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Mercury Menu  

Fourteen Facts about Mercury

Situated almost on the doorstep to the Sun, Mercury whizzes around its nearby star while getting barbecued on one side and freezing on the other. It may be small, but it's also fascinating. So fascinating in fact that here are 14 fascinating facts about it!


Mercury is the solar system's smallest planet. It has a diameter of 4,879 kilometres (3,302 miles). It is about a third of the size of Earth. There are even two moons in the solar system larger than Mercury. These are Ganymede, which orbits Jupiter and has a diameter of 5,268 kilometres (3,272 miles) and Titan, which orbits Saturn and has a diameter of 5,149 kilometres (3,200 miles). That's an exciting fact to start with, isn't it? Here's a picture to make it even more exciting.


Mercury is one of the brightest objects in the sky. Only the Sun, the full Moon, Venus, Jupiter and Mars are brighter. Okay, that's half of the solar system, but Mercury is brighter than the other half (Saturn, Uranus, Neptune and so on) and any stars that lie beyond the solar system. Despite it's apparent magnitude (that's the fancy way of saying brightness), it isn't always easy to spot. As it is so close to the Sun, it rises and sets with it. This means it can only be seen just before the Sun has risen or just after it sets.


The planet Mercury has been observed since ancient times since it is visible from Earth with just a pair of eyes. You don't need instruments like binoculars or telescopes or trumpets to see it. The Romans named it after their god of commerce and communications, and all the things that are linked to it. Stuff like shopkeeping, buying and selling, messaging, goods deliveries, and even stealing. The equivalent god in Greek mythology is Hermes.


Mercury does not have a moon. It is one of only two planets in the solar system without one. The other is Venus.


Despite being the closest planet to the Sun, Mercury is not the warmest. Venus is warmer. In fact, Mercury can be one of the coldest planets in the Solar System. Temperatures on Mercury can reach 430 °C (800 °F) on its sunny side, but drop down to a finger and toe numbing -180 °C (-290 °F) on its night side. Venus however is 471 °C (880 °F), no matter where you are on it. And if you are there, you'd be cooked.


Mercury is the most cratered planet in the solar system. There are thousands of them on its surface, and 414 have so far been named. The discovery of craters on Mercury's surface occurred in 1974 when Mariner 10 became the first spacecraft to fly past it and take pictures. Prior to that, astronomers weren't sure whether or not Mercury would be cratered. What they saw from Mariner 10's photos was a surface which looked very similar to Earth's Moon.

Mercury from MESSENGER spacecraft, taken by its Mercury Dual Imaging System cameras on 21st November 2013, image credit NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury has no atmosphere. The lack of atmosphere means that sunlight and heat cannot be spread through it. This gives the planet a dark sky regardless of whether it's day or night. Not having an atmosphere is also partly why the planet gets so cold at night and why it has so many craters. On planets with atmospheres, meteors heading for them usually burn up in it. If they're heading for Mercury, they head straight for an impact with its surface.


Mercury's craters are mostly named after people who have achieved fame in the world of arts, such as in music, literature, painting and acting. The rules around naming craters are that a person needs to no longer be living and they need have been famous for over 50 years. Bit of a shame that they'll never know that they were well-known enough to become a crater on Mercury.


Mercury is the least visited of the Inner Planets. Only two space craft have so far been there. Mariner 10 went there in 1974 when it flew past it three times, twice in 1974 and once in 1975. It's now likely to still be orbiting the Sun. MESSENGER reached Mercury in 2011 and remained in orbit of it until 30th April 2015. It now lies in pieces on the surface of Mercury after being sent to crash into it after running out of fuel. Mercury is due to receive a third visitor soon when BepiColombo enters orbit of it in December 2025.


As Mercury is the closest planet to the Sun, it has the shortest orbit and travels the fastest of all planets. It travels at a speed of 48 kilometres a second or 172,800 kilometres per hour (30 miles per second or 108,000 miles per hour). It takes only 88 days for Mercury to orbit the Sun, meaning that a year on Mercury is 88 days long.


Mercury's days are very long, and also very confusing! It takes 58 days for Mercury to spin once on its axis but as the planet will also have travelled quite a distance around the Sun by the time this has happened, it affects how the Sun rises and sets there. After the Sun rises on Mercury at the start of a new day, it appears to linger over the horizon for a while before setting again. It then rises higher into the sky, and depending on your location on Mercury, might appear to go back and forth and side to side before setting again. This cycle take 176 days as Mercury has to travel around the Sun two times and rotate on its axis three times for everything to be back in their starting positions. I'm so confused. This is all because Mercury is tidally locked in a 3:2 spin-orbit resonance. That's it, I have a headache. I think the next fact needs to be an easier one before I need to go for a long lie down.


If you were able to stand on Mercury, the Sun would appear three times larger than it does on Earth.


Mercury is about 100 times closer to the Sun than dwarf planet Pluto.


Mercury has a much larger core than one would expect of such a small world. It is made up of partially molten iron which gives the planet a weak magnetic field.

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