As the Earth Turns

I am sure you are aware of the two basic movements of the Earth.

Axis

One of them is the Earth's 365 1/4 day movement around the Sun -- the Earth's orbit. The Earth's orbit marks out a year in time, an Earth-year. (Jupiter in comparison takes almost 12 Earth-years to move in its orbit around the Sun. Nevertheless Jupiter does take one Jupiter-year to move around the Sun. When talking about years in the solar system it is best to distinguish between different kinds of years.)

Earth also spins on its axis. One complete turn about an axis is termed one day. It takes about 24 hours for Earth to spin on its axis.

The tilt of the Earth's axis (23.5 degrees) creates the seasons, with its varying length of days. The amount of sunlight any region gets in a day is determined by where the Earth is positioned in its orbit.

Some of you may be aware of a thing called the precession of the equinoxes. Precession of the equinoxes is actually the movement of the Earth's axis. A complete circuit takes about 25780 years. It is caused by the Sun and Moon pulling at the bulges at the equator. The main thing precession of the equinoxes does is change where due north (or due south) is on the celestial sphere. Currently the star Polaris, more commonly called the North Star, is (very nearly, 9/10 of a degree) at the north celestial pole. Most of the time during a precession cycle there is no star at north or south celestial pole. However, 3000 years ago, the star Kochab (Beta Ursa Minor), was at the north celestial pole.

In addition, in the precession there is a wobble. This wobble, called nutation, is a tiny wave that repeats itself every 19 years. Nutation is caused by changes in the Moon's pull on the Earth. Changes in the Moon's pull on the Earth are caused by the Moon being closer or farther away in its orbit.

Another movement in the Earth axis is a varying in the tilt of the Earth's axis. The tilt changes in a cycle lasting 41,000 years. In 1900 the tilt was 23.45229 degrees; in August 1977 the tilt was 23.44229 degrees; in the year 2000 the tilt will be 23.43928 degrees.

The Earth's poles also move. The Chandler wobble is a roughly circular wobble of 10 meters, that is completed in 430 days. This wobble is caused by shifts in the distribution of matter in the Earth. What causes these shifts? Earthquakes! The larger the earthquake the larger the wobble.

Another wobble, just discovered in the last decade or so, is a very tiny wobble is the Earth's axis. This wobble moves the axis in a small circle anywhere from two weeks to a couple of months and varies in size from 2 1/2 inches to 2 feet. This wobble is caused by shifts in the distribution of matter on the surface of the Earth. These shifts are caused by the winds in the atmosphere, storms, and varying snow cover. Isn't it incredible that scientists can detect a wobble that is caused by varying snow cover? Currently lasers that bounce off the Moon can determine the Earth's position to within an inch, making detection of these tiny wobbles possible.

Orbit

The Earth's orbit is not a perfect circle. It can't be. All stable orbits are ellipses. At it's closest point the Earth is 5,020,000 km or 3.3% closer to the Sun then when it is furthest away. Really that is not much -- on a map with Earth's orbit you would be hard pressed to see it as an ellipse. Yet it does make a difference. For one thing, the Earth receives 7% more heat when it is closest. For another thing, this orbit changes in slight ways.

One slight way the Earth's orbit changes is the movement of Earth's perihelion. Perihelion means the closest point in an orbit; aphelion means the furthest point in an orbit. The Earth's perihelion moves in a complete circle about the Sun in 21,310 years. Because of this the Earth's orbit can be visualized as a perpetual spiral, never quite visiting the same place twice in space.

The Earth's eccentricity also wobbles in a cycle of 92400 years. The eccentricity is a measure of how far an orbit is off a circular shape. Currently the Earth's orbital eccentricity is 0.016, which causes the Earth's closest point to be 5,020,000 km closer as said above. The eccentricity is decreasing at a rate of approximately 0.0004 per century. In total, the eccentricity varies from 0.0033 to 0.0211. That translates into the Earth's perihelion being 990,000 to 6,310,000 km closer than aphelion.

The wobble in the Earth's eccentricity has an interesting effect on far future total solar eclipses. The Earth and the Moon is practically a double planet. The Moon doesn't really move around the Earth, it is too far away. Rather the Moon moves around the Sun. Because of this tidal braking from the Earth tends to slow down the Moon and make it move farther and farther from the Earth every year. Since the Moon is getting more distance from the Earth every year, the Moon will gradually appear smaller in Earth's sky. Eventually the Moon will look so small from the Earth, the Moon will no longer cover the Sun during an eclipse. There will come a time when the Earth will no longer see total solar eclipses -- just annular and partial ones. Before the time comes, the total solar eclipses will tend to become rarer and rarer.

However, because of changes in the Earth's eccentricity, the probability of having a total solar eclipse will not gradually decrease evenly. In fact there may be periods of 70,000 years when total eclipses are impossible and then for the next 20,000 years or so, a few eclipses will be possible and then eclipses will impossible for the next 70,000 years. The very last total solar eclipse will occur when the Earth is very close to its' maximum orbital eccentricity, the Moon's maximum perihelion, and the Earth's maximum aphelion from the Sun some time a few 100 million years in the future.

Another effect of the cyclical changes in the Earth's eccentricity and movement of the perihelion are ice age cycles.

The Earth's moves in a lot of ways doesn't it? This is not a complete list and doesn't include all the movements that the solar system makes as a whole. For example, I have not included Earth's movement around the galaxy or in galaxy clusters and so on. The list also doesn't include even smaller effects such as changes in the wobble of the Earth's eccentricity. Over a long period of time many factors will become unpredictable; chaotic. We live in a chaotic solar system -- eventually the orbits of the other planets, moons, asteroids, and comets will have inherently unpredictable effects on our home planet. I doubt that the orbits of the gas giants will be the same in a billion years. Eventually we will slam into a brick wall of unpredictability if we try to go too far in predicting the future.

Yet isn't it amazing what we can know?

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