The Southern Cross

Crux is the smallest of the 88 constellations — 68 square degrees, a third the area of the Great Square of Pegasus — and it contains two first-magnitude stars. The long axis runs Gacrux at the top to Acrux at the foot. That line is the instrument; everything else here is context.

Acrux (α, 0.77) is blue-white, a B-type pair 320 light-years off. Gacrux (γ, 1.59) is an M-type red giant at 88 light-years, and the colour difference between the two ends of the Cross is obvious once you look for it. Mimosa (β, 1.25) makes the eastern arm, Imai (δ, 2.79) the western. Long axis: 6.0°.

The two brilliant stars east of the Cross are the pointers, 4.4° apart. Rigil Kentaurus (α Cen, −0.27) is the nearest star system to the Sun at 4.37 light-years; Hadar (β Cen, 0.61) only looks like its partner and is actually 390 light-years behind it. The nearest star of all, Proxima Centauri, is in this field at the marked point — 4.24 light-years, with a planet in its habitable zone, and magnitude 11. You will never see it. That is what a red dwarf looks like from next door.

The southern sky has no pole star — nothing brighter than magnitude 5.5 within 1° of the pole. So you construct one. Extend the Cross's long axis past Acrux by 4.5 times its own length: 6.0° becomes a run of 26.9°, and the far end of that line is the south celestial pole. Drop straight down from there and you have true south on the horizon.

Cross-check it, because the first method has a well-known trap. Take the midpoint of the two pointers and drop a perpendicular; where that meets the Cross's extended axis is the pole. Two independent lines agreeing is worth far more than one line remembered confidently.

The trap: Avior, Aspidiske, Alsephina and Markeb form the False Cross, 30° west of the real one. It is larger, dimmer, more symmetrical, and has no pointers beside it. Use it by mistake and your "south" is out by about 15°. Two tells: the true Cross is compact and lopsided, and it has Alpha and Beta Centauri sitting next to it.

From the southern temperate latitudes Crux is circumpolar — the altitude readout, which is tracking Acrux, never reaches zero — and it wheels clockwise around the pole all night, standing upright only when it is due south. Acrux is at −63.1°, so it stays up from anywhere south of 27°S; Gacrux at the top of the Cross is 6° nearer the equator and needs 33°S before the whole shape stops setting. That is the other half of the skill: the Cross is a pointer at any angle, but it only looks like the flag when it crosses the meridian.

A degree south-east of Mimosa sits NGC 4755, the Jewel Box: magnitude 4.2, a naked-eye knot, and in binoculars a scatter of blue-white stars with one red supergiant among them. Beside it the Coalsack — the dark patch that makes the Cross look like it is cut out of the Milky Way — is not empty sky but a dust cloud 600 light-years away, blocking everything behind it.