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The science of spring equinox

By Brett Shepard 11:30 AM March 19, 2014

Three factors cause Alaska days to exceed 12 hours of daylight prior to the Spring Equinox.

ANCHORAGE – Tuesday, Anchorage topped 12 hours of sunlight and today it’s up to 12 hours and 9 minutes of sunlight.

The spring equinox, though, isn’t until March 20. Equinox is Latin for “equal night,” so why don’t Alaskans see an equal length of day and night on the first day of spring? Why does it happen a few days earlier in Alaska?

There are three factors at play that cause Alaska days to exceed 12 hours of daylight prior to the spring equinox. The first is that the geometric center of the sun on March 20 is above the horizon for 12 hours. But the sunrise and sunset are not measured by when the geometric center of the sun reaches the horizon: Sunrise begins the instant the upper edge of the sun becomes visible and sunset happens the instant the upper edge of the sun disappears below the horizon.

In this instance, the center of the sun is below the horizon, therefore the length of day is longer than 12 hours on the spring equinox. The second factor at play is refraction, which causes the upper edge of the sun to be visible before and after the edge of the sun actually reaches the horizon. The amount of refraction depends on factors such as temperature and pressure.

The final aspect to all of this is latitude. The higher the latitude, the closer you will be to the actual date of equal day and night. In other words, the closer you are to the equator the earlier in the year you will see a 12-plus hour day north of the equator. At 15 degrees north, the first 12-plus hour day in 2014 happened March 14. Anchorage topped the 12-hour mark on March 18.

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