Some people are very observant. The other day, I got this email from Charles Thompson. He wrote:
“I enjoyed your column today on the length of the days. Most people do not realize that on Dec. 11, the evenings start to increase. By the time the 22nd rolls around the evenings are already three minutes longer. A wee bit of a pick-me-up in the dead of winter.”
That’s a great observation, Charles. The reason for that is something called the “equation of time.” It’s a complex balance sheet incorporating earth’s rotation, orbit, and tilt with respect to our distance from the sun and the earth’s velocity. That’s a mouthful, so let me try to explain.
If the sun was to culminate every day at 12 noon you would be correct to assume that the times for sunrise and sunset should be symmetrical around noon. However, since the Earth is not moving around the sun in a perfect circle, the angular velocity over the sky as seen from a fixed observer on Earth, varies through the year giving rise to what is called Equation of Time, meaning the sun will not culminate at the same time of day through the year. Our clock is based on what is called Mean Solar time which fits the average yearly angular speed of the sun.
So, in a nutshell, the Equation of Time is the time interval between noon – when the sun is highest – and midday – when clocks read 12:00. The word equation is used in a historical sense, not to mean an algebraic expression but rather a correction.
As solar noon increasingly occurs later, sunrises and sunsets also steadily occur later each day after the winter solstice. This is why a location’s earliest sunset occurs before, and its latest sunrise occurs after the winter solstice.
Using Charlottetown as an example:
The earliest sunset of the year was well before the solstice. The sun set at 4:26 p.m. from Dec 4 to Dec. 16; on Dec. 17, the sun set at 4:27 p.m.
The latest sunrise came during the first week of January, well after the winter solstice.
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Cindy Day is the chief meteorologist for SaltWire Network
