Of all the 12 constellations of the Zodiac, the faintest is Cancer (the crab) – and this is the best month to spot the dim crustacean for yourself. It’s high in the south, nestled between the feline shape of Leo (the lion) and the twin stars of Gemini, Castor and Pollux, which you’ll find near brilliant Jupiter.
In ancient Greek myth, Cancer attempted to nip the hero Hercules when he was fighting the multi-headed monster Hydra. Undeterred, Hercules crushed the crab under his foot, but the goddess Hera – a sworn enemy of Hercules – took pity on Cancer and raised it to the heavens. (You can also find Hercules and Hydra as constellations in the sky this month.)
None of the stars in this constellation is outstanding, but it contains one claim to fame. Find a really dark location, well away from light pollution, and you’ll see a faint glowing cloud in the centre of Cancer: what the ancient Greek astronomer Ptolemy called "a nebulous mass in the breast of the Crab". Its traditional name is Praesepe, meaning "the manger," with the stars to either side being asses that were eating from it.
In 1609, Galileo turned his pioneering telescope to Praesepe, and exclaimed "it contains not one star only but a mass of more than 40 small stars". Binoculars or a small telescope will show you why these swarming stars have given Praesepe its nickname, the "Beehive Cluster".
The brightest stars appear as diamonds and rubies scattered on black velvet. The diamonds in this cosmic jewel box are stars like Sirius – hotter and brighter than the Sun – while the rubies are more massive stars that have swollen up to become red giants.
Praesepe was born about 600 million years ago, making it a youngster on the cosmic stage – little more than one-tenth the age of the Sun. Intriguingly, it’s the same age as the Hyades, a star cluster in Taurus that we see behind the bright star Aldebaran. Probably these two clusters formed from denser knots within the same primordial cloud of gas and dust, and have now drifted their own ways. Of the two, the Beehive Cluster appears fainter because – at 600 light years – it’s four times further away than the Hyades.
On the latest count, Praesepe contains 1004 stars spread over 70 light years. But it was born with more members, and many of its original stars have been stripped away by the gravity of our galaxy as Praesepe follows its orbit within the Milky Way. A survey with the Gaia spacecraft, which accurately measures the distances and motions of billions of stars, has identified 389 stars that have been ripped away from Praesepe and now form two "tidal tails" reaching out from the cluster itself.
These tails are immense, stretching over 500 light years to either side of Praesepe. If we could see them in the sky, these scattered stars would form a band from the Beehive Cluster down to the horizon in the south; and upwards from Praesepe to the Pole Star, Polaris.
What’s Up
All hail the Evening Star! Venus roars upwards in the dusk twilight this month, dominating our view of the sky after the Sun has set, and shining more brilliantly than anything bar the Moon. At the start of March, Venus is setting around 7pm, but by the end of the month it stays above the horizon until 9.30pm.
If you have good binoculars or a low power telescope, look out for the faint Neptune near to Venus on 7 March. The following night – 8 March – Venus lies near to Saturn, a conjunction that’s visible to the naked eye low in the sunset glow. And don’t miss the glorious sight of the crescent Moon pairing up with Venus on 20 March.
High in the southern sky, Jupiter is brighter than any of the stars and is up for most of the night. You’ll find the Moon nearby on 26 March.
Near Jupiter lie Castor and Pollux, the twin stars of the constellation Gemini. To their lower right, the gaudy star-patterns of winter are beginning to set with the changing seasons: Orion the great hunter, with his two dogs Canis Major and Canis Minor, along the celestial bull Taurus.
In the east are rising the less showy spring constellations, featuring Leo the lion, Virgo the virgin and the herdsman Boötes – not to mention a long straggly water-snake, Hydra, with a crow (Corvus) perched on its back.
On 29 March, keep a careful eye on the Moon around 8.20 pm, and try to spot the Leo’s principal star Regulus appearing from the behind the lunar orb. Through binoculars, you’ll see the star’s fainter companion (Regulus B) emerge first, followed a few minutes later by brilliant Regulus itself.
Diary
6 March: Moon near Spica
7 March: Venus near Neptune
8 March: Venus near Saturn
11 March, 9.38am: Last Quarter Moon
19 March, 1.23am: New Moon
20 March, 2.46pm: Spring Equinox; Moon near Venus
25 March, 7.18pm: First Quarter Moon
28 March: Moon near Jupiter
29 March, 1.00am: British Summer Time begins
29 March, 8.20pm: Regulus occultation ends
