How Hot is Mercury?
As the closest planet to the Sun, it should come as no surprise that Mercury gets very very hot. Temperatures on the sunlit side of Mercury reach a sweltering 430 °C (800 °F). It's very toasty there. It's a different story on the opposite side of the planet though. On the side facing away from the Sun, where day is night and the Sun shines no light, temperatures plummet to a chillingly chilly -180 °C (-290 °F). This means that Mercury is simultaneously one of the hottest AND one of the coldest planets in the solar system. It makes choosing what to pack for a trip there very difficult! Even though Mercury is the closest planet to the Sun and therefore roasts in the heat it receives from the nearby star, it isn't the hottest planet in the solar system. That honour goes to Venus, the second planet out, where temperatures are a constantly scorching 471 °C (880 °F).
There is one key reason why Mercury experiences such extremes in temperature and also why it isn't quite as hot as its sweaty neighbour Venus. It's because there is no atmosphere on Mercury. Although the day side of Mercury bakes under a constant stream of heat from the Sun, the lack of atmosphere means none of this heaty goodness can be retained and spread over to its night side. Atmosphere is a bit like a blanket that covers a planet. It traps heat inside it and spreads it around. Having no blanket means that daytime on Mercury is very very hot but night on it is extremely cold.
Mercury also rotates very slowly, taking 59 days to spin once on its axis. This means that the side facing towards the Sun stays facing it for weeks. Meanwhile, the opposite side remains in the shadow of the Sun for just as long. As the planet turns away from the Sun and day turns to night, the surface temperature drops rapidly and stays that way.
Unlike Mercury, Venus does have an atmosphere. In fact, it has the thickest atmosphere of all of the inner planets. Venus is twice the distance as Mercury is from the Sun so receives less heat from it. But its thick atmosphere prevents this heat from escaping from the planet. Instead, the heat that Venus receives gets trapped in its atmosphere and stays there, spreading equally all around the planet. Even the night side of Venus is just as hot as its day side. This is despite the fact that Venus rotates even slower on its axis than Mercury so has even longer nights.