Navigators also draw the sky as a celestial sphere: a huge glass sphere with the Earth at its center, with every body painted where its direction meets the glass. It is the same picture as the ground position, seen from outside. The Earth's poles and equator, projected outward, become the sky's celestial poles and celestial equator; a body's declination is its angle north or south of that equator, and its GHA is measured west along it from the Greenwich meridian, projected out.

Predict A star has declination 0°. Where is it on the celestial sphere?

May 21, 15:00 UT. Earth fixed: the sky turns west. Drag to turn the sphere; drag the body to run the clock. Not to scale: the real sphere is so large the Earth would be a point.

Celestial sphere

An imaginary sphere around the Earth, so large that the Earth is a dot at its center. Each body is drawn where its direction meets the sphere: a navigator needs only directions, never distances.

Body: Sun
Declination: N 20°13.3'
GHA: 45°51.0'

Body

Time: 15:00 UT

Date: May 21Your latitude: N 40°00.0'Your longitude: 10° E

Pick each term above and watch it on the sphere, then press Play: the sky turns west and GHA grows, while declination stays almost still. The line from the body to the center of the Earth crosses the surface at the ground position, which is why its latitude is the declination and its longitude (counted west) is the GHA.

The celestial sphere

Celestial sphere