Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry
The length of day on Mercury is 58.646 Earth days. This is a sidereal day, which is the amount of time it takes Mercury to turn once on its axis compared to the background stars. If you could look down from above Mercury’s north pole, it would take almost 59 days to turn in a counter-clockwise direction.
Things get unusual because a year on Mercury only lasts 88 days. And a solar day on Mercury lasts 176 Earth days. In other words, if you could stand on the surface of Mercury, it would take 176 days for the Sun to return to the same spot in the sky.
Astronomers used to suspect that Mercury was tidally locked to the Sun, and always showed the same face to the Sun; similar to how the Moon is tidally locked to the Earth. But radar measurements demonstrated that Mercury is actually rotating compared to the Sun.
We’ve written many articles about Mercury for Universe Today. Here’s an article about the length of day for all the planets, and here’s an article about how long is a day on Mars.
If you’d like more info on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.
We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.
Comments on this entry are closed.