When equal is not equal
Frank Watson, Meteorologist
August 2004

This is the time of year that we've all worked for, harvest time. Whether you've planted a garden of flowers, a small family plot of vegetables or acres of a main cash crop it is time to see the fruit of our labor.

Hopefully the weather has cooperated and rains were timely. This is the month of the Harvest moon, September 28th. This month also ushers in the crossing of the sun's rays that have been centered north of the Equator to south of Equator.

This event is known as the Autumnal Equinox and it occurs on the 22nd. No longer will we have longer days than night. For the roughly the next 90 days the amount of sunlight will get shorter and shorter until December 21.

All over the world the sun will rise at roughly at 6am and set at 6am but because we observe daylight saving times here in the United States that sunrise and sunset time will shift the time forward by one hour. The sun will be due south on this date at 1am.

One thing that had always puzzled me about the Autumnal Equinox as well as the Vernal Equinox (beginning of Spring) was why the day and night weren't really the same length. 12 hours of sunlight and 12 hours of darkness.

The word equinox comes from the Latin word meaning "equal night." But yet looking at sunset and sunset times this was not true. This month we have around 12 hours and 9 minutes of sunlight on the 22nd. Its not until a few days later do we really have 12 hours of daylight and 12 hours of darkness.

What is going on?

No one really could give me a good answer. I heard one TV meteorologist say it was because we are "so many" degrees north of the Equator. I dugout my sunrise and sunset charts and the sunrise and sunset lines are parallel to each other so there is no way that this way true.

Digging further I found my answer. All I needed to do was look at the definition of sunrise and sunset.

The definition of sunrise is when the very top of the sun breaks the horizon. The definition of sunset is NOT when the bottom of the sun dips below the horizon giving us equal day and night but the definition of sunset is when the top of the sun dips below the horizon. This is the added extra time that we see on our sunrise sunset charts.

With that burning question answered for me a few years back I hope given you a bit of trivia you can use to impress your family and friends.

Frank Watson is a White Bear Lake, Minnesota meteorologist and can be found on the web at WeathermanWatson.com