Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax" - Philip Plait (2002)

Part II. From the Earth to the Moon

Chapter 5. A Dash of Seasons: Why Summer Turns to Fall

ome examples of bad astronomy are pernicious. They sound reasonable, and they even agree with some other preconceived notions and half-remembered high school science lessons. These ideas can really take root in your head and be very difficult to get out.

Perhaps the most tenacious of these is the reason why we have seasons.

Seasons are probably the most obvious astronomical influence on our lives. Over most of the planet it's substantially hotter in the summer than in the winter. Clearly, the most obvious explanation is our distance from the Sun. It's common sense that the closer you are to a heat source, the more heat you feel. It's also common sense that the Sun is the big daddy of all heat sources. Walking from underneath the shadow of a shade tree on a summer's day is all you need do to be convinced of that. It makes perfect sense that if somehow the Earth were to get closer to the Sun, it could heat up quite a bit, and if it were farther away our temperatures would dip. And hey, didn't you learn in your high school science class that the Earth orbits the Sun in an ellipse? So sometimes the Earth really is closer to the Sun, and sometimes it's farther away. This logic process seems to point inevitably to the cause of the seasons being the ellipticity of the Earth's orbit.

Unfortunately, that logic process is missing a few key steps.

It's true that the Earth orbits the Sun in an ellipse. We know it now through careful measurements of the sky, but it isn't all that obvious. For thousands of years it was thought that the Sun orbited the Earth. In the year 1530, the Polish astronomer Nicolaus Copernicus first published his idea that the Earth orbited the Sun. The problem is, he thought the Earth (and all the planets) moved in a perfectly circular path. When he tried to use that idea to predict the positions of the planets in the sky, things came out wrong. He had to really fudge his model to make it work, and it never really did do a good job predicting positions.

In the very early part of the 1600s, Johannes Kepler came along and figured out that planets move in ellipses, not circles. Here we are 400 years later, and we still use Kepler's discoveries to figure out where the planets are in the sky. We even use his findings to plan the path of space probes to those planets; imagine Kepler's reaction if he knew that! (He'd probably say: "Hey! I've been dead 350 years! What took you so long?")

But there's a downside to Kepler's elliptical orbits; they play with our common sense and allow us to jump to the wrong conclusions. We know that planets, including our own, orbit the Sun in these oval paths, so we know that sometimes we're closer to the Sun than at other times. We also know that distance plays a role in the heat we feel. We therefore come to the logical conclusion that the seasons are caused by our changing distance from the Sun.

However, we have another tool at our disposal beside common sense, and that's mathematics. Astronomers have actually measured the distance of the Earth to the Sun over the course of the year. The math needed to convert distance to temperature isn't all that hard, and it is commonly assigned as a homework problem to undergraduate-level astronomy majors. I'll spare you the details and simply give you the answer. Surprisingly, the change in distance over the course of the seasons amounts to only a 4-degree Celsius (roughly 7 degrees Fahrenheit) change in temperature. This may not surprise people from tropical locations, where the local temperature doesn't vary much over the year, but it may come as a shock to someone from, say, Maine, where the seasonal temperature change is more like 44 degrees Celsius (80 or so degrees Fahrenheit).

Clearly, something else must be going on to cause such a huge temperature variation. That something else is the tilt of the Earth's axis.

Imagine the Earth orbiting the Sun. It orbits in an ellipse, and that ellipse defines a plane. In other words, the Earth doesn't bob up and down as it orbits the Sun; it stays in a nice, flat orbit. Astronomers call this plane the ecliptic. As the Earth revolves around the Sun, it also spins on its axis like a top, rotating once each day. Your first impression might be to think of the Earth's axis pointing straight up and down relative to the ecliptic, but it doesn't. It's actually tilted by 23.5 degrees from vertical. Have you ever wondered why globe-makers always depict the Earth with the north pole pointing at an angle from straight up? Because it is tilted. It doesn't point up.

That tilt may not seem like a big deal, but it has profound implications. Here's an easy experiment for you: Take a flashlight and a piece of white paper. Darken the lights in a room and shine the flashlight straight down on the paper. You'll see a circle of bright light. Now tilt the paper so that the light shines down at about a 45-degree angle. See how the light spreads out? It's now an oval, not a circle. But more importantly, look at the brightness of the oval as you change the illumination angle. It's dimmer. The total light hitting the paper hasn't changed, but you've spread the light out by tilting the paper. More of the paper is lit, but each part of the paper has to share all the light, so there is less light for each part. If you tilt the paper more, the light gets even more spread out, and dimmer.

This is exactly what's happening to the Earth. Imagine for a moment that the Earth is not tilted, and that the axis really does point straight up and down relative to the ecliptic. Now pretend the Sun is a giant flashlight shining down on the Earth. Let's also say you are standing in Ecuador, on the Earth's equator. To you the Sun would be straight up at noon, with the sunlight hitting the ground straight on. The light is highly concentrated, just like it was when the paper was directly facing your flashlight in the experiment.

But now let's pretend you are in Minneapolis, Minnesota, which happens to be at 45 degrees latitude, halfway between the equator and the north pole. Again, sunlight is spread out like it was when you tilted the paper in the experiment. Since the Sun's light is what heats the Earth, there is less heat hitting the ground per square centimeter. The ground there doesn't get as much warmth from the Sun. The total light hitting the ground is the same, but it's spread out more.

To take matters to the extreme, imagine you're at the north pole. The sunlight there is hitting the ground almost parallel to it, and it gets spread out tremendously. Another way to think of this is that at the north pole, the Sun never gets very high off the horizon. This is like tilting your paper until the flashlight is shining almost along it. The light gets spread out so much that it barely does any good at all. That's why it's so cold at the north and south poles! The Sun is just as bright down there as it is in Ecuador and Minneapolis, but the light is spread out so much it can barely warm up the ground.

The seasons are caused by the Earth's tilt, and not because of its distance from the Sun. In the northern hemisphere, it's summer when the Earth's north pole points most toward the Sun, and winter when it points away. Note that the Earth is closest to the Sun during the northern hemisphere winter.

In the summer, the Sun is higher in the sky. Its light is more concentrated on the Earth's surface. In the winter, the Sun is lower, and the light gets spread out, heating the Earth less efficiently.

In reality, the Earth's axis is tilted, so matters are a bit more complicated. As the Earth orbits the Sun, the axis always points in the same part of the sky, sort of like the way a compass needle always points north no matter which way you face. You can imagine that the sky is really a crystal sphere surrounding the Earth. If you were to extend the axis of the Earth until it intersects that sphere, you'd see that the intersection doesn't move; to us on the surface of the Earth, it always appears to point to the same part of the sky. For those in the Earth's northern hemisphere, the axis points very close to the star Polaris. No matter what time of year, the axis always points in the same direction.

But as the Earth orbits the Sun, the direction to the Sun changes. Around June 21 each year the axis in the northern hemisphere is pointing as close as it can to the Sun. Six months later, it is pointing as far away as it can from the Sun. This means that for someone in the northern hemisphere the Sun is very high in the sky at noon on June 21, and very low in the sky at noon on December 21. On June 21, the sunlight is concentrated as much as it can be, and so it heats the ground efficiently. On December 21 the light gets spread out and it doesn't heat things up well. That's why it's hot in the summer and cold in the winter, and that's why we have seasons. It's not our distance from the Sun, but the direction to the Sun and therefore the angle of the sunlight that makes the difference.

Take a look at the diagram showing the Earth's axis relative to the Sun. Note that when the northern-hemisphere axis points toward the Sun, the southern-hemisphere axis points away, and vice versa. That's why people in the southern hemisphere celebrate Halloween in the spring and Christmas in the summer. I wonder if the song "I'm Dreaming of a Green Christmas" is popular in Australia ...

There's an added tweak, too: because of our axial tilt the Sun gets higher in the sky in the summer, as we've seen. That means the path the Sun appears to travel in the sky is longer, so the Sun is up longer during the day. This, in turn, gives the Sun more time to heat up the Earth. Not only do we get more direct sunlight, the sunlight also lasts longer. Double whammy! In the winter the Sun doesn't get up as high, and so the days are shorter. The sun also has less time to heat up the ground, and it gets even colder. If the Earth were not tilted, days and nights would be 12 hours each, no matter where you were on the Earth, and we'd have no seasons at all.

Take another look at the figure on page 51. It shows that the Earth is actually closest to the Sun in January. This is the final nail in the coffin of the misconception that distance to the Sun is the main reason we have seasons. If that were true, we should have summer in January in the northern hemisphere and winter six months later in June. Since the opposite is true, distance must actually be a bit player in the seasons game.

However, it is not completely negligible. Distance does play a role in the seasons, although a minor one. For the folks in the northern hemisphere it means that winters should be a couple of degrees warmer on average than they would be if we orbited the Sun in a circle because we are closer to the Sun in the winter. Conversely, the summers are a couple of degrees cooler because we are farther away. It also means that people in the southern hemisphere should have hotter summers and colder winters than do those living in the northern hemisphere.

However, in reality, things are even more complicated. The southern hemisphere is mostly water. Check a globe and see for yourself if you like. Water is slower than land to heat up and cool off. This plays a role in the heat budget of the Earth, too. As it turns out, summers in the southern hemisphere are about as hot and winters are about as cold as they are in the northern hemisphere. The huge amount of water south of the equator acts as a kind of insulator, protecting that hemisphere from big temperature swings.

Amazingly, there is even more to this story. I said earlier that the Earth's axis is fixed in space, but I lied. Forgive me; I didn't want to make this too complicated at that point. The truth is, the Earth's axis does move, slowly, across the sky.

A slight digression: When I was a kid, my parents bought me a toy top. I used to love to spin it, watching it move across the floor in funny patterns. I also noticed that as it began to slow its spin, it would start to wobble. I was too young to understand it then, but I now know that the wobble is due to the interplay of complicated forces on the spinning top. If the axis of the top is not exactly vertical, gravity pulls the top off-center. This is called a torque. Because the top is spinning, you can think of that force being deflected horizontally, making the top slowly wobble. The same thing would happen if the top were spinning in space and you poked it slightly off center. The axis would wobble, making little circles; the bigger the poke, the bigger the circle it would make.

This wobble is called precession, and it is caused by any tug on the top that is not lined up with the axis. It happens for any spinning object that experiences some kind of force on it. Of course, the Earth spins, too, just like a top, and there does happen to be a force on it: the Moon's gravity.

The Moon orbits the Earth and pulls on it with its gravity. The Moon's tug on the Earth acts like an off-axis poking, and, sure enough, the Earth's axis precesses. It makes a circle in the sky that is 47 degrees across, exactly twice the size of the Earth's axial tilt, and that's no coincidence. The amount of the Earth's tilt with respect to the ecliptic, the orbital plane, doesn't change; it's always 23.5 degrees. However, it's the direction in the sky that changes with time.

The effect is slow; it takes about 26,000 years for the Earth's axis to make a single circle. Still, it's measurable. Right now the Earth's northern axis points toward Polaris, the Pole Star, which is how the star got that name in the first place. But it wasn't always pointed that way, nor will it be. As the axis precesses, it points to a different part of the sky. Back in 2600 B.C. or so it pointed toward Thuban, the brightest star in the constellation Draco. In A.D. 14,000 or so it will point near the bright star Vega.

For astronomers, precession is a bit of a headache. To measure positions of astronomical objects, astronomers have mapped out the sky in a grid much like the way cartographers have mapped the surface of the Earth into latitude and longitude. The north and south poles on the sky correspond to those same poles on the Earth, but the sky's north pole moves due to precession. Imagine trying to figure out directions on the Earth using north, south, east, and west if the north pole kept wandering around. You'd need to know just where the north pole was to know in which direction you needed to go.

Astronomers have the same problem on the sky. They must account for the precession of the Earth's axis when they measure an object's position. The change is small enough that most sky maps need to be updated only every 25 to 50 years. This is particularly important for telescopes like the Hubble Space Telescope, which must point with incredible accuracy. If the precession is not included in the calculation of an object's position, the object might not even be in the telescope's field of view.

Precession has an immediate impact on astronomers but a much slower one on the seasons. Right now, the Earth's north axis points toward the Sun in June. But due to precession, 13,000 years from now-half a precession cycle-the Earth's north pole will be pointed away from the Sun in June and toward it in December. Seasons will be reversed relative to our current calendar.

Remember, too, that we are closest to the Sun on our elliptical orbit in January. So half a cycle from now the northern hemisphere of the Earth will experience summer when the Earth is closest to the Sun, amplifying the heat. It'll also be winter when we're farther from the Sun, amplifying the cold. Seasons will be more severe. In the southern hemisphere, the seasons will be even milder than they are now, since they'll have summer when we are farther from the Sun and winter when we are closer.

This works the other way, too: 13,000 years in the past, the seasons were reversed. Summers were hotter and winters were colder in the northern hemisphere. Climatologists have used that fact to show that things might have been profoundly different back then. The slow change in the direction of the Earth's axis might have even been the cause of the Sahara becoming a desert! On a yearby-year basis precession is barely noticeable, but over centuries and millennia even small changes add up. Nature is usually brutal and swift, but it can also display remarkable subtlety. It just depends on your slant.