Mercury
What is Mercury?
Planets· Diameter 4,879 km. The planet closest to the Sun. With almost no air, its surface swings from 427 °C by day to −173 °C at night. BepiColombo, launched in 2018, is due to enter orbit around it in November 2026.
📍 Visible from where you are right now — computed in your browser
Orbit and size
| Diameter | 4,879 km |
|---|---|
| Distance | 0.39 AU |
| Year | 88 d |
| Temp | -173~427 °C |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Related events
- BepiColombo launch — to Mercury
- BepiColombo enters Mercury orbit(planned)
- Transit of Mercury (2032)(planned)
On Mercury, one sunrise to the next takes two Mercury years
Mercury spins once every 59 days and circles the Sun once every 88 days. The two motions are locked in a 3-to-2 ratio, so from one sunrise to the next at any spot takes 176 days. On Mercury a 'day' is twice as long as a 'year'. Daylight lasts 88 days, long enough for the ground to heat up to 427 °C.
Stranger things happen when Mercury comes closest to the Sun. Its orbit is the most stretched of any planet's, so near the Sun it speeds up a great deal. For a few days the orbit gets ahead of the spin. Seen from the ground, the Sun stops in the sky, moves backward for a while, then carries on. In some places this gives a morning with two sunrises. Replaying this stretch slowly in the orbit view shows Mercury speeding up as it swings close to the Sun.
Mercury is hard to see from Earth. It never gets more than 28 degrees from the Sun, so it appears only briefly, low in the sky just before sunrise or just after sunset. Sometimes it passes in front of the Sun as a small black dot. This happens about 13 times a century. The most recent time was 11 November 2019, and the next is 13 November 2032.
The planet in depth
Mercury is the closest planet to the Sun, but not the hottest. That title belongs to Venus, with its thick atmosphere. Mercury has almost no air to hold heat. In sunlight its ground reaches 427 °C, and after sunset the heat escapes straight to space, so the temperature drops to −173 °C. The difference between day and night is about 600 degrees.
On the outside Mercury looks like the cratered Moon, but inside it is very different. It is the densest planet after Earth, because a heavy iron core fills about 85 percent of its diameter. Why its rocky outer layer is so thin is not yet known. One idea is that a huge collision long ago knocked much of it away, and BepiColombo is designed to help test such ideas.
Even this close to the Sun, Mercury has ice. The floors of deep craters near its poles lie in permanent shadow, where sunlight never reaches. The MESSENGER spacecraft, which orbited Mercury from 2011, confirmed ice in those shadows. After mapping the whole planet over four years, MESSENGER ran out of fuel in 2015 and ended its mission by crashing into the surface.
Frequently asked
- How big is Mercury?
- The diameter of Mercury is 4,879 km.
- How far is Mercury from Earth?
- The distance of Mercury is 0.39 AU.
- In one line, what is Mercury?
- Planets — The planet closest to the Sun.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.