![]() In addition the tilt, or obliquity, of the Earth's axis is not constant but changes in a cycle of its own. In 25,772 years, the points are once again at the same point in the sky where observations began. The resultant motion of the Earth's axis is called general precession and the equinox points in the ecliptic move westward along the ecliptic at the rate of about 50.3 seconds of arc per year as a result. ![]() A similar conical motion can also be observed in a gyroscope that is subjected to lateral forces. Ī slow conical motion of the Earth's polar axis about its normal to the plane of the ecliptic is caused by the attractive force of the other heavenly bodies on the equatorial protuberance of the Earth. The alignment of the axis is maintained throughout the year so that the point of sky above the north or south poles remains unchanged throughout the Earth's annual rotation around the Sun. The Earth's axis of rotation is not set perpendicular to this plane but at a present angle of 23.5 degrees to the perpendicular. It is the Earth's orbital motion about the Sun which causes this apparent motion to occur. The plane of the ecliptic is the plane described by the apparent motion of the Sun against the starry background. The data in red show the polar point in past epochs and the constellation the vernal equinox was in at that time. The path of the North Pole projected on the sky is a circle which takes 25,772 years to complete. Almost any period can be found sometime or somewhere honored with this name. The difficulty with the term "great year" lies in its ambiguity. ![]() He called this time period the Great Year and suggested that such a unified return would take place about every 36,000 years.īy extension, the term "Great Year" can be used for any concept of eternal return in the world's mythologies or philosophies. Plato hypothesized that winding the orbital motions of the Sun, Moon and naked eye planets forward or back in time would arrive at a point where they are in the same positions as they are today. The Platonic Year, which is also called the Great Year, has a different more ancient and mystical meaning. But as the direction of the axis is changing, this is a passing coincidence which was not always so and will not be so again until a Great Year has passed. The position of the Earth's axis in the northern night sky currently almost aligns with the star Polaris, the North Star. In the heliocentric model, the precession can be pictured as the axis of the Earth's rotation making a slow revolution around the normal to the plane of the ecliptic. Thus the time it would take the equinox to make a complete revolution through all the zodiac constellations and return to its original position would be approximately 25,920 years. Ptolemy reported that his teacher Hipparchus, by comparing the position of the vernal equinox against the fixed stars in his time and in earlier observations, discovered that it shifts westward approximately one degree every 72 years. It is defined by scientific astronomy as "The period of one complete cycle of the equinoxes around the ecliptic, or about 25,800 years". The term Great Year has more than one major meaning. The tilt of the Earth's polar axis remains constant but describes a circular path in space during a period known as The Great Year. ![]()
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