A 23 Hour Day

Robert Bigelow

Yes, Daylight Saving Time is upon us. For most of the United States, on March 14, 1:59 a.m. will be followed by 3:00 a.m. as clocks are officially set forward one hour. So, March 14 will only be 23 hours long. Of course, Earth’s rotation knows nothing of this timepiece manipulation, so observers will note that sunrise occurs an hour later as measured by our altered clocks.

As I visit schools, I have found that some students have a surprising misconception. They think Daylight Saving Time results in more daylight hours. I suppose this comes about as they notice that sunset occurs an hour later. But Daylight Saving Time shifts both sunrise and sunset times. So, we experience the same amount of daylight before and after Daylight Saving Time.

Well, almost . . .

A close look at sunrise and sunset times for Salt Lake City on March 13 and 14, 2010 reveals that March 14 has 2 minutes more daylight than March 13.

Saturday, March 13, 2010         Mountain Standard Time
Sunrise                    6:42 a.m.
Sunset                     6:32 p.m.
Sunday, March 14, 2010         Mountain Daylight Time
Sunrise                    7:41 a.m.
Sunset                     7:33 p.m.

This is NOT the result of Daylight Saving Time. Instead, it comes about as Earth orbits the Sun. Earth rotates on its axis once a day. Earth also orbits, or revolves around the Sun once each year. Earth’s rotational axis is tilted by about 23.4º and points in a nearly constant direction as Earth circles the Sun. This is evidenced by the northern axis pointing toward Polaris, the North Star.

Diagram of Earth's orbit around the Sun.

Diagram of Earth's orbit around the Sun.

While the axis continues to point in the same direction, it’s orientation to the Sun changes. Back on December 21, Earth was at the place in its orbit where the northern axis leans most away from the Sun. On this day, Salt Lake City experiences about 9 hours of daylight. As Earth continues to move around the Sun, the angle between the axis and the Sun decreases. This results in an increase in the hours of daylight. This continues until June 21, when Earth reaches the place in its orbit where the northern axis leans most toward the Sun. On that day, Salt Lake City will experience about 15 hours of daylight.

Earth will continue in its orbit and eventually Daylight Saving Time will end with a 25 hour day on November 7, 2010.

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One thought on “A 23 Hour Day

  1. Pingback: Is the Sun directly overhead? « Clark Planetarium

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