Seasons (or Why My Front Door is Hotter in Winter than Summer)

We moved to CO a few months ago and when we first moved into our new place, I noticed that our front door, which is south facing, became very hot on sunny days. This was March and so I was worried what it was going to be like in summer. Even in March, I was afraid my daughter was going to hurt her hands is she touched it. Interestingly, now it is June, and it is regularly 90 deg and sunny, but the front door stays cool to the touch. This got me thinking about the sun and seasons…

If you have not taken an Astronomy course, you may not spend a lot of time thinking about the solar system and how the Earth moves in relation to the Sun. You may also not have thought much about what causes summer and winter and why the days are longer in the summer.

This week is the summer solstice for the northern hemisphere. The solstice this year is June 20. So what is the solstice, what does it mean about the Earth and the Sun and why do we have seasons? Why does the Southern Hemisphere have opposite seasons?

A common misconception is that summer occurs when the Earth is closest to the Sun and winter occurs when the Earth is farthest from the Sun. Let’s think about that a little more. I found this chart in Lecture-Tutorials for Introductory Astronomy by Edward Prather et al.:

Month

Earth-Sun Distance

December

147.2 million kilometers

June

152.0 million kilometers

September

150.2 million kilometers

March

149.0 million kilometers

There are a couple of things I can learn from this. First, the Earth is not always the same distance from the Sun. Second: It is June now and we seem to be at our farthest point from the Sun. Wait a second! It’s hot now. It’s summer, right? Shouldn’t we be closest to the Sun?

The Southern Hemisphere has winter in June when the Sun is at its farthest point, but the Northern Hemisphere is enjoying summer in June. If the seasons were caused by our distance from the sun, both hemispheres would have to experience summer (and winter) at the same time. But we have opposite seasons, so the distance from the Sun can not be what causes our changing seasons. Well, then, what does?

We know that the Earth is warmed by radiation from the Sun – sunlight. The Earth is also tilted with respect to the Sun at about 23.5°, so that it looks like this as it orbits:

Sometimes the northern hemisphere is tilted toward the sun and sometimes it is tilted away from the sun. When the north is tilted toward the sun, we get more direct sunlight hitting us. The Sun appears to be higher in the sky in the summer when we are tilted toward the sun. Direct sunlight gives us more heat. Think about going for a walk in the summertime. Does the sun feel hotter in the middle of the day when it is directly above or in the early evening when it is low in the sky? The sun definitely feels most intense to me in the middle of the day. I tend to go on walks with my daughter in the morning and the evening.

This more direct sunlight gives us more heat and causes the change in seasons that we experience.

At the north pole, this tilt is enough that in the summer, there is always sunlight hitting the ground and there is no night at the peak of summer. The farther north you are (or south in the southern hemisphere), the more dramatic the change in light and temperature is. Near the equator, there is always plenty of direct sunlight regardless of the season and so the temperature remains warm year round. However, far from the equator, the amount of sunlight hitting the ground changes dramatically due to the tilt of the Earth and so the temperature also changes quite a bit from season to season.

The summer solstice on June 20 is the time when the tilt of the Earth and the rotation around the Sun causes the northern hemisphere to get the most direct sunlight. The sun is highest in the sky in the north and daylight lasts longer than any other day of the year.

So what does this all have to do with my front door? In the spring and winter, the sun is low in the sky to the south and so my front door gets a lot of direct sunlight. However, now that it is summer, the sun is high in the sky and the eaves on the house block the sun from hitting the door. Here is my silly exaggerated picture of this:

Phew! So I do not have to worry about my daughter burning her hand on the door. Of course, it is hotter outside and the sunlight is generally more intense, so I have to worry about other things like sun hats, sunscreen, sun shades on the car, etc. She is very fair of skin (like her mother) and will likely burn easily with the intense summer sun.

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