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Spotlight on Mercury

The closest planet to the Sun can often seem tricky to observe, but with keen eyes and a clear horizon, you can spot this alien world. Read on to find out more about Mercury, the small, rocky planet skirting the heart of our Solar System.

Mercury in History

Mercury, so named for the god of messages and commerce in Roman mythology, has been known to astronomers since antiquity. Before the invention of the telescope, it was known along with the other naked eye planets as a ‘wandering star’. Astronomers observing these ‘stars’ noticed that they moved differently to the rest of the stars in the sky. It wasn’t until the 16th Century that astronomers began to distinguish planets as something different to the stars.

In 1631, Thomas Harriott and Galileo Galilei were the first to see Mercury through a telescope. Just over 340 years later, the spacecraft Mariner 10 became the first to visit the planet in 1974. Scientists have learned all manner of things about Mercury from Mariner 10, NASA’s 2008 MESSENGER mission and the countless observations from Earth. In 2018, ESA launched BepiColombo, their first mission to Mercury. It hopes to enter orbit by 2025 – and many more discoveries will come from it!

What do we know?

Mercury is about 1/3 the size of the Earth, and resembles our own Moon in apperance. It takes 88 Earth days to orbit the Sun, thus defining the length of one Mercurian year. But one full day-night cycle takes 176 days – roughly 2 Mercurian years. This is due to Mercury’s incredibly slow rotation period. There are also no seasons, because there is very little tilt on its axis. A very different world to Earth!

As the planet closest to the Sun, it’s reasonable to assume that Mercury would be the hottest. The Sun itself appears three times as big in the sky from the surface of Mercury, and the sunlight is seven times brighter than on the Earth. However, Mercury has very little atmosphere. In fact, what it does have is instead classed as an exosphere, because the density is too low for atoms to collide, like in a gas. This is similar to the upper most layer of the Earth’s atmosphere, where it merges with the vacuum of space. One of the characteristics of an atmosphere is that it traps heat, so Venus’ extremely dense atmosphere makes it much hotter than Mercury, despite it being further away from the Sun.

How to find it

Beware of the Sun! When looking for Mercury, ensure you never look directly at the Sun without specialist equipment. Sunglasses will NOT protect you.

Because of its proximity to the Sun, Mercury is only ever seen in the sky around sunrise and sunset. Most of the time, the light from the Sun makes it impossible or very difficult to see, but sometimes the planet appears just far away enough in the sky for you to catch a glimpse of it. On Sunday 31st May, it’ll have the added benefit of reaching its highest point in the sky, therefore being visible later than usual. Don’t be fooled though; Mercury still only reaches 16° above the horizon. Make sure you have as little obstruction around the horizon as possible.

You can view it in the north west after sunset, where it will remain until it sets, about 23:30. It will still be visible in the same place until about the 13th June, though setting earlier. If you’re not sure how to distinguish Mercury from the stars, look as close to sunset as possible. This will allow you to spot Mercury before many of the stars begin to appear. Look for a bright stationary light close to the horizon.

Mercury is easily visible with the naked eye, but from the 29th May to the 19th June, binoculars and telescopes will allow for a view of the planet at dichotomy. You’ll be able to see Mercury at half phase, and particularly bright magnitude.

Sources: NASA Solar System, ESA Science & Technology