This Sunday 16th December the second of two bright-green comets will be making an appearance in the night sky.

A comet is a small icy body composed of frozen rock, dust and gases such as carbon dioxide, carbon monoxide, cyanogen, methane, and ammonia. These balls of ice – or dirty sun balls as they are sometimes known – orbit the sun in highly elliptical orbits.

When the comet reaches perihelion – that is when it reaches the closest point to the Sun in its orbit – the heat from the sun turns the ice into gas. The small 10-30 km icy nucleus is therefore expanded into a gaseous ‘coma’ with a tail extending millions of miles pointing in a direction away from the Sun. The UV light from the Sun interacts with the gaseous ‘coma’ causing the comprising atoms and molecules to be ionized and/or excited. As the atoms relax into their ground state, they emit light, which is the glow that we see. The colour of this glowing light depends on the composition ratios, which for green is predominantly cyanogen CN2 and diatomic carbon C2.

On the 13th December comet 46P/Wirtanen reached perihelion passage. Since then it has been increasing in visibility and will reach maximum visibility on Sunday 16th December when it will be closest to the Earth. Discovered by the American astronomer Carl Wirtanen in 1948, this periodic comet follows a 5.4-year cycle around the Sun and made its last visit in 2013. However, this time it will be much closer to Earth at only 7.3 million miles away compared to 564 million miles in 2013. Reaching a magnitude of 3, comet 46P will thus be visible with the naked eye, so all you need to do is look towards the constellation of Taurus.

Also on show this month are the annual Geminids meteor shower, which are active every December when Earth passes through a massive trail of debris left by the rocky object known as 3200 Phaethon. The exact nature of Phaethon is not known for certain and is currently believed to be either a near-Earth asteroid or an extinct comet, sometimes called a rock comet.

To find out more details and information on how and where to view in both real and virtual time see the articles below.

By Amira Val Baker