This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Precession of the Earth

by Dr Jamie Love Creative Commons Licence 1997 - 2011

Throughout this course I have tried to start with the simple and move to the complex. In this lesson you will learn a few things that are more difficult and perhaps appear at first to be at odds with what you've learned so far. Please understand that I am not trying to confuse you but to give you an education that is complete including the exceptions and the details you need to appreciate our universe.

For example, the Earth's axis points to Polaris and you can use that important "landmark" to get your bearings and develop your now considerable knowledge about positions on the Celestial Sphere. During your lifetime, you can always use Polaris as your North Star. However, Polaris wasn't always our North Star and it will not be so in the far future.

What!? Why? How?

The Earth's daily rotation causes our nights and days, and its rotation has been compared to the spinning of a top. [You know, the toy.] That's a fair analogue. Both the Earth and a top continue to spin because they conserve their (angular) momentum. It's this momentum that causes a top to spin long after it has started spinning and it's the same force that will keep the Earth spinning for billions of years into the future. This analogue can be extended to precession as well.

Precession is an added motion given to a spinning object causing it to change the direction of its axis of rotation.

A spinning top experiences a downward force due to gravity and, unless the top has been spun perfectly vertical (very unlikely), it will tend to learn down towards the ground. But the action of the spinning cause its "down" to move around its body so instead of falling over it precesses- it slowly turns its rotational axis around an invisible pivot point and in the process the axis of the top moves in a circle instead of remaining pointed in a fixed direction. The top undergoes many rotations (tops spin many times a second) but the time it takes to complete one precession circle is much longer (usually several seconds). The precession of a spinning top is a very good example of this complex rotational physics. If you have never noticed a top's precession I recommend you get a top, give it a spin and observe what I am talking about.

The Earth's rotation is noticeable - one rotation takes one day (by definition).
The Earth's precession, on the other hand, is very slow. While a top's precession is measured in seconds, the Earth's precession is measured in thousands of years!

It takes 25,800 years for the Earth's precessional motion to return to its starting point. This movement is so slow that you will never notice it, but you should understand that over the centuries the direction of our planet's axis will move away from Polaris and not return for 25,800 years.

The Earth's precession is caused by the gravitational forces of the Sun and Moon on the Earth's "equatorial bulge".

[Note: the Earth is NOT a perfect sphere. Its equator is slightly further from the center of the planet than its poles. This was (and is) caused by the Earth's spin too! Spinning causes a "spinning force" called centripetal force that moves a bit of the Earth's mass outward at the point of maximum spin force - the equator. So the Earth has an "equatorial bulge".]

Let's take a look at the northern sky to see how precession will affect the "North Star" and learn some more constellations!

Here I've labeled some important stars and drawn in some constellations including two new ones.

CEPHEUS, the King, lies immediately to the west of his queen, CASSIOPEIA, who is just out of sight in this image.
(Although one of her bright stars, Chaph, is visible. Besides you know that CASSIOPEIA can always be found by imagining a line from the pointers in the Big Dipper to Polaris and extending the line a similar distance to CASSIOPEIA.)

CEPHEUS is not a particularly bright constellation. His brightest star, Alderamin, is only 2.4 in magnitude. And he is not shaped like a king - more like a house with a peaked roof. Maybe that's supposed to be his castle. The tip of his castle is a dim star between Polaris and Chaph, although slightly closer to Polaris. Just below the peak of the roof is a star nearly as bright as the one at the peak.

The other new constellation is DRACO, the Dragon. He is one of our largest constellations but made of fairly dim stars. To find him, look between the bowl of the Little Dipper and LYRA for a group of stars that form the dragon's small diamond-shaped "head". (If you can only see the three brightest stars, the Dragon's head is triangle-shaped.) Eltamin¸ DRACO's brightest star, is in the dragon's right eye and the slightly dimmer Alwaid is in his left eye. The dragon's long body snakes first northeast and then bends sharply southwest until is seems to settle in a long arc that takes it between the two Dippers until it finally ends near Dubhe. It's easy to lose his body between the Dippers because the only bright star there is Thuban.

Now that you are familiar with the northern sky let's get back to the precession of the Earth's axis.

I've drawn in a few declination and RA lines to give us some reference and I've drawn the precession circle in red with an arrow showing the direction in which the axis moves.

The Earth's precession circle is 47o in diameter meaning the axis will carve out a circle (over the course of 25,800 years) through a large section of the sky.

If you look carefully you will see that the precession circle does not actually touch Polaris. That's not a mistake.

In fact, Polaris has a declination of 89o 15' 15'' (89 degrees, 15 minutes and 15 seconds). That missing fraction of a degree doesn't concern us.

Notice the way the axis changes over time.

In 5,000 years the axis will have swung about a fifth of the way around the circle (that's 5,000 divided by 25,800 - roughly) and Alderamin will be our new "North Star". During the interval (of 5,000 years) we will be hard pressed to find a decent "North Star". So enjoy Polaris as the North Star while you can because it won't be in this position again for 25,800 years!
On the other hand in about 11,000 years the "North Star" will lie between brilliant Vega and less brilliant Eltamin.

Look at the circle in the opposite direction and think about the past.

About 5,000 years ago the Earth's axis pointed to Thuban.
When the Ancient Egyptians built their pyramids they were using this piece of the Dragon to guide their work.

That makes for an interesting point about coordinates.
Any Egyptian buildings constructed to be aligned 5000 years ago with the "North Star", Thuban, will now point to Polaris. In other words, true "north" is defined by the position of the Earth's axis (on the Earth's surface) not the direction in which it points (in the sky).

Precession does not change the position of the Earth's axis on the Earth's surface but precession does change the direction in which the Earth's axis points into the sky. Therefore, precession changes only the star positions with respect to the North Pole. Also, the Earth's orbital plane will not change during the ages so the path of the Sun will continue to go through the Zodiac in much the same way as today. Therefore, the ecliptic is unaffected.

The only thing precession changes are the celestial coordinates.
As the axis swings away from Polaris the declination of the stars will change. Today Polaris is at a declination of (almost) 90o and Alderamin is at a declination of 62o. In 5,000 years, Alderamin will be at a declination of 90o and Polaris will be at a declination of 62o. All the other stars will have a different declination too because declination is a celestial coordinate and those coordinates are based upon the direction in which the Earth's axis points. At any time, now or thousands of years from now, the 90o declination must be over head at the North Pole and the 0o declination must be the celestial equator which is the Earth's Equator projected into the sky.

What about right ascension? Does it change?

Well, that depends on how you measure it.

The RA lines run from celestial pole to celestial pole. While the stars at the poles will change, the orientation of the RA lines will not. They continue to run between the celestial poles regardless of what stars are at the celestial poles.
As long as we agree to keep the 0h RA line at the same point in PISCES, all the RA's will be constant.

BUT, you will recall that astronomers chose to put the 0h RA through the point in PISCES because that's where the ecliptic and the Celestial Equator meet. Specifically, the Sun should cross the Celestial Equator (declination 0o) at the 0h RA line during the Vernal (Spring) Equinox (as viewed in the Northern Hemipshere). As I just explained, the declinations of all the stars will change as the Earth goes through its precession. That means the apparent Celestial Equator (0o) will change - but not the ecliptic! It requires some three dimensional geometry to understand why precession causes the intersection of the (unchanging) ecliptic with the (changing) Celestial Equator to move through the Zodiac. [Sorry, but I just can't draw it! ] Regardless, it's a fact that the point at which these two lines intersect will move through the Zodiac at the same speed as the precession circle of the "North Star". That means the Vernal Equinox moves through the Zodiac at a rate of 2,150 years per Zodiac constellation. (That assumes all these constellations are of equal size, 30o, so all 12 are divided into 25,800 years to give 2,150 years for each constellation of the Zodiac.) Another way to think of that is that the Vernal Equinox moves slightly more than one hour of right ascension every thousand years.

Today the ecliptic and the celestial equator intersect at a point in PISCES, but a couple thousand years ago they intersected in ARIES. That would have been about the time that astronomers (called "astrologers") started to map the night sky. Back then, when the Vernal Equinox occurred, the Sun crossed the Earth's Celestial Equator through a point in ARIES (not PISCES). Astronomers called that point the First Point of Aries. Now, thousands of years later, precession has moved the First Point into PISCES but, for historical reasons, astronomers still call it the "First Point of Aries"! Don't let the name confuse you. The First Point of Aries is in PISCES. Today.

It's easy to get confused here. Is this stuff important? Precession is so slow. Who cares?

Precession is so slow that it will not affect your observations. However, it is important that any astronomy student understand precession because it affects the way we imagine the celestial coordinates. If you use a table of star coordinates (say, to align a telescope in order to find a particularly dim object) you will discover that the table will tell you (somewhere) for what year the coordinates were calculated. Some books will even have tables for several years - for example 1950, 1975, 2000 and 2025. (It's easy to calculate where the First Point will occur because the precession that causes it to move is very precise.) This "confusion" is due to precession and now you know why it is there.

Movement of the First Point will continue as the Earth continues to precess and future astronomers will move their RA lines with it. Maybe they will rename it the "First Point of Pisces". The moving Vernal Equinox against the night sky will continue to occur very slowly over the centuries.
And, of course, as precession swings the North Star point through the night sky, declinations will change as well.

Ancient civilizations (living in the Northern Hemisphere) knew that when the Sun crossed the First Point, summer was on the way, so the First Point held great significance to the Ancients. Some historians are aware of the movement of the First Point and have used it in their ideas about ancient cultures.

For example, about three or four thousand years ago the Vernal Equinox occurred in TAURUS. At that time there was a religious sect in the Middle East that worshipped the power and strength of the bull. Perhaps TAURUS, with its blood-red eye (Aldebaran) and the arrival of spring acted as a "sign" to help this cult to flourish.
Biblical historians point out that the disappearance of the bull worshippers and the rise of Christianity occurred as the Vernal Equinox moved from TAURUS to AIRES. Christ is sometimes associated with lambs, so ARIES the Ram may have helped early Christians to recruit "bull worshippers" when their "god" TAURUS lost its spring influences to the new "god" ARIES. It's an interesting idea.

Of course, all of the celestial calendar changes with the precession, not just the Vernal Equinox.
Three months before the Vernal Equinox the shortest day of the year occurs - the Winter Solstice. Three or four thousand years ago the Sun would have been in LEO during the Winter Solstice. (Note that LEO is a big constellation so the years need not line up too much in order for this to make sense.) Some Egyptologists (historians of Ancient Egypt) wonder if the Sphinx, the half-man half-lion monument that sits within sight of the ancient pyramids, was built to celebrate the end of winter and the arrival of longer days. The Sphinx faces east (towards the Nile). Three or four thousand years ago the sunrise on the shortest day of the year would have occurred between the paws of the Sphinx. (Roughly.) At the same time the Sun would have been in LEO (obviously) and very close to its brightest star Regulus, which happens to be in LEO's paw! Hmmm.

In about a thousand years the First Point will move into AQUARIUS. Back in the 1970's there was a popular song with the lyrics "This is the dawning of the Age of Aquarius". That dawn is a thousand years away, so those singers will have to be patient! But it does illustrate how even today the First Point makes an impression (especially among "hippies" ).

You have learned only two new constellations this month but they are important ones. DRACO is a very large constellation and it's visible all year round (unless you live VERY far south). CEPHEUS is also visible each night and delta-Cephei is an important variable star whose magnitude you can actually "measure" yourself. (You'll read about that in your next lesson.)

This image shows the view overhead as seen from the North Pole. (Here's a reverse color image that will be easier on your printer.) Notice that Polaris is right in the center. I haven't copied the entire sky, just the view from 90o to 45o, so the edge of the circle is the 45o declination line.
This is different from other views you have seen in this course, but it serves well to illustrate the stars of the northern sky. By now you should be able to identify these stars and constellations. (There are other small constellations that fill in some of the gaps but we won't discuss them.)
If you live north of the 45o latitude all of these stars can be seen any clear night. They are said to be "circumpolar".

You'll recall that a circumpolar star goes around the sky (around Polaris) and never goes below the horizon. You can determine the circumpolar stars for your home by subtracting your latitude from 90o. Any stars with a declination higher than the number you calculated will be circumpolar.
For example, Chicago is at latitude 42o (north). Subtracting that from 90o leaves 48o. Therefore, any star with a declination of +48o or more will be a circumpolar star from Chicago. Also, any star with a declination less than -48o will never rise above the Chicago horizon so you will never see it from Chicago.

Speaking of the far south, next month you will learn about those bright stars to the south of the Zodiac and I will complete your observational astronomy lessons with a brief tour of the Southern Celestial Hemisphere. You may never see the southern stars but any astronomy student should be aware of them because there is some wonderful astronomy going on "down under".

This would be a good time to take a break and study your starmap of the northern sky.
However, if you are keen, you can continue on to your next lesson where you will learn how astronomers determine distances in space.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.