Visual Angular Velocity
of Earth Satellites

This table provides a very rough guide to identifying a satellite's orbital height based upon its observed angular velocity.

The apparent speed of a satellite across the night sky varies according to the satellites orbital height, the altitude above the horizon, and the direction of the orbit relative to the observer at a location on the earth's surface.

For example, the International Space Station currently orbits at about 220 miles above the mean surface of the earth. From the table we find that the ISS is therefore orbiting at 17,219 miles per hour, or 4.78 miles each second. When passing directly overhead at the observer's location (at the zenith) the angular velocity is 1.25 degrees per second, or 0.80 seconds per degree.

The angular velocity gets slightly more complicated when a satellite is passing a viewer at an altitude angle less than at the zenith. The rest of the table consider a satellite at a 45 degree elevation above the horizon, or half way up to the zenith. When moving directly toward or away from the observer a satellite at this altitude will have a smaller angular velocity than when moving across the sky, or parallel to the horizon. These various combinations of directions, along with angular velocity expressed as both degrees per second and as seconds per degree, account for these last four columns of data.

Some spy satellites are low enough in height to appear to be moving much faster than the ISS. Similarly, compared to the Hubble Space Telescope, at 380 miles orbital height, the ISS appears to be moving 1.8 times as fast.

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