Definition
The exact middle point of the sun's visible disk, used as the reference for defining official sunrise and sunset times in aviation. Sunrise occurs when this point crosses the horizon going up; sunset occurs when it crosses going down.
Plain English
The dot in the middle of the sun. Aviation rules use this point — not the top edge or the bottom edge of the sun — to mark the official moment of sunrise and sunset.
Context Anchor
Seen in discussions of sunrise, sunset, and civil twilight, especially when determining whether it is legally day or night for aviation purposes.
Derivation
Geometric' comes from Greek roots meaning 'earth-measuring' and refers here to pure shape and position rather than appearance. The geometric center is the mathematical middle of the sun's disk, which is a more precise reference than 'when the sun looks like it's touching the horizon.'
Why Pilots Care
Provides the reference point needed for accurate sunrise, sunset, and solar bearing calculations without small errors from the sun's size.
Grounding Statement
For aviation timing, the sun is treated like a round disk, and its position is measured from the disk’s exact middle.
Intuition Check
Do not read this as the first or last visible edge of the sun. In this context, it means the exact middle point of the sun’s visible circle.
Example Sentence 1
Official sunset is defined as the moment the geometric center of the sun passes below the horizon, which is why the sky still looks bright at that time.
Example Sentence 2
When planning the route, the crew used the geometric center of the sun to determine precise solar bearings.