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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Solar eclipse

There is going to be a solar eclipse visible in the Western United states this evening (and in east Asia, but I live in the US). This will be the first solar eclipse for my daughter, so hopefully the weather cooperates and we can go out and see it. Of course, she probably won’t even notice it and I should not encourage her to look at the sun anyway.

What is a solar eclipse and what causes it?

We know that the Earth orbits the sun and the moon orbits the Earth. Sometimes these orbits are lined up so that the moon passes between the Earth and the sun, blocking the light of the sun from reaching us. This causes a solar eclipse.  Below is a picture from Wikipedia Commons illustrating this effect.

 The umbra is the area on the Earth where the light from the sun is completely blocked. This is a total eclipse. The penumbra is the area where the light from the sun is only partially blocked. Outside these areas, an eclipse will not be viewed – the moon will not block the light from the sun at all. 

The eclipse this weekend is going to be annular eclipse for some areas. This is what you see when the moon passes directly in front of the sun, but is not large enough to block the entire sun. Some light makes it past the edges of the moon and is seen on Earth.

Below are some rough drawings of what the different types of eclipses look like:

The type of eclipse depends on the exact path of the moon and where on Earth you are viewing the eclipse.

Why does the moon sometimes block all of the light from the sun and sometimes only some of the light? The amount of light blocked depends on the moon’s distance from the Earth. When the moon is closer to the Earth, it casts a bigger shadow, and when it is far from the Earth, it casts a smaller shadow. To understand this, close one eye and hold your thumb up in front of the other eye. Now look at something far away.  If you hold your thumb close to your eye, it likely blocks the entire object you are looking at. However, if you move your thumb as far away from your eye as you can, it blocks a much smaller area of the object.

The moon does not orbit the Earth in a perfect circle. The orbit is an ellipse (oval shaped). That means that sometimes the moon is close to the Earth and sometimes it is farther away.  The closest point is called the perigee of the orbit and the farthest point is the apogee of the orbit. When there is a full moon at the perigee (or close to it), it is sometimes called a ‘supermoon’ because the moon seems very large and bright. This happened earlier this month, on May 5 (the perigee was on May 6). A calendar of apogee and perigee dates can be found here.

Eclipses only occur during a new moon. The new moon is when the moon is dark – the opposite of the full moon. A full moon occurs when the the light side of the moon is facing the Earth. This is caused by the light of the sun reflecting off of the moon and hitting the Earth where we see it since the moon does not emit visible light on its own. If the moon is blocking the sun, as during an eclipse, the light side must be facing the sun (and the light reflects away from us)and the dark side must be facing the Earth.

Today, there is a new moon close to the lunar apogee (May 19), so the moon will be almost as far away as it gets. Therefore, the shadow it casts will not completely block out the sun, allowing for some people on Earth to see an annular eclipse.

Will I get to see the eclipse?

Solar eclipses are fairly common on Earth, but each eclipse can only be seen by a relatively small area of the Earth. Therefore, eclipses at a particular location are not that common. If you live in the Western US, there will be an annular eclipse today, May 20, 2012. Much of the Western US will only see a partial eclipse. If you would like to find out what time the eclipse will occur at your location, or when the next solar eclipse will be, check out NASA’s Solar Eclipse page.  This page requires you to either pick the city closest to you, or enter the exact coordinates of your location. If you would like to enter your coordinates, you can find most towns and cities on Wikipedia, with the coordinates listed on the right hand side of the page. Or you can use Google Earth to find the exact coordinates of your house. NASA also has a special page with information on today’s solar eclipse.

We will not be in the area where the annular solar eclipse will be visible, but I am excited about being able to see the partial solar eclipse tonight.