Altitude and degrees

The altitude of a star, a planet, or the International Space Station is a measure of how high it is in the sky. Altitude is measured in (angular) degrees. If an object is on the horizon, it has an altitude of 0 degrees. If it is right above your head (in the 'zenith'), it has an altitude of 90 degrees.

ISS passes at which the maximum altitude is less than 10 degrees are considered unfavourable, since they're difficult to observe.

Twisst classifies ISS passes depending on the maximum altitude:
* Relatively low in the sky:    maximum altitude less than 20 degrees
* Moderately high in the sky:   maximum altitude between 20 and 50 degrees
* Very high in the sky:         maximum altitude more than 50 degrees

It's easy to get an idea of how much 10 degrees is. Find an open space, with an unobstructed view toward the horizon. Stretch your arm and make a fist. Close one eye. Move your arm up or down until the bottom of your fist rests upon the horizon. The top of your fist is now at an altitude of approximately 8 degrees.