Moon of Saturn · Solar System

Iapetus

Above the horizon all nightHighest after dark 00:49 · south · 40°

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Where is Iapetus?

Times for LondonChange city

Look south, about 40° up at 00:49 in Cetus.

Rises: 18:34
Highest: 00:47
Sets: 06:59
Best after dark: 00:49 · 40°

Can I see Iapetus tonight?

Iapetus always stays within a small fraction of a degree of Saturn, so its position tonight is Saturn’s position.

From London tonight, Iapetus is above the horizon all night. It’s highest after dark at 00:49, in the south at 40°.

This is position and timing only: clouds, haze and light pollution aren’t included.

What does Iapetus look like?

Eyes

Not visible.

Binoculars

Not visible.

Telescope

A telescope of about 100–150 mm (4–6 inches) shows it as a faint point well away from Saturn, brightest when it is west of the planet. At its faintest it needs a larger telescope.

What you’re likely to see, not what a space telescope photographs.

About Iapetus

Iapetus is Saturn’s third-largest moon and one of the oddest. One hemisphere is as dark as coal and the other is bright ice, a contrast Giovanni Cassini noticed soon after discovering it in 1671.

Because of that two-tone surface, Iapetus changes in brightness as it orbits. It looks brightest when it is on the western side of Saturn, showing us its icy face, and several times fainter on the eastern side.

Iapetus also has a strange ridge running along its equator, giving it a walnut-like shape, and it orbits far from Saturn, taking about 79 days to go around once.

Its dark side faces forward as it orbits, sweeping up dust from Saturn’s outer moons, including material from a huge, faint ring associated with the moon Phoebe. The dark surface warms in sunlight, driving off ice and making the contrast even stronger.

A ridge up to about 20 km high runs along much of its equator. Iapetus’s orbit is tilted about 15° to Saturn’s rings, so it would offer one of the best views of the rings from any of Saturn’s large moons. Cassini flew past it at close range in 2007.

Saturn and its moons

Questions

Why does Iapetus change brightness?

One side is dark and the other is bright ice. As it orbits Saturn, we see different amounts of each.

When is Iapetus easiest to see?

When it is near its farthest point west of Saturn, showing its bright side to Earth.

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Sources: SkyMate; star chart: Wikidata (CC0). Positions computed for your place and time with astronomy-engine. Data & Licences