Starwatch: Mercury reaches its greatest western elongation

This week, on 5 September, Mercury will reach the astronomical configuration known as greatest western elongation. This is the point at which Mercury will be at its furthest point west from the sun, as viewed from Earth. It offers a prime opportunity to catch a glimpse of the elusive planet.

A greatest western elongation event means that the planet will be visible in the sky before sunrise. The chart shows the view looking east-north-east at 05.30 BST on the morning of 5 September. This is about 50 minutes before sunrise. Find a spot with a clear view of the horizon, and as far away from any street lights as you can.

Mercury is a challenging target because it never rises high into the sky before the sun arrives but it will outshine all the other stars around it in the pre-dawn sky.

The planet is also visible from the southern hemisphere, although its altitude above the horizon is not as favourable for this particular elongation.

After this elongation, Mercury will travel through its orbit behind the sun. It will next become visible in November, reaching its greatest eastern elongation on 16 November. In this configuration, it will be visible in the evening sky.