Measuring a star’s altitude above the horizon requires two things: the star must be visible, and the horizon must be visible. If it is dark, the horizon is not visible. If it is too bright, only the Sun is visible among the stars. The Sun is therefore a very important star.
The stages of twilight are:
The center of the Sun is less than 6° below the horizon, and the Sun is otherwise completely below the horizon.
The center of the Sun is more than 6° but less than 12° below the horizon.
The center of the Sun is more than 12° below the horizon.
The center of the Sun is more than 18° below the horizon.
The Sun rises when its upper limb appears above the horizon. The Sun sets when its upper limb drops below the horizon. Star altitude measurements can be made after sunset and are best performed around the middle of civil twilight. By the end of nautical twilight, the horizon is already too poorly visible for reliable altitude measurements.
During astronomical twilight, the human eye already perceives night as dark, but the faintest celestial objects such as nebulae are still not visible due to sunlight scattered in the atmosphere.
Hamburg VII 2026