Alaskan Trees, 9/21/13The sun rose this morning at 7:38, and will set 12 hours, 8 minutes and 33 seconds later at 7:46 this evening. We’re still losing 6 minutes and 37 seconds a day.

Wait a minute. Yesterday at 12:44 pm was the Autumnal Equinox, the time when day and night are supposedly equal at 12 hours each. What are we doing with days still longer than 12 hours?

Actually, the 12 hour days on the equinox would be true if (a) the sun were a point source of light and (b) the earth had no atmosphere. Since neither is true, the days are still longer than 12 hours today and tomorrow. Why?

The sun actually has an apparent diameter of half a degree, and sunrise and sunset are defined as when the upper edge of the sun is just on the horizon. Further, the air is densest near the ground, which means that the light rays are to a certain extent bent around the Earth’s curvature. This last means that there is actually a day-to-day variation in the difference between the geometrical sunrise and the observed sunrise, so the times I give are only approximate. Specifically, they are taken from a website that calculates them based on a standard atmosphere (which is certainly not the case here in the winter!) To quote from the website I use:

Sunrise and Sunset

My yard, 9/20/13The times for sunrise and sunset are based on the ideal situation, where no hills or mountains obscure the view and the flat horizon is at the same altitude as the observer. Sunrise is the time when the upper part of the Sun is visible, and sunset is when the last part of the Sun is about to disappear below the horizon (in clear weather conditions).

If the horizon in the direction of sunrise or sunset is at a higher altitude than that of the observer, the sunrise will be later and sunset earlier than listed (and the reverse: on a high mountain with the horizon below the observer, the sunrise will be earlier and sunset later than listed).

The Earth’s atmosphere refracts the incoming light in such a way that the Sun is visible longer than it would be without an atmosphere. The refraction depends on the atmospheric pressure and temperature. These calculations use the standard atmospheric pressure of 101.325 kilopascal and temperature of 15°C or 59°F. A higher atmospheric pressure or lower temperature than the standard means more refraction, and the sunrise will be earlier and sunset later. In most cases, however, this would affect the rising and setting times by less than a minute. Near the North and South Poles it could have greater impact because of low temperatures and the slow rate of the Sun’s rising and setting.

For locations north of 66°34′ N or south of 66°34′ S latitude, the Sun is above the horizon all day on some days during the summer and below the horizon all day on some days during the winter.

Technically, sunrise and sunset are calculated based on the true geocentric position of the Sun at 90°50′ from the zenith position (directly above the observer).

In case you’re wondering, my house is at 64° 33′ North latitude.

The photos above, by the way, were taken Friday. Saturday night and yesterday we had mixed snow and rain all day. I’ll add a current photo below as soon as it’s light enough to take one, but I fully expect an inch or so of snow.

9:23:13 Nyard

This morning, 7:50 am

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