Course / The circle of equal altitude

Part III · The line of position · Page 2 of 4

Zenith, horizon and altitude

On the celestial sphere the sextant's angle has a simple picture. The celestial horizon is the great circle 90° from your zenith. A body's altitude is its angle up from that horizon; the rest of the way to the zenith, 90° − altitude, is the zenith distance.

Predict The Sun's altitude is 60°. How far is its ground position from you?

SunZenithN poleS polealtitude 38°44.0'zenith distance 51°16.0'ZenithDayNESWSunalt 39°Zn 265°
Your sky: drag to walk round; drag the body along its path to change the time. May 21, 15:00 UT. Earth fixed: the sky turns west. Drag to turn the sphere; drag the body to run the clock; drag your zenith to move yourself. Not to scale: the real sphere is so large the Earth would be a point.

Altitude and zenith distance

Altitude is the angle from the celestial horizon up to the body, along the vertical circle through it. The zenith distance is the rest of the way, 90° − altitude: it is also your distance from the ground position, in degrees.

Altitude
38°44.0'
Zenith distance
51°16.0' = 3,076 nm to the GP

The zenith distance on the sphere and your distance from the ground position on the Earth are the same angle, seen from the center. That is why one altitude puts you on a circle around the GP. "Your sky" shows the same altitude as you would see it from the deck, with the bearing (azimuth) along the horizon; drag the Sun along its path to watch both change through the day.