Sun-synchronous orbit
Published 19 October 2026 · All guides
In short: a sun-synchronous orbit is a near-polar low orbit, tilted at about 98 degrees, whose plane turns once a year in step with the Sun. The result is that the satellite crosses every latitude at the same local time each day, so every picture of a place has the same lighting. Most Earth-imaging satellites use one.
The trick behind it
The Earth bulges slightly at the equator, and that extra mass pulls on every orbit, slowly turning its tilted plane around the Earth's axis. How fast it turns depends on the orbit's height and tilt (see orbital mechanics without the maths).
Choose the combination where the plane turns about one degree a day, eastward, and it keeps pace with the Earth's yearly trip round the Sun. The orbit then keeps the same angle to the Sun all year. At 700 km that takes a tilt of about 98.2 degrees, slightly past polar, so the satellite travels a little against the Earth's rotation.
Why imaging satellites want it
Because the satellite crosses each latitude at the same local time, the sun is at the same angle in every picture it takes of a place (allowing for the seasons). Changes from one image to the next are then real changes, not shadows moving. Examples:
- Landsat 8 and 9 fly at 705 km and cross the equator at about 10 a.m. local time, repeating their ground track every 16 days.
- Sentinel-2, Europe's land-imaging pair, fly at 786 km and pass at about 10:30 in the morning.
Mid-morning is a deliberate choice: the sun is high enough to light the ground well and low enough to cast the shadows that show height, and the clouds that build up over land in the afternoon have usually not formed yet.
The dawn–dusk variant
Put the orbit along the line between day and night and the satellite rides the terminator, over sunrise on one side of the Earth and sunset on the other. It is then in sunlight almost all the time, so its solar panels never rest. Radar satellites, which bring their own illumination and do not care about the light on the ground, often use this.
The catch
Predictability works both ways. A satellite that always passes at 10:30 is a satellite whose visits anyone can schedule around. And because a sun-synchronous orbit is low and near-polar, it sees any one place only briefly and only every few days, so a single satellite gives consistency, not constant watch.
Where Celestrike comes in
In Celestrike the sun is in the model: an imaging satellite collects best when the sun is low over its target and slows down in the dark, while a signals satellite ignores the light entirely. Celestrike is a free spy-satellite strategy game for iPhone and Android, played on real orbits — on the App Store and Google Play.