The Science Behind the Solstice

Winter sunset near Little Harbor (Sturgeon Bay).

The Winter Solstice this year is on Sunday, Dec. 21 at 5:02 pm. What does this mean?

To many folks, the Winter Solstice means the shortest daylight day of the year, which is true. So how could it have a specific time of day?

The answer involves the tilt of Earth’s axis in relation to its orbit around the sun, the location of Earth on its orbit at specific times, and the fact that the plane of Earth’s equator is not the same as the plane of Earth’s orbit.

Let’s say you wanted to demonstrate the concept using a globe (or an orange with a skewer through it) plus a lamp or other object to represent the sun. Place the “sun” where you can walk around it. Stand facing the “sun” with the axis tilted toward the sun. Look across the room past the “sun” and make note that the axis is also tilted toward that spot. Rotate the globe on its axis so that Wisconsin is closer to the floor than to the ceiling. Using the orange, represent Wisconsin by placing a sticker about half way between the “equator” and the skewer.

Wisconsin is experiencing summer, because rays from the sun are striking Wisconsin more directly than the same distance below the equator, where the rays would be at a shallow angle like they were just glancing off the Earth. So that spot below the equator is experiencing winter. Rotate the globe on its axis so that Wisconsin is now facing toward you and away from the “sun.” It’s now nighttime in the summer.

Walk one quarter of the way around the “sun” with the globe still tilted toward that spot across the room. Now the plane of Earth’s axis is at 90 degrees to the direction between the “sun” and the globe. This is the Vernal Equinox.

Continue around the “sun” until the tilt of the Globe’s axis is till pointing in the same direction in relation to the room, but directly away from the “sun.” At the exact time that this occurs, that’s the Winter Solstice. At this time, rays from the sun in daytime are largely “glancing” off the part of the globe which represents Wisconsin.

What is there to see? Well, forget the binoculars and telescopes as far as “seeing” the Winter Solstice is concerned. By coincidence, however, it will be a New Moon this year so a good time to see the stars, galaxies, nebulae and clusters.

But you can appreciate the Winter Solstice if the sun shines that day. Note and record where the sun rises in relation to your horizon and where it sets. Then every day or week afterward, notice where it rises and sets. You’ll see how those places move progressively southward until the Summer Solstice. Or, on the day of the Solstice, mark the shadow of the top of a tree or pole. Then at noon every day or week after, mark the same shadow and see how it progressively gets closer to the pole or tree. The sun appears to be getting higher in the sky because Earth is coming closer to being tilted directly toward the sun.

Thousands of years ago, in many cultures, people knew about the solstice and equinox.