Day and Night of East Lansing Autumnal Equinox Not Quite Equal

Every fall, the autumnal equinox comes to East Lansing. This year the equinox will be Monday, the 22nd of September, although the sun will be visible from East Lansing for more than twelve hours.

A little background: There are two equinoxes in each year. The spring, or vernal, equinox comes on or about March 21 and the autumnal equinox comes on around September 22. The word equinox refers to the roughly equal (Latin: aequalis) duration of day and night (Latin: nox) on the equinoxes.

While the day and night of Monday, September 22, will be nearly equal in East Lansing (and nearly everywhere else on earth), the sun here will be visible slightly longer than it will be invisible. ELi’s eagle-eyed readers of available charts will notice that on Monday the sun is due to rise at 7:26 am and set at 7:36 pm. The time between sunrise and sunset on Monday will therefore be twelve hours and ten minutes, and not the exact twelve hours that would be expected from the word “equinox.”

Why? According to common earthly conventions, sunrise is defined as the appearance of the first edge of the sun above the horizon in the morning, and sunset is defined as the disappearance of the last edge of the sun below the horizon in the evening. Thus the time it takes the disk of the sun to pass the horizon is counted twice in the duration of the day and never counted as a part of the night. If the sunrise and sunset were timed from the passage of the geographic center of the sun over the horizon in the morning to the center of the sun passing below the horizon in the evening, then the sunrise and sunset would be at the same (for example, 7:30 am sunrise and 7:30 pm sunset) and the day and night would both be twelve hours.

September 25 will actually be East Lansing’s closest approach to a twelve-hour day in 2014. On that day, sunrise will be at 7:29 am and sunset at 7:30 pm.

Astronomers think about the question of when the equinox occurs with extraordinary precision. Astronomically speaking, the equinox occurs when the geographic center of the sun passes the equatorial plane of the earth. The earth spins at a 23.4-degree angle compared to its orbit around the sun. For East Lansing residents, the tilt of the earth points toward the sun in the summer (creating longer daytimes with the sun higher in the sky), and the tilt points away from the sun in the winter (creating shorter daytimes with the sun lower in the sky). As the earth orbits the sun, the orientation of the tilt moves between these extremes and passes the midpoint on the equinoxes. For Fall, 2014, the center of the sun will pass the earth’s equator at 10:29 pm Monday night, well after sunset. So that is when our true autumnal equinox will occur.

Now here is your ELi horoscope:* Based on an informal poll of neighbors, the autumnal equinox will leave many people in East Lansing, especially those who lost power last Winter, disappointed that the astronomical Summer has ended and Fall has begun.

*A horoscope is a prediction of human events based on the positions of celestial bodies. By policy, ELi reports only facts.

Photo courtesy of NASA.

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