Astronomical Explanation of the Summer Solstice

Summertime! Does it remind you of the lyrics to the Eagles’ song Ventura Highway? “When the days are longer, the nights are stronger than moonshine.”

We all look forward to the longer days of summer, perhaps without giving much thought to the fact that the days actually start getting longer in the middle of winter. Then, at the height of summer, they start getting shorter again.

It seems so unfair. Just as we really get into the swing of summer, it starts waning on us. What’s responsible for this?

Actually, it’s a cosmic conspiracy. The orbit of the earth, and the plane of the earth’s axis in relation to the sun create our warm and cool seasons.

And as difficult as it is to imagine, while we’re celebrating Christmas and hoping for snow, the folks in Australia and South Africa are heading to the beaches because summer for them is in December. Needless to say, “I’m Dreaming of a White Christmas” is not so popular in Perth.

To understand these seasonal variations, you have to imagine how the earth rotates while it revolves around the sun.

If you were to insert a pencil through the center of an orange and then hold both ends of the pencil to spin the orange, you’d have a model of the earth turning on its axis.

Then if you were to put a camp lantern on a table in the middle of your living room, and walk around it while spinning the orange around the pencil, you’d have a model of the earth spinning on its axis while orbiting the sun.

As you’re making a circle around the camp lantern, if you’re holding the orange’s pencil axis straight up and down, you’re being unfaithful to cosmic reality. You need to pick something in the room, such as a light switch, and tilt the pencil at a 45-degree angle so the top point is aimed at the light switch.

(A 45-degree angle is exaggerated reality, but that helps to better understand the principles involved. And instead of a light switch, you can pick a door or a window or a photograph or any other stationary object in the room.)

Now, as you walk around the lantern, keep the top point of the pencil pointed toward the stationary object. What you’ll notice is when you’re on the opposite side of the lantern from your stationary object, the top tip of your pencil is pointed toward the lantern. But when you’re on the side of the room closest to your stationary object, the top tip of your pencil is pointed away from the lantern.

When the top of the axis is pointed toward the lantern, the top of the orange receives more light. On the other side of the room when the top of the axis is pointed away from the lantern, the bottom of the orange receives more light.

Comparing this to the earth’s orbit, it takes six months for you and your orange to get from one side of the living room to the other, then another six months to get back to your starting point.

When you reach that point in your orbit around the camp lantern that you are closest to your stationary object and the top of the pencil is pointed directly away from the lantern, it is the summer solstice in the Southern Hemisphere. Johannesburg is having its longest summer day. At the same time, Toronto residents are in the grips of the longest night of winter.

As you circle back around to the opposite side of the lantern and the top of the pencil is now pointing exactly toward the light, Keflavik, Iceland is seeing the official sunset at 12:03 AM and the official sunrise at 12:04 AM! A day that is 23 hours and 59 minutes long.

On the same day, Ushuaia, Tierra Del Fuego, Argentina (the city farthest south on the dark, bottom side of your orange) is celebrating the Festival of the Longest Night.

Our universe is a fascinating place, and the seasons we take for granted are amazingly different on many other worlds. Take the moon for example. The moon’s rotation is such that one side always faces the sun. There is no sunrise, no sunset only eternal summer and daytime on one side, and eternal winter and nighttime on the other. Other worlds have many other wild seasonal differences from ours.

As for me, I’ll take what we have and be happy.

Sum, sum, sum, sum sum, sum, summertime! Summer time, ooo.