Sidereal Clock Screensaver
The Sidereal Clock screensaver shows Local Sidereal Time — the clock astronomers use — on a slowly turning zodiac wheel showing which stars are overhead right now, computed from your longitude. It's free, runs in your browser, and ESC exits.
How the Sidereal Clock screensaver works
The saver computes Local Sidereal Time from the date and your longitude — the time kept by the stars rather than the Sun.
Sidereal time runs about four minutes faster a day than clock time, because Earth turns relative to the stars slightly quicker than relative to the Sun.
A zodiac wheel turns so the constellation currently crossing your meridian sits at the top.
Choose your city; everything is computed locally.
A worked example
The readout shows a sidereal time of 21:47:05 for Toronto, the zodiac wheel turned so Aquarius rides the meridian overhead — the sky's own clock, ahead of your wall clock by a few minutes more each night.
Settings & tips
- Location sets your longitude, defaulting to Toronto.
- Background sets the starry backdrop.
- The wheel marks the sign now crossing your meridian.
Frequently asked questions
- What is sidereal time?
- It's time measured by the stars — when the same star returns to your meridian. Astronomers use it to point telescopes.
- Why does it differ from my clock?
- Earth turns relative to the stars about four minutes faster a day than relative to the Sun, so sidereal time slowly pulls ahead.
- Does it need a connection?
- No. It's all computed from your longitude on your device, so it runs fully offline.
- Does my location matter?
- Yes. Local Sidereal Time depends on your longitude, so picking your city makes the overhead constellation correct.
- Is it free?
- Yes — free, no download, no key, in your browser.