Leo & the Spring Sky

Most constellations require an act of faith. Leo does not: a backwards question mark for the head and mane, a right-angled triangle for the hindquarters, and a lion lying down between them. The question mark is called the Sickle, and it is the part you learn.

At the base of the Sickle, the full stop under the question mark, sits Regulus — "the little king", magnitude 1.36, 79 light-years away. It lies 0.46° from the ecliptic, closer than any other first-magnitude star, so the Moon and the planets keep walking over it. The Moon occults Regulus outright in runs of a dozen or more events, and any bright object you find inside that ring is worth a second look. Regulus is also spinning at 86% of break-up: a flattened, 4-solar-mass star with a white dwarf in tow.

24.6° east of Regulus is Denebola — dhanab al-asad, the lion's tail — with Zosma and Chertan making the triangle of the hindquarters. Regulus at the front foot, Denebola at the tail: that is the length of the Lion, and it is a useful ruler. Two Leos laid end to end is roughly the span of the Big Dipper plus its pointers.

In the Sickle's curve, Algieba (γ Leonis, magnitude 2.01) looks single and is not. It is two orange-gold giants 4.6 arcseconds apart, orbiting in about 550 years, and a 60 mm telescope separates them cleanly. It is the best colour-matched double in the spring sky, and a much better first target than anything faint.

Step back and Denebola turns out to be a corner of something bigger: the Spring Triangle, with Arcturus 35.3° away and Spica 35.1° from Denebola on the other side. Three stars, three constellations, sides of 35°, 35° and 33° — almost equilateral, and the easiest way to hold the whole spring sky in your head at once.

If you already know the Big Dipper, you already know how to get here. Punch a hole in the bottom of the bowl: from Megrez, where the handle joins, down through Phecda, and carry on for eleven times that 4.5° gap — 51° — and the line drips onto Regulus. The bowl's other pair, Merak through Dubhe, goes the opposite way to Polaris.

Every 17 November the Leonids appear to stream out of the point marked here, just north of Algieba, as Earth crosses the dust trail of comet Tempel–Tuttle. Most years give 10–15 meteors an hour. Every 33 years, when the comet has recently passed, the rate can go to thousands: in 1833 observers in the eastern United States reported meteors falling like snow, and the shock of that night is a large part of why meteor astronomy exists at all. The next likely outburst is in the 2030s.

Leo crosses the meridian around midnight in March and earlier every week after that, which is the whole basis of "spring constellation" — nothing about Leo changes, only when you happen to be outside. Watch the altitude readout peak and fall away: that peak is the moment the Lion is highest, and it arrives four minutes earlier every night.