What happens if there is no sunrise or sunset for that date and location?

The result reports 24 hours of daylight for polar day, or 0 hours for polar night, directly, rather than returning an error or a misleading partial figure.

Why does the equinox not produce exactly 12 hours everywhere?

Because the NOAA equations account for atmospheric refraction and the sun's actual disk size, both of which nudge the real sunrise and sunset moments slightly earlier and later than a perfectly geometric 12-hour split would suggest.

Daylight Duration Calculator

Enter a date, latitude, and longitude. See the total daylight duration, or a polar day/night notice if the sun never sets or never rises.

Comparing daylight between two dates or locations? See Daylight Change Comparator.

Worked example

At the equator on the March equinox, daylight duration comes out to approximately 12 hours and 7 minutes, slightly over the naive geometric expectation because of atmospheric refraction near the horizon. Move that same date to 60 degrees north latitude instead, and daylight duration stretches out to roughly 12 hours and 16 minutes, a small but real increase driven purely by the higher latitude even on a day when day and night are supposed to be roughly equal everywhere.

Common mistakes

Expecting exactly 12 hours everywhere on the equinox. Latitude and atmospheric refraction both introduce small but real variation even on that date.
Treating the result as a guarantee of visible sunlight. Local weather, terrain, and obstructions are not part of this astronomical calculation at all.

Limitations

This is an astronomical estimate using the standard NOAA solar equations. It is not a guarantee of actual visible sunlight, since local weather and terrain can block sunlight the calculation still counts as present.

Frequently asked questions

What happens if there is no sunrise or sunset for that date and location?: The result reports 24 hours of daylight for polar day, or 0 hours for polar night, directly, rather than returning an error or a misleading partial figure.
Why does the equinox not produce exactly 12 hours everywhere?: Because the NOAA equations account for atmospheric refraction and the sun's actual disk size, both of which nudge the real sunrise and sunset moments slightly earlier and later than a perfectly geometric 12-hour split would suggest.