The Sun Will be With You Shortly

CreditVictoria Roberts

A. What goes into the forecast is based on calculation, not observation, and the calculation is done without reference to local topography or visibility.

The National Weather Service gets its sunrise and sunset times from data computed by the United States Naval Observatory.

The conventional definition for both is the time at which the upper edge of the disk of the Sun meets the horizon. Atmospheric conditions are assumed to be average, and the observer is assumed to be at sea level with an unobstructed view.

The observatory’s calculations do include corrections for visibility and for the average amount of refraction, or light bent by the atmosphere. But it is not practical to include exact local conditions in routine computations of sunrise and sunset.

Unpredictable atmospheric conditions, for example, affect the amount of light refraction at the horizon, so that even under ideal conditions, the times may be off by a minute or more.

Local topography, like mountains on the horizon, and the height of the observer can affect the times even more. At high latitudes especially, small variations in refraction can alter the observed times by many minutes.

If you want calculations of estimated sunrise and sunset times for a specific location, you can enter the coordinates of latitude and longitude on a website maintained by the observatory.

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