The Earth's Orbit
The shape and orientation of the Earth's orbit around the Sun are also changing, under the influence of the other planets.
The Orbital Eccentricity
The eccentricity of the Earth's orbital ellipse is currently decreasing from a peak of about 0.02 some 14,000 years ago toward a low of about 0.003 some 25,000 years from now (Figure 10). It passes through cycles of varying amplitudes with a period of about 95,000 years. During the ice ages, the peak eccentricity reached 0.05-0.06. At that time, the difference between perihelion and aphelion distances was about 16 million km.
Figure 10. The shrinking eccentricity of the Earth's orbit. Illustration courtesy of author.
The Longitude of Perihelion
The perihelion and aphelion are precessing, advancing eastward along the ecliptic about 11 arcseconds per year in relation to the fixed stars. This is the difference in arcseconds between the sidereal year and the anomalistic year. At this rate, they will take about 114,000 years to make a complete revolution.
The perihelion is currently in the middle of Sagittarius. In the Pyramid Age, it was near the western edge of Sagittarius, its border with Ophiuchus (Figure 11). It and the Spring equinox, advancing westward about 50 arcseconds per year, move about 1 arcminute per year in relation to each other. At this rate, the time between coincidences of the perihelion and equinox is about 21,000 years. This interval, rather than the sidereal movement of the perihelion, is sometimes called the precession of the perihelion. It has been confused with the precession of the equinoxes. The perihelion and Spring equinox last coincided about 16,500 years ago, almost on the border of Ophiuchus and Scorpius. They will next coincide in about 4,500 years in Sagittarius, near the border of Capricornus (see Figure 12).
Figure 11. The precession of the perihelion of the Earth's orbit, 2500 BC to AD 2000. Illustration courtesy of author.
The change in eccentricity, plus the changing relation between the perihelion and aphelion of the Earth's orbit and the equinoxes and solstices, change the duration of the seasons. Today, Summer is the longest season and Winter the shortest. In the Pyramid Age, Spring was the longest and Autumn the shortest.
Figure 12. The combined precessions of the equinoxes and perihelion. Illustration courtesy of author.
Precession in Human History
None of the changes in the Earth's motions that we have considered are apparent to the unaided eye. Even a lifetime of observing would not necessarily reveal them. But some astronomical positions and events preserved in "social memory" -- either oral teaching or documents -- for several generations may provide clues to the astute observer that something in the heavens is changing. (Then it may have been necessary to overcome human inertia and ideology to convince others of the changes.)
The most rapid change is lunisolar precession, which affects the positions of the stars and the precession of the equinoxes. This change can be detected, over a century or two, in several ways: stars moving closer to or farther from the poles, stars rising farther north or south, and stars rising heliacally (just before the Sun) or culminating around sunset later in the year.
To observers as far south as the Nile delta, the seven main stars of Ursa Major were fully circumpolar in the northern sky from the 4th millennium BC to the birth of Christ. However, in the 3rd millennium, they began to move away from the NCP. They also rotated, the eastern end today's Big Dipper moving south faster than the western. Alkaid, the star at the Dipper's eastern tip, moved south most rapidly. This change should have been detectable to observers armed with records going back several generations. A document from ancient Egypt suggests that it was. The Book of Day and Night (12th century BC) speaks of binding the Leg (their name for Ursa Major) to mooring posts, as though seeking to keep it from moving farther from the NCP.
One of the brightest stars to change its position significantly at rising was Arcturus (Alpha Boötes). In the 2000 years following the Pyramid Age, it shifted southward along the horizon about 0.7° in azimuth per century. Natural or artificial alignments used to mark its rising would gradually have become useless.
The equinoxes and solstices shift even more rapidly -- about 1.4° per century. Since these events were observed by people responsible for the calendar, their changes were likely to be noticed. The ancient Chinese apparently keyed their earliest calendars to the culmination of certain stars such as Antares at sunset on these dates. Because of precession, they found it necessary to revise this system every few centuries.
Hipparchus of Nicaea (2nd century BC) is the first known astronomer to have made careful observations and compared them with those of earlier astronomers to conclude that the fixed stars appear to be moving slowly in the same general direction as the Sun. Confirmed by Ptolemy (2nd century AD), this understanding became common in medieval Europe and the Near East, although a few astronomers believed that the motion periodically reversed itself. The Chinese astronomer Yü Hsi (4th century AD) was the first known in east Asia to take official note of precession.
The Motions of the Earth and the Climate
Many climatologists have looked to the changing motions of the Earth to explain the recurring ice ages of the Pleistocene geological epoch, which began an estimated 1.7 million years ago. The prevailing theory holds that the changing obliquity of the ecliptic (41,000-year cycle) and a combination of the changing eccentricity of the Earth's orbit (about 100,000-year cycle) and the relative movement of the perihelion and the spring equinox (complex cycle, averaging about 21,000 years) account for the cycles of glaciation. These changes affect the seasonal and latitudinal distribution of sunlight on the Earth's surface, and this presumably causes the ice ages, although the mechanism is not fully understood. The 100,000-year cycle of glaciation has been difficult to relate to the eccentricity cycle.
A competing theory, recently revived and redefined, holds that a major climatic factor in the last one million years is the changing inclination of the Earth's orbit with respect to the invariable plane (a 100,000-year cycle). Around maximum, this inclination is thought to dip the Earth's orbit low enough that a dust cloud between the Earth and the Sun reduces the amount of sunlight reaching the Earth and triggers a glacial cycle.
Our knowledge of the Earth's motions has come far from the theology of medieval and early modern Europe, which took literally Psalm 104:5, and cited it against Galileo.
The Earth does move, and its motions vary in ways that the early Copernicans could not have predicted or detected. The changes are responsible for changes in the length of the day and the year, and for changes in the relative durations of the seasons. They may be responsible for the ice ages. Knowledge of these motions and their changes is useful not only to astronomers but also to historians and climatologists-indeed, to any profession concerned with the changing behavior of the Earth and with records of it in history and geology.
Donald V. Etz is a retired technical writer and a non-retired amateur astronomer and historian who lives in Dayton, Ohio. He gives lectures on astronomical subjects to his local astronomical society and other organizations and has published a couple of articles on these subjects. His particular astronomical interests are the history of astronomy and the behavior of the Solar System. His particular non-astronomical interests are his grandchildren and church activities. His email address is firstname.lastname@example.org.
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