Jupiter's Moons Screensaver
The Jupiter's Moons screensaver shows Io, Europa, Ganymede and Callisto shuttling around Jupiter exactly as a small telescope would show them tonight — all computed on your device. It's free, runs in your browser, and ESC exits.
How the Jupiter's Moons screensaver works
The saver computes each Galilean moon's position from its orbital period, projected to this exact moment.
It draws the classic edge-on view: Jupiter's banded disc in the centre with the four moons strung out along the equator.
Moons in front of Jupiter shine bright; those swinging behind dim and slip out of view, just as through a telescope.
The whole configuration matches the real sky right now, and it needs no connection.
A worked example
Tonight Io and Europa might huddle close on the left of the disc while Ganymede and Callisto swing wide to the right — the same line-up Galileo sketched in 1610, computed for this very hour.
Settings & tips
- Label the moons toggles the moon names.
- Background sets the night-sky backdrop.
- Positions are computed locally, so it runs offline.
Frequently asked questions
- Are the moon positions real?
- Yes. They're computed from each moon's orbital period, closely matching the real configuration you'd see in a telescope.
- Why don't the moons seem to move?
- They orbit over hours and days, so motion is slow; the diagram always reflects the real current line-up.
- What are the Galilean moons?
- Io, Europa, Ganymede and Callisto — the four largest moons of Jupiter, discovered by Galileo in 1610.
- Does it need a connection?
- No. Everything is computed on your device, so it runs fully offline.
- Is it free?
- Yes — free, no download, no key, in your browser.