is a thermometer, and 18.6° apart is close enough to judge one against the other without moving your head. Betelgeuse is also the brightest variable star in the sky: it sank from magnitude 0.5 to 1.6 in the winter of 2019–20, when a cloud of dust it had thrown off drifted across our line of sight.

The Belt is 2.7° end to end and lies along the celestial equator. Mintaka, the western star, sits 18 arcminutes from it — closer than any other star this bright. That has a use: Mintaka rises within a third of a degree of due east and sets within a third of a degree of due west, from anywhere on Earth. It is a compass that needs no calibration and no batteries.

Run the Belt south-east and 21.7° later you arrive at Sirius — magnitude −1.44, the brightest star in the night sky, and the fifth-nearest system to the Sun at 8.6 light-years. It is not an extraordinary star. It puts out about 25 Suns' worth of light. It is simply very close.

Reverse the same line and 21.8° to the north-west — almost exactly the same distance again — brings you to Aldebaran, the orange eye of Taurus. One straight line, two directions, two first-magnitude stars. Learn this and you can find a quarter of the winter sky from the Belt alone.

Because the Belt straddles the equator, Orion is the constellation the whole inhabited world shares: it rises due east and sets due west from Reykjavík to Ushuaia. Eight hours pass here. The azimuth readout swings east to west, the altitude climbs and falls, and the Hunter goes over the top and down.

Pull back, and Orion turns out to be the anchor of something bigger: six first-magnitude stars from six different constellations, enclosing 1,940 square degrees — a tenth of the sky above your horizon. That is the Winter Hexagon, with Betelgeuse burning orange off-centre inside it.

The scorpion that killed Orion was put on the far side of the sky, and the myth is load-bearing. Antares, the Scorpion's heart, is 152° from Betelgeuse: the measuring arc leaves Betelgeuse and drives straight into the ground, because that is where Antares is — the readout gives you the number. Six months from now the two will have traded places, and it will be Orion under your feet.