Seeing Satellites

Orbiting satellites are visible from the ground only because they reflect sunlight. Sometimes it is not the satellite we see, but the much larger rocket used to put it in orbit and which also orbits.

The requirements for seeing an orbiting object are that it be in sunlight and that there be no sunshine on the lower atmosphere near the observer. We can, of course, see the moon in the daytime, but it subtends a large angle to our eyes. Satellites normally can be seen only during the times of nautical or astronomical twilight. Civil twilight is the time when the sun is 0 to 6 degrees below the horizon; nautical twilight is when it is 6 to 12, and astronomical twilight is when it is 12 to 18 degrees. When the sun is below 18 degrees, it no longer shines on the high atmosphere (below about 400 km). A very high satellite, one above 400 km, could perhaps be seen during the night.

The best times for viewing satellites are those marked AT (astronomical twilight) or NT (nautical twilight) in the diagram. This diagram was prepared twenty years ago by C. T. Elvey, then director of the Geophysical Institute. That was when satellites were first being seen.