The Night Sky in March
As was the case last January, there are two full moons this month, one on March 2 and one on March 31. The second full moon in a month has popularly become known as a blue moon. The occurrence of two full moons in January followed by two in March takes place approximately four times every century.
During March, Venus becomes more prominent in the evening twilight. On March 4, it is joined by Mercury. Both planets are separated by a little over one degree after sunset. This is roughly equal to the width of your little finger viewed at arm’s length. Both Venus and Mercury will appear very bright at this time, although Venus easily outshines its neighbour. Elusive Mercury remains visible in the evening for much of the month.
The familiar shape of the Plough can be seen almost directly overhead around midnight. Despite its prominent appearance, the Plough is an unofficial shape referred to as an asterism. It forms part of the constellation known as the Great Bear, Ursa Major, and in legend represents the bear’s backside and elongated tail.
All of the stars in the Plough, with the exception of Dubhe and Alkaid, belong to an association of stars moving through space together. Known as the Ursa Major Moving Group, its members have a common age of around 500million years.
The two stars forming the side of the blade furthest from the handle are Dubhe and Merak, known as the Pointers because they point at the Pole Star, Polaris. Once Polaris has been identified, dropping a vertical from it to the horizon locates north. A useful characteristic of the Plough is that, from the UK at least, it is circumpolar, meaning it never sets. As long as it is dark and clear, it is easy to find the Plough and thus work out your cardinal directions.
The second star in from the end of the Plough’s handle is Mizar. Look closely at Mizar and you should see a fainter companion nearby. This is the only binary system in the entire night sky where both components are named, the fainter star being known as Alcor. Together, these are sometimes referred to as the Horse and Rider. Things are more complex than they seem though, because a telescope reveals Mizar to be binary again. Spectroscopic analysis of Mizar-a in 1890 revealed that it was also a very close binary. Then in 1908 it was discovered that Mizar-b was also binary. Alcor is now also known to consist of two stars, raising this seemingly simple naked-eye binary to the impressive status of a sextuplet system.
A number of interesting deep sky objects can be found around the Plough. Located south of a point one-quarter of the way from Merak towards Phecda is a dim planetary nebula known as Messier 97, the Owl Nebula. Through a telescope it looks like a ghostly disc with two owl-like eyes staring back at you.
Extending a line from Phecda through Dubhe for the same distance again brings you to the vicinity of a distinctive pair of galaxies known as M81 and M82. Visible in binoculars or a telescope, they are striking because of their different shapes. M81 or Bode’s Galaxy, is a spiral galaxy with a classic bright core surrounded by spiral arms. M82 or the Cigar Galaxy is a starburst galaxy, exhibiting a high rate of star formation. Visually, M82 looks like a thin, elongated smudge seen through an average back garden telescope. Both galaxies lie nearly 12million light years from Earth.