In the picture above, taken from a satellite orbiting Earth, you can see little Mercury in front of the gigantic Sun. This small planet is so close to the Sun that it only takes it 88 days to orbit the star, compared with Earth's 365 days and Pluto's 248 years. Because of Mercury's closeness to the Sun, the planet is also very hot. But, surprisingly, it isn't the hottest planet in the Solar System, and temperatures on parts of the surface can be colder than the coldest temperatures possible on Earth and many of the Outer Planets.
All planets spin around an imaginary axis running through the middle of them. Earth takes about 24 hours to do so, Mars takes 24 hours and 37 minutes and Jupiter takes 9 hours and 50 minutes. Mercury however takes 59 days. This means that during its 88-day journey around the Sun, Mercury spins only once (maybe twice) on its axis. Earth makes 365 rotations a year, meaning that there is a sunset and a sunrise everyday. Because Mercury travels so fast around the Sun but spins so slowly on its axis, it takes 176 days for the Sun to have fully risen and fallen on the planet. Confused? Hopefully the picture below will help to explain this! It shows a year on Mercury. Imagine that on the first day, you are standing on the side of the planet which isn't receiving any light or heat from the Sun. Follow the orbit of the planet around the Sun.
After 88 days, the planet is back to where it had began, and a year has been completed. But notice that you are now standing on another side of the planet, opposite to the side you were standing on 88 days earlier. You are now directly below the Sun, with temperatures of 427 °C (800 °F) hitting you! It won't be for another 29 days that the Sun will set, when you will be on a side of the planet that will not receive any heat or light for over 70 days! The temperature will drop below freezing, to about -183 °C (-297 °F). Temperatures do not reach any colder than this until you go as far away from the Sun as Uranus! So, Mercury may be the closest planet to the Sun, but it can also be one of the coldest. Even when you are on the side of Mercury that is receiving heat and light directly from the sun, temperatures on Venus are even hotter. This is because Venus is covered by a suffocating layer of thick cloud which traps heat from the Sun, causing temperatures there to reach over 480°c!
Mercury has no clouds, air or atmosphere to absorb or trap any heat. The sky on
Mercury is also always dark, just like the Moon's sky because the lack of
atmosphere means that light cannot be spread. Instead, sunlight hits the
surface, lights it up and reflects off it.