Bad Astronomy. Misconceptions and Misuses Revealed, from Astrology to the Moon Landing 'Hoax'

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.

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.

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.

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 that page. 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