Astronomy and Groundhog Day

Astronomy and Groundhog Day

February begins with an odd holiday that is really based on astronomy. The holiday is Groundhog Day, February 2. It has also been called Candlemas. By either name, it is a cross-quarter day. Let’s find out what that means.

Models can help make new ideas easier to understand. To better understand cross-quarter days, get a group of 8 or 9 people to work together to build a model. Have eight people join hands to make a circle. If you have enough people, put one more in the center of the circle to be the sun. Give one person in the circle a sign that says Summer Solstice. Give the next person a sign that says Lammas (August 1). The next person should get a sign that says Fall Equinox. The next person’s sign should say Halloween. The next person will hold a sign that says Winter Solstice. The next person gets a sign that says Groundhog Day. The next person gets a sign saying Spring Equinox. The last person gets the May Day sign.

Now, give a ball to the person who has the Summer Solstice sign. The ball is the earth. Have the summer person pass the ball to the person with the Lammas sign to show that the earth is moving around the sun, from one part of the year to another. Keep passing the ball along and you will see that the earth goes around the sun, moving from one special day to another.

The two solstices and the two equinoxes mark the start of the major seasons of the year. The four cross-quarter days are halfway between the start of the major seasons.

The Summer Solstice is when the sun gets to its highest point in the sky at noon and the day has the most hours of sunlight. The Summer Solstice in the northern half of the earth comes in late June. The days leading up to the solstice have gradually had more and more hours of daylight. After this day, the number of hours of daylight each day will begin to decrease. 

The Winter Solstice is when the sun is at its lowest point in the sky at noon and the day has the fewest hours of daylight. In the northern half of the earth, the Winter Solstice is in late December. The days leading up to this one have gradually had fewer and fewer hours of sunlight. After this day, the number of hours of daylight each day will begin to increase.

At the Fall or Autumnal Equinox, there are 12 hours of daylight and 12 hours of night. In the northern half of the earth, this is in late September. At the Spring or Vernal Equinox, there are also 12 hours of day and 12 hours of night. In the northern hemisphere, the Spring Equinox is in late March.

So, how does Groundhog Day fit in? Goundhog Day is the cross-quarter day about half way between the Winter Solstice and the Spring Equinox. That means that no matter what the groundhog sees, the earth has traveled about one half of the way between winter and spring. The days will keep getting longer and the earth will slowly begin to warm.

There are three other cross-quarter days. We know them as May Day on May 1, Lammas on August 1, and Halloween on October 31. The four cross-quarter days have had interesting celebrations in different parts of the world and in different times in history. You may enjoy finding out about some of them. Maybe you can have a party on each of these important days in the solar year.

For more about Groundhog Day, visit our Groundhogs and Shadows page. To see a photograph of how the sun's position in the sky seems to change through the year visit the Astronomy Picture of the Day for December 21, 2008.
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