Definition
The point where a line from the body's centre to the Earth's centre pierces the surface. Its latitude equals the body's declination and its longitude corresponds to the Greenwich Hour Angle, converted by subtracting from 360 degrees when GHA exceeds 180. Because the Earth rotates about 15 degrees of longitude per hour, the sun's GP moves roughly 15 minutes of arc westward each minute of time, which is why timing a sight to the second matters.
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