AuroracastUKLearn
Mercury
The hardest of the bright planets to catch, because it never strays far from the Sun.
Seeing it from the UK
The one that takes some effort. Mercury never appears far from the Sun, so it is only ever visible in twilight, low down, for a few weeks at a time before it disappears again. You need a clear flat horizon with no buildings or trees, either west just after sunset or east just before sunrise, and you need to know roughly when to look. It is bright enough for the naked eye once you find it. Plenty of experienced observers have never knowingly seen it.
Mercury is the smallest planet and the closest to the Sun. It looks like the Moon at a glance and for the same reason: almost no atmosphere, so nothing has ever worn the craters away and nothing spreads heat around. The bright streaks are rays of material thrown out when something struck it.
That lack of air makes it a planet of extremes. In sunlight the surface reaches about 430 degrees, and on the night side it falls to around minus 180. It is not the hottest planet despite being nearest the Sun, because it has no atmosphere to hold the heat in.
Strangest of all, there is almost certainly water ice on it. Some craters at its poles are angled so that their floors have never seen sunlight in billions of years, and radar and MESSENGER both found ice sitting in them, on the planet closest to the Sun.
The numbers
- Distance from the Sun
- About 36 million miles, a third of ours
- A day there
- 176 Earth days from one sunrise to the next
- A year there
- 88 Earth days, the shortest of any planet
- Temperature range
- About 430 degrees in sunlight, minus 180 at night
- Moons
- None
- Surprising feature
- Water ice in polar craters that sunlight has never reached
To find out whether it is up tonight, and when it gets properly dark where you are, see Sky Tonight.