The inner planet Mercury reaches its greatest western elongation from the sun this week.
This means that from the point of view of Earth, Mercury will be the furthest it can possibly be from the sun. For many around the world, especially the southern hemisphere, this means it will be at its easiest to pick out from the early-morning twilight sky.
The chart shows the view looking east from Cape Town, South Africa, at 05.20 SAST. Mercury is located in the constellation of Sagittarius, the archer. Even though it is a small planet, Mercury will appear bright because it is so close to the sun. The nearest bright star will be blood-red Antares in Scorpius, the scorpion. Unfortunately, from the UK and other similar northern latitudes, it will prove a much more challenging observation to make. This is because the ecliptic plane, where the planets are located, makes a shallow angle with the horizon at this time of year. So, although Mercury is widely separated from the sun, they rise at a similar time.
In the southern hemisphere, the ecliptic makes a steeper angle to the horizon and so Mercury climbs higher into the sky before the sun appears. In the UK we will have to wait until March and April for a better view of the planet.
