Mercury
The smallest planet, the fastest, and the one with almost no sky. The planet itself, turning at its real rate — how big and how heavy, how long its day and its year, and what is still unexplained.
Mercury on its axis
Turning here at one rotation every 12 seconds, which is 422,280 times real speed. What is not sped up: which way it turns, how far the axis leans, and how its day compares with every other planet's. Craters are placed by their real coordinates and sized in real kilometres — Caloris really is 1,550 km across and really is at 30 degrees north. What is invented is the small stuff between them. Mercury is the most heavily cratered of the four rocky planets, and the flattest in contrast: the whole disc varies by less than a factor of two.
Mercury has no moons, so there is no system of its own to watch here — the globe above is the planet itself, turning at its real rate. To see where it is in the solar system today, and how fast it goes round compared with everything else:
Mercury by the numbers
The mass is worked out from Mercury’s gravitational parameter and the gravity from that and its radius; the year comes from Kepler’s third law and the width is the same figure the simulator draws with. None of them is typed in beside the picture, so none of them can disagree with it.
When Earth and Mercury are next closest
Both planets are moving, so the gap between them swings enormously — and the closest approaches are not all equal, because the orbits are ellipses rather than circles. These are the next four, solved from the orbits rather than looked up, and the widest is 1.2× the distance of the closest.
Click a date to take the simulator there. These are minimum-distance moments, which fall near — but not exactly on — opposition.
Mercury: things worth knowing
- Mercury turns three times for every two orbits, locked into a 3:2 resonance with the Sun. The consequence is that a single day — sunrise to sunrise — lasts 176 Earth days, which is two Mercury years.
- Its iron core takes up about 85% of its radius. Proportionally, Mercury is the most metallic planet in the solar system, and nobody is sure why.
- There is water ice at the poles. Mercury's axis is barely tilted at all, so some crater floors have never seen sunlight, and radar and MESSENGER both found ice sitting in them a few hundred kilometres from rock at 430 °C.
- The planet is shrinking. Long cliffs called lobate scarps run for hundreds of kilometres, and they are wrinkles: the whole globe has contracted by several kilometres in radius as its core cooled.
- Mercury broke Newton. Its perihelion drifts by 43 arcseconds per century more than Newtonian gravity allows, and explaining that was the first observational success of general relativity, in 1915.
What we still don’t know about Mercury
Every one of these is genuinely unsettled — not simplified for the page, not waiting on a textbook update.
Why does Mercury have no moon?
The usual answer is that it is too close to the Sun: the region in which Mercury's gravity beats the Sun's — its Hill sphere — is small, so a captured body has very little room to be stable in, and one that formed alongside would be dragged out of that room by solar tides. The unsatisfying part is that this is an argument about probability, not a piece of evidence. Nothing tells us whether Mercury ever had a moon and lost it, and the same reasoning is used for Venus, which is much further out and has a Hill sphere a moon could comfortably live in.
Why is it so metal-rich?
Three families of explanation compete: a giant impact stripped off most of the original rocky mantle; the young Sun vaporised the outer layers; or Mercury simply formed from material that was already metal-heavy. MESSENGER found volatile elements like potassium and sulfur still present on the surface, which is awkward for both of the violent explanations — you would expect them to have been driven off.
Where did the polar ice come from?
Comet and asteroid impacts are the leading candidate, with solar-wind hydrogen combining with oxygen in the rock as a second source. The proportions are unknown, and they matter, because the same question is asked about the Moon's polar ice and about the water on Earth.
Recently learned about Mercury
Findings reviewed August 2026. Space science moves; a date on a finding is part of the finding.
The rest of the system
Outward: Venus. Or step out: all the planets, a picture and a paragraph each, and the solar system simulator, where every orbit runs at once.
The other simulator
This one is about the whole system. If the question is where the sun and the moon are from where you are standing — what time the sun comes up, why tonight's moon is the shape it is — that is the Sun, Earth & Moon movement simulator, which has a page for every city and a slider over a day, a week or a month. Between the two is the three bodies moving together — Earth going round the sun, the moon going round the Earth, on one screen and openly not to scale, keeping only the real ratio between the two periods.
And for why any of it stays up: the orbital velocity simulator takes one planet and lets you set its distance and its sideways speed by hand, so you can watch the balance that holds every orbit here — and break it, into an ellipse, an escape, or a fall into the sun.
Also: the classroom guide · sunrise & sunset by city · moon phase & moonrise · lunar eclipses · how the positions are worked out
Mercury: questions without settled answers
Why does Mercury have no moon? The usual answer is that it is too close to the Sun: the region in which Mercury's gravity beats the Sun's — its Hill sphere — is small, so a captured body has very little room to be stable in, and one that formed alongside would be dragged out of that room by solar tides. The unsatisfying part is that this is an argument about probability, not a piece of evidence. Nothing tells us whether Mercury ever had a moon and lost it, and the same reasoning is used for Venus, which is much further out and has a Hill sphere a moon could comfortably live in.
Why is it so metal-rich? Three families of explanation compete: a giant impact stripped off most of the original rocky mantle; the young Sun vaporised the outer layers; or Mercury simply formed from material that was already metal-heavy. MESSENGER found volatile elements like potassium and sulfur still present on the surface, which is awkward for both of the violent explanations — you would expect them to have been driven off.
Where did the polar ice come from? Comet and asteroid impacts are the leading candidate, with solar-wind hydrogen combining with oxygen in the rock as a second source. The proportions are unknown, and they matter, because the same question is asked about the Moon's polar ice and about the water on Earth.