Science & history · How the Sun moves across your sky
When exactly is sunset?
Sunset is a definition. The Sun's centre is already below the horizon when the published time arrives.
The convention
USNO defines sunrise and sunset as the moments the upper edge of the Sun's disc is on the horizon, with a standard allowance for refraction, the air bending sunlight round the curve. In that convention the centre of the Sun is about 0.83° below a level horizon at the published time.
Why the minute can be wrong
Refraction depends on the air on the day. USNO warns that even at sea under a clear sky computed times may be in error by a minute or more, and at high latitudes by many minutes. Hills, buildings and your own height above the ground move it again. The page uses the standard definition and says so.
Sources
- US Naval Observatory Rise, Set, and Twilight Definitions (opens in a new tab) for sunrise/sunset and the three twilights; refraction and accuracy caveats · checked 2 October 2026