Found with a pencil before anyone pointed a telescope at it. Where it is right now, how big and how heavy, what goes round it, 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 Neptune’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.
Neptune has 16 confirmed moons, of which the 4 below are large enough to be worth drawing. The rest are mostly a few kilometres of captured rubble on distant, tilted orbits. Every distance and period here is real, and they are not independent: each pair has to satisfy Kepler’s third law against Neptune’s mass, which is how this table is checked rather than trusted.
| Moon | Distance | Orbit | Width | |
|---|---|---|---|---|
| Larissa | 73,548 km | 13.3 h | 194 km | Battered and irregular, inside the ring system |
| Proteus | 117,647 km | 1.12 d | 420 km | About as large as a body can get and stay this lumpy |
| Triton | 354,759 km | 5.88 d ↺ | 2,707 km | Orbits backwards, so it was captured — and is spiralling in |
| Nereid | 5,513,818 km | 360.1 d | 340 km | The most eccentric orbit of any large moon: 1.4 to 9.6 million km |
↺ marks a moon going round backwards. Watch this system move →
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.
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.
Same size, same composition, same era — and one has an internal heat engine while the other does not. Every explanation offered (a different impact history, layered convection, helium raining out of the interior) is a hypothesis without a decisive measurement, and only a mission to one or both of them would change that.
Neptune receives about 0.1% of the sunlight Earth does, and generates the strongest winds anywhere. Internal heat is the presumed engine, but the mechanism that turns it into 2,000 km/h zonal jets has not been demonstrated.
Capturing a body that large normally requires shedding an enormous amount of energy. The favoured route is that Triton arrived as half of a binary and its partner was flung away, leaving Triton bound. Whatever happened, it would have destroyed Neptune's original moons — which is consistent with the odd, sparse system that is there now.
Findings reviewed August 2026. Space science moves; a date on a finding is part of the finding.
Inward: Uranus. 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 is Neptune so much warmer inside than Uranus? Same size, same composition, same era — and one has an internal heat engine while the other does not. Every explanation offered (a different impact history, layered convection, helium raining out of the interior) is a hypothesis without a decisive measurement, and only a mission to one or both of them would change that.
What powers those winds? Neptune receives about 0.1% of the sunlight Earth does, and generates the strongest winds anywhere. Internal heat is the presumed engine, but the mechanism that turns it into 2,000 km/h zonal jets has not been demonstrated.
How was Triton captured without wrecking the system? Capturing a body that large normally requires shedding an enormous amount of energy. The favoured route is that Triton arrived as half of a binary and its partner was flung away, leaving Triton bound. Whatever happened, it would have destroyed Neptune's original moons — which is consistent with the odd, sparse system that is there now.