ANCHORAGE, Alaska (KTUU) — Saturday is the autumnal equinox and the official start of fall. The equinox occurs at 5:54 p.m. AKDT on Sept. 22. Though we tend to treat the equinox as a day-long event, it actually occurs at the moment the Sun crosses the celestial equator – the imaginary line in the sky above Earth’s Equator.
Equinox means “equal night” to indicate the day and the night are the same length—i.e.,12 hours of daylight and 12 hours of darkness—but that’s not really the case. In most locations, there will be more daylight than darkness. This is because of the way the sunrise and sunset are calculated.
On the equinox, the sun’s geometric center is above the horizon for 12 hours but the sunrise is calculated from the moment the top edge of the sun’s disc moves above the horizon. Conversely, sunset is measured at the point the sun’s top edge drops below the horizon. The time that it takes for the sun to fully rise and fully set are added into the day’s length.
For example on the equinox, the sunrise in Anchorage will occur at 7:44 a.m. and set at 7:58 p.m. for a total of 12 hours, 14 minutes and 32 seconds of daylight. Utqiagvik will have a sunrise at 8:07 a.m. and sunset at 8:29 p.m. for 12 hours, 21 minutes and 53 seconds.
There is a day when locations will have almost equal daylight and darkness and it’s called the equilux. This is determined by the latitude of the location. Utqiagvik’s equilux will occur on Sept. 24 with 12 hours, 3 minutes and 30 seconds. Anchorage’s equilux is Sept. 25 with 11 hours, 57 minutes and 34 seconds.