Q: Why does the rate for change of day length vary?
A: We define the length of the day as the time between sunrise and sunset, so that we can apply some simple mathematics.
Atmospheric conditions can make the actual sunrise and sunset vary slightly from the calculated times. As the Earth moves around the Sun, the length of the day changes.
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The length of day at a particular location on Earth is a periodic function of time. This is all caused by the 23.5-degree tilt of the Earth’s axis as it travels around the sun. In the Northern Hemisphere, days are longest at the time of the summer solstice in June, and the shortest days are at the winter solstice in December. At the two equinoxes in March and September, the length of the day is about 12 hours, a mean value for the year.
The length of a day changes far more during the year at higher latitudes than at lower latitudes. At the poles the daytime length varies from 0 to 24 hours, while at the tropics the daytime length varies little.
There is essentially no change in length of day from one day to the next at the time of the solstices. There is more change at the equinoxes. At those times the day-to-day changes can be a few minutes. The day length is changing fastest at the equinoxes.
Consider the percentage change — the change in day length from the previous day divided by the length of day on that day. That can give the impression of how quickly we lose daylight.
The shortening of the length of day in terms of percentage change is at its peak in late October and early November around our latitude.
"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.
The length of a day changes far more during the year at higher latitudes than at lower latitudes.