ASK TOM: When the sun is setting, at what point is considered sunset?

Dear Tom,
When the sun is setting, at what point is considered sunset? Is it when the bottom of the sun or the top of the sun dips below the horizon?
—Daniel Shutty

 

Dear Daniel,
Sunrise and sunset are both defined as the moment the upper edge of the sun’s disc touches the horizon.
At the official moment of sunrise or sunset, the full disc of the sun is actually below the horizon, but because the Earth’s atmosphere bends or refracts light we are able to see the sun earlier in the morning and later in the evening than if we were viewing it through a vacuum.
Because a person on top of a high-rise building or on a mountain will be able to see a more distant horizon, he or she will actually experience an earlier sunrise and later sunset than someone at ground level.