Question 1

A celestial body’s geographical position (GP) is…

Accepted Answer

The point on earth directly beneath it at that instant — Stand at the GP and the body is overhead. Altitude falls one degree per 60 nm from it — the whole basis of the sextant fix.

Question 2

Greenwich Hour Angle (GHA) is measured…

Accepted Answer

Westward from Greenwich, 0–360° — GHA is the sky’s longitude, always westward from Greenwich. It grows ~15°/hour as the earth turns.

Question 3

A chronometer error of 4 seconds shifts a celestial longitude by about…

Accepted Answer

One nautical mile — 15°/hour = 1′ of arc every 4 seconds — and a minute of arc is a mile. Time IS longitude.

The sky as a clock

This lesson is part of a paid course