The Precession Of Perihelion

Refer back now to Figure A-3. This diagram is a slanted view to allow the orientation of the Earth's spin axis to be clear, but even if it were drawn looking straight down from above the very low eccentricity would make the orbit's deviation from a circle difficult to identify. Note that a small cross is drawn on the terrestrial orbit. This cross indicates the position of the Earth at perihelion, equivalent to a date around January 4 in the current epoch, soon after the winter solstice on December 22. The perihelion point is slowly moving, however, owing to tugs imposed by the other planets, and this motion is called precession of perihelion.

The date of perihelion moves later by about one day every 60 years, so that 4,500 years into the future it will align with the spring equinox. About 750 years ago, perihelion and the winter solstice coincided. A full rotation of the perihelion point around the orbit takes about 21,000 years. These gradual alterations in relative alignment affect our climate, and are thought to be one of the causes of the Ice Ages. To demonstrate clearly what is meant by precession of perihelion, Figure A-4 depicts an imaginary precess-ing orbit with a large eccentricity. In the four-and-a-half orbits

FIGURE A-4. Under the influence of various gravitational perturbations successive orbits precess (swivel around in their orientation) compared to the fixed stars. For clarity a highly eccentric (meaning noncircular) orbit is shown here. Both the perihelion point q and the aphelion point Q move counterclockwise from one orbit to the next in this diagram. Similarly both the terrestrial orbit about the Sun and the Moon's orbit about the Earth undergo precession in the counterclockwise direction.

displayed the perihelion point (labeled q) has turned through about 45 degrees, this movement being more obvious in the case of the aphelion point (Q).

The time taken for the Earth to return to perihelion, termed the anomalistic year, is 365.2596 days, almost one-hundredth of a day longer than 365 and a quarter. This might be considered the period to complete an orbit, but there are problems. If that year were used to design a calendar, then because it is longer than 365.25 days one would need not only a quadrennial leap year, but also an additional day every century, maybe a super-leap year with 367 days. If such a calendar were employed, predicated upon keeping the date of perihelion constant, then the dates of the equinoxes and the solstices would progressively move earlier in the year, and that would not do.

A better definition of the time taken to complete an orbit might be how long it takes the Earth to execute a 360-degree arc around the Sun. Because perihelion is moving counterclockwise, the Earth must traverse a little more than 360 degrees to reach it again. If one instead asked that the stars return to their previous positions in the sky, then the planet will have circuited through precisely 360 degrees, occupying a length of time called the sidereal year, which lasts for 365.2564 days. Again this is not really the sort of year wanted for setting up a calendar because the stars do not affect such things as our climate and seasons. The fundamental reference points we use are the equinoxes and solstices, but again those are not stationary, as we will see below.

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