Solar altitude is the sun's height above the horizon in degrees: 0° at sunrise and sunset, 90° straight overhead, negative through the night.

Sol computes it from your latitude, longitude, the date and the minute, to about a tenth of a degree, the same way for every instrument. The drawing is the altitude scale the twilights are defined on; the dot is the sun now. Refraction near the horizon lifts the apparent sun a little, and the twilight definitions use the geometric altitude.

Every hour of light Sol names — golden, blue, the twilights — is a band of altitude, not a time.