The Universe in the Classroom

This Restless Globe

Precession of the Ecliptic

The relation of the celestial equator and poles to the ecliptic and its poles is complicated by the fact that the ecliptic is not stationary. The gravitational force of the Solar System planets, especially Jupiter and Venus, causes the ecliptic to precess, wobbling as its poles describe an arc counterclockwise in space, around the poles of the invariable plane. The ecliptic is like a second wobbly plate atop the same juggler's pole. However, it wobbles much less -- it precesses much more slowly than the celestial equator, and its poles describe a much smaller arc (see Figure 8).

This motion is called planetary precession. It tends to shift the ecliptic westward along the celestial equator, counteracting a small part of the effect of lunisolar precession. The path of the ecliptic poles, like that of the celestial poles, is not a circle in space but a loop or spiral.

The Combined Effects

Figure 8. The precession of the ecliptic north pole through the background of stars. Illustration courtesy of author.

The effects of these changes, in combination, are general precession, the observed precession of the equinoxes westward along the ecliptic, and a change in the obliquity of the ecliptic (its inclination in relation to the celestial equator) and its inclination in relation to the invariable plane.

The Precession of the Equinoxes

Today the equinoxes are in Pisces and Virgo, and the celestial poles in Ursa Minor and Octans. In the Pyramid Age, the equinoxes were in Taurus, not far from the border with Aries, and Scorpius, along the border with Libra (Figure 9). Because the equinoxes precess, moving westward along the ecliptic opposite the apparent eastward progress of the Sun during the year, they advance to meet the Sun. The Sun returns to an equinoctial point in less time than it takes to return to a fixed star. This gives us two different astronomical measures for the year: the return of the Sun to the Spring equinoctial point (tropical year, about 365.2422 days), and its return to the longitude of a fixed star (sidereal year, about 365.2564 days). For most human affairs, we are more interested in the tropical year, because it is in step with the seasons. Neither astronomical year consists of a whole number of days, but human calendars do. Hence, calendar-makers have struggled for several thousand years to determine exactly how long the year is, and to devise systems for allocating the fractional day.

The rate of general precession is currently about 50.29 arcseconds per year. At this rate, the equinoxes would make one revolution in about 25,770 years. However, the rate has been increasing slightly, decreasing the tropical year even more.

Figure 9. The precession of the equinoxes, 2500 BC to AD 2000. Illustration courtesy of author.

A Decrease in Obliquity

Planetary gravitational influences are also the primary factor in changing the obliquity of the ecliptic. Today, it is about 23.44°, as mentioned. In the Pyramid Age, it was about 24.02°. It has been decreasing from a peak of about 24° some 8,000 years ago toward a low of about 22° some 13,000 years hence. The obliquity goes through cycles of varying amplitudes with a period of about 41,000 years. The rate varies-currently it is about 0.47 arcseconds per year.

A Decrease in Inclination

The inclination of the ecliptic in relation to the invariable plane also goes through cycles of varying amplitude, with a period of about 100,000 years. It is currently 1.58°, as noted, and decreasing. Its last maximum, about 30,000 years ago, was about 2.6°, and it is expected to decrease to a minimum of about 0.8° in about 20,000 years. The greater the inclination of the ecliptic, the greater the extremes of the Earth's travels north and south in relation to the Sun.

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