SpaceAbout Mercury
Mercury is the smallest planet in the solar system and the nearest to the Sun. NASA describes it as only slightly larger than Earth's Moon. It is also the fastest planet, going round the Sun every 88 Earth days. Even so, it is not the hottest planet: that is Venus, because of its dense atmosphere.
Source: NASA, read on 12 November 2026.
Mercury compared with Earth
Size. Mercury is 4,879 km across. That is 38% of the width of Earth (12,756 km). You would need about 2.6 of it in a row to match the width of Earth. Its mass is 0.055 times Earth's.
Gravity and your weight. Gravity at the surface is 3.7 m/s², against 9.8 on Earth. So things weigh 38% of what they do on Earth. Someone who weighs 50 kg on Earth would feel like 18.9 kg there. Try your own weight in the weight calculator.
Day. Mercury turns once on its axis in 58.6 Earth days. From one noon to the next, its day lasts 175.9 Earth days. The two figures differ because it also moves along its orbit while it spins.
Year. One trip round the Sun takes 88 Earth days, which is 0.24 Earth years. A ten-year-old on Earth would be 41.5 years old there, counting in that world's years. See the age calculator.
Distance. Mercury is on average 57.9 million km from the Sun, between 46 million km at its closest and 69.8 million km at its farthest. Sunlight takes 3.2 minutes to arrive. That is 0.39 times Earth's distance.
Tilt and temperature. Its axis is tilted by 0.034°. It is so close to upright that it has almost no seasons. The mean temperature in the fact sheet is 167 °C.
Moons, rings and magnetism. Mercury has no moons. Rings: no. A global magnetic field: yes.
All the figures
| Diameter | 4,879 km |
|---|---|
| Mass | 0.33 × 10²⁴ kg |
| Density | 5,429 kg/m³ |
| Gravity | 3.7 m/s² |
| Escape velocity | 4.3 km/s |
| Rotation period | 1,407.6 hours |
| Length of day | 4,222.6 hours |
| Distance from the Sun | 57.9 million km |
| Closest | 46 million km |
| Farthest | 69.8 million km |
| Orbital period | 88 days |
| Orbital velocity | 47.4 km/s |
| Orbital inclination | 7° |
| Orbital eccentricity | 0.206 |
| Tilt of axis | 0.034° |
| Mean temperature | 167 °C |
| Surface pressure | 0 bar |
| Moons | 0 |
| Rings | No |
| Global magnetic field | Yes |
Figures: NASA NSSDCA Planetary Fact Sheet, last updated by NASA on 18 March 2025, read on 12 November 2026. NASA rounds these values. Counts of moons change as new ones are confirmed.
Common questions
How big is Mercury compared with Earth?
Mercury is 4,879 km across, 0.38 times the diameter of Earth.
How long is a day on Mercury?
From noon to noon, a day on Mercury lasts 175.9 Earth days.
How many moons does Mercury have?
The NASA fact sheet read on 12 November 2026 gives none.