The smallest planet, the fastest, and the one with almost no sky. Where it is right now, how big and how heavy, and what is still unexplained.
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Distances from the sun are in AU — one AU is the Earth’s average distance, 149,597,870 km — and the angle is where the body sits around its orbit, measured from the March equinox direction.
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.
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.
Every one of these is genuinely unsettled — not simplified for the page, not waiting on a textbook update.
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.
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.
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.
Findings reviewed August 2026. Space science moves; a date on a finding is part of the finding.
Outward: Venus. Or go back to the whole system, where every planet is on screen at once.
The date, the zoom, the span, the speed, the layers and any flight path are all in the address bar, so copying the URL shares exactly what is on screen. Set it up above, then take the link — it is the quickest way to hand a class one specific thing to look at.
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.
Also: the classroom guide · sunrise & sunset by city · moon phase & moonrise · lunar eclipses · how the positions are worked out
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.