As we’ve discussed the seasons, you will have noticed that we’ve said several times that Earth’s axis remains pointed toward the Polaris (the North Star) “throughout the year.” Perhaps you’ve wondered why we haven’t said that it “always” stays pointed in the same direction, and you are now ready to learn the reason: because it doesn’t.
Instead, Earth’s axis very gradually changes its orientation over time. The reason is something called precession , which might be a new word for you, but is something you’ve probably seen before with spinning tops (Figure 2.21a). If you watch carefully, you’ll notice that as a top spins rapidly, its axis also sweeps out a circle at a slower rate, at least until the top falls over. That slow sweep (sometimes called a “wobble”) is the precession of the top’s axis.
Activity
Precession of a Top
Find a top or a jack that you can spin. Make it spin as fast as you can, and then watch it until it falls over. Can you see the motion that we call precession? Explain.
Teacher Notes. This is a simple activity to make sure students can actually see precession with a top or jack.
Earth’s axis precesses in much the same way, but far more slowly (Figure 2.21b) and, unlike a top, it never falls over. Each cycle of Earth’s precession takes about 26,000 years and gradually changes the direction in which the axis points in space. For example, Figure 2.21b shows that while the axis points nearly toward Polaris today, making it our North Star, in about 13,000 years the axis will point nearly toward the star Vega, so Vega will be our North Star at that time. In other words, Polaris is only a temporary North Star, though in this case "temporary" means several centuries. If you study Figure 2.21b, you will see that at most times, there is no bright star near the north celestial pole (meaning the direction in which Earth’s axis points), just as there is no bright star near the south celestial pole today.
I was wondering...
What causes precession?
Precession is caused by the way gravity affects tilted, rotating objects that are not perfectly round. It does not occur with perfectly round balls. So it is the fact that a top (or jack) is not round, along with the tug of Earth’s gravity, that makes it precess when spinning. Earth’s axis precesses for similar reasons. Earth is not perfectly round; instead, it bulges around the equator, and the equator is tilted along with Earth’s axis. The gravity of the Sun and Moon therefore tug on the equatorial bulge, causing Earth to precess. Earth never “falls over” like a top because there is no friction in space.
Teacher Notes. A more detailed explanation would get into conservation of angular momentum, but that is beyond the scope of middle school science. If you want your students to learn more about angular momentum and precession, you can get an inexpensive toy gyroscope, which should have instructions for activities you can do to learn from it.
Because changes in the orientation of Earth’s axis also mean changes in the orientation of Earth’s equator (because the equator is always 90° away from the poles), precession gradually shifts the positions in Earth’s orbit where the equinoxes and solstices occur. (On the celestial sphere, this corresponds to shifting the points where the ecliptic and celestial equator cross.) If you look back at Figure 2.16, you’ll see that this therefore changes the constellations in which the Sun appears to be at those times. For example (as discussed in the box “I was wondering… What’s my sign?”), a couple thousand years ago the March equinox occurred when the Sun appeared in the constellation Aries, even though it now occurs when the Sun appears in Pisces.
This also explains an interesting fact about the naming of the Tropics, which you can see on any world map:
- If you look back at Figure 2.16, you’ll see that, today, the June solstice occurs when the Sun appears to be in Gemini and the December solstice occurs when the Sun appears to be in Sagittarius.
- But on a map or globe you’ll see that the latitude at which the Sun is directly overhead on the June solstice (23½°N) is called the Tropic of Cancer, and the latitude at which the Sun is directly overhead on the December solstice (23½°S) is called the Tropic of Capricorn.
- The explanation is that back when the Tropics got their names, the Sun did appear to be in Cancer on the June solstice and in Capricorn on the December solstice. But that was a couple thousand years ago, and precession has since shifted the locations of the solstices to their current positions.
Activity
Effects of Precession on the Equinoxes and Solstices
Step 1: Place an object to represent the Sun in the middle of your classroom.
Step 2: Draw each constellation of the zodiac on a large piece of paper, and then place them in the correct order (matching Figure 2.16) around the perimeter of your classroom.
Step 3: Now get a globe with the correct axis tilt, and put it in the position around your Sun that correctly corresponds to the June solstice today; that is, so that the Sun appears to be in Gemini as seen from Earth at the June solstice position.
Step 4: Keep the axis still pointing in the same way (toward Polaris), and identify the constellations that go with where the Sun appears to be on the equinoxes and the December solstice.
Step 5: Return to the June solstice position, and while staying in that place, change the orientation of your globe to represent the way it precesses over 26,000 years. Then orient the globe approximately as it was about 2,000 years ago, and walk over to the new position of the June solstice. If you went the correct direction, you should now see that the Sun appears to be in Cancer at this position, as it was for the June solstice when the Tropic of Cancer got its name. You can then continue to explore how precession shifts the constellations in which the Sun appears at different seasons.
Teacher Notes. This is an optional and fairly complex activity, but can be fun if students really want to understand how precession has affected zodiac “signs” and the naming of the Tropics, and other related topics. Two more examples that some students may find of interest:
- The March equinox now occurs when the Sun appears to be in Pisces, but it is often called the “first point of Aries” because that is the constellation that the Sun appeared in on the March equinox a couple thousand years ago
- if you consider the above fact and then look at Figure 2.16, you’ll see that at some point roughly about a thousand years from now, the Sun will appear to be in Aquarius on the March equinox. This is what is sometimes called “the Age of Aquarius.” The exact time at which it begins depends on whether you used the official astronomical borders of the constellations or more traditional borders used by astrologers. Note: If you cover this, be sure to note how silly it is to ascribe great meaning to something that is simply a visual effect (remember that the Sun is never actually “in” a constellation, it just looks that way from our perspective on Earth) resulting from Earth’s axis precession.
Precession also has other effects besides just changing the way we view the sky from Earth. In particular, the 26,000-year cycle of precession is one of three subtle cycles of Earth’s rotation and orbit that scientists think are tied to long-term cycles of ice ages and warm periods on Earth. Later in this book, we will discuss those cycles – and how and why today’s global warming is different from the natural cycles of climate change that result from precession and other similar factors.
Discussion
Precession and Climate
As we’ve discussed, precession by itself does not affect the amount of axis tilt (23½°). So how can precession affect Earth’s climate over its 26,000-year cycle? Think about all you’ve learned, and hypothesize about possible answers.
Teacher Notes. This discussion will help students “think like a scientist” in coming up with a viable hypothesis. If they are having difficulty, you can help guide them to a strong hypothesis as follows.
- To begin, be sure that students understand why simply changing the orientation with precession would not be expected to have any effect on the seasons if Earth’s orbit were a perfect circle.
- Then remind them of what we discussed earlier about how we might expect Earth’s varying distance from the Sun to affect the seasons, even though we saw that the distribution of land/oceans turned out to be the stronger effect (discussed in the subsection on why the seasons are milder in the Southern Hemisphere).
- Now they should realize that, for example, in 13,000 years the June solstice will be occurring when Earth is near its closest orbital point to the Sun, rather than the farthest as it does today. Discussing the effects of this should help students realize why that would lead the Northern Hemisphere seasons to become even more extreme at that time.
- You could also discuss how “more extreme” seasons might tie in with Ice Ages, though the complete answer is quite complex since there are other orbital/rotational cycles that are also playing a role (together, these are the so-called Milankovitch cycles). But it should be clear that more extreme seasons (warmer summers and colder winters) will have at least some climate effects.