The ISS orbit, explained

Why 51.6 degrees?

The inclination — the angle between the orbit and the equator — was set by geography and politics. Russian crew launches historically flew from Baikonur Cosmodrome at 46°N, and a 51.6° orbit is the most efficient one reachable from there without dropping spent rocket stages over China. The angle turned out to be a gift for the rest of us: it carries the station over every point on Earth between 51.6°N and 51.6°S, where more than 90% of humanity lives — which is why almost everyone gets visible passes.

Why the ground track looks like a wave

The orbit itself is (almost) a fixed circle in space — it's Earth that rotates underneath it. Project a tilted circle onto a flat world map and you get the familiar sine-wave line. Each orbit takes about 92 minutes, during which the planet turns roughly 22.9° eastward, so every successive track is shifted that far to the west. After about a day the pattern nearly repeats, a little offset — over weeks, the station passes over every spot in its latitude band at every hour of the day.

Why the orbit decides when you can see the ISS

You can only see the station when it flies over you while your sky is dark but the station is still in sunlight — shortly after sunset or before sunrise. Because of the westward-shifting ground track, the times of day the ISS crosses your area drift continuously. The result is the famous ~2-week visibility cycle: a stretch of evenings with bright passes, then a period with none while the passes happen in daylight or the middle of the night. The pass predictor computes exactly where you are in that cycle, and email alerts catch the next window for you.

Key orbit numbers

• Inclination: 51.6° — covers latitudes 51.6°N to 51.6°S
• Period: 90–93 minutes (~15.5 orbits per day)
• Altitude: ~400 km, nearly circular
• Speed: ~28,000 km/h (7.66 km/s)
• Westward shift: ~22.9° of longitude per orbit