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Equinox precession

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The equinoxes happen when the sun's rays shine directly on the equator, the imaginary line around the middle of the earth. In this picture, the dotted lines show where the sun's rays hit the earth directly at different times of the year.

Equinox precession is the observable phenomenon that every year on the morning of the spring equinox (March 21, one of two days in the year that the day and night are of equal duration), the sun appears to rise at a slightly different place relative to the background stars than it did the year before.

Currently, the sun rises in the constellation of Pisces on the morning of the spring equinox. Prior to about 200 BC, it rose in the constellation of Aries. After 2015, it will rise in the constellation of Aquarius.

It takes 24,000 years for the sun to make a complete cycle through the stars, and rise in exactly the same place twice.

Equinox precession was well-known to the ancients. Plato described the 24,000 year cycle as the "Great Year." Ancient Indian and Mesoamerican calender-makers were also aware of this phenomenon.

Important Definitions

A few definitions necessary to understand precession:

  • The Tropical year is the time it takes from spring equinox to spring equinox;
  • The Sidereal year is the time it takes for the sun to realign with the background stars;

If the sun were immobile and the Earth were not wobbling, then the tropical year and sidereal year would be identical every year: Each spring equinox, the sun would line up in exactly the same way with the background stars as it did the year before.

But this is not the case. Every year, the tropical year is approximately 20 minutes shorter than the sidereal year. In other words, every year, the spring equinox occurs twenty minutes before the sun aligns itself with the background stars. Or to say it a third way, every year on the spring equinox, the sun is "running behind" the background stars by 20 minutes.

The result is that every spring equinox, the sun appears to have "moved" 50.29 arc-seconds from where it appeared on the spring equinox the year before.

Precession Models

There are two theories to explain equinox precession.

  • The Lunisolar model holds that the axis of the Earth is wobbling slowly;
  • The Binary model holds that the Earth is not wobbling significantly, but instead that precession is caused by the entire solar system moving in a curved line through space;

In the Lunisolar model, the Earth travels 360 degrees around the sun in a Sidereal year, but the Tropical year is shorter because the Earth's axis moves a little each year, making the equinox come before the Earth has made a full 360 degree turn. This has been the scientific consensus since Copernicus.

In the Binary model, the Earth travels 360 degrees around the sun in a Tropical year, but the Sidereal year is longer because the Sun is moving in a curve, so that the sun is in a different place every year relative to the background stars, and the Earth must travel more than 360 degrees before the sun lines up with the background stars. This was the view of ancient Indians as reported in the 19th century by Sri Yukteswar.

Evaluating the Models

The key issue is to determine which is the "True Year," during which the Earth passes 360 degrees around the sun. Is the tropical year the true year because the Earth's axis does not move but the sun moves, or is the sidereal year the true year because the Earth's axis moves but the sun does not?

Lunar calculations

The most reliable way to determine which is the "True year" is through lunar calculations.

The Synodic month (the period of the phases of the moon) and Tropical month (the period from one lunar equinox to the next) are different. This difference is caused by the motion of the Earth relative to the sun. Every time the moon revolves around the Earth, the Earth has moved a bit too. Consequently, the angle from Sun to Moon to Earth is different than it was the last time the moon was at that location in our sky. The phases of the moon are determined by by the angle of the sun to the moon to the Earth. Consequently the moon is at a different phase because of the changed sun-moon-Earth angle, even though it is at the same location in the sky.

The difference between the synodic and tropical months is fairly significant. The tropical (moon relative to background stars) month is 27.321582 days long, and the synodic (moon phase) month is 29.530589 days long. Because of this difference in periods, the moon passes the same place relative to the background stars (tropical month) 13 times every year, but returns to the same shape (synodic month) only 12 times every year.

It is possible to determine how long it takes for the Earth to make one 360 degree orbit around the sun by determining how long it takes for the difference between these two to add up to one full revolution of the moon, corresponding to the revolution of the Earth during that year. In other words, when the moon has gone through X synodic months and X+1 tropical months, we know that the Earth has made one full revolution around the sun to cause there to be one more tropical month than synodic month during the year.

According to the 2002 Astronomical data almanac, [1] this takes place after the moon has gone through 12.368266 synodic periods, and 13.368266 tropical periods. 12.368266 synodic months of 29.530589 days each totals 365.2421 days.

The tropical year is 365.2421 days. The sidereal year is 365.2425 days. The revolution of the moon corresponds to the tropical year, and not the sidereal year.

Therefore, the tropical year is the true year, and the sidereal year is not.

This means that equinox precession is not caused by the Earth's axis moving and causing the tropical year to be shorter than the true year, but that equinox precession is caused by the curved motion of the sun causing the sidereal year to be longer than the true year.

This unambiguously proves that the sun is moving in a curve relative to the background stars.

Objects within the solar system

The locations of objects outside the solar system are calculated with respect to the sidereal year, but the locations of objects inside the solar system are calculated with respect to the tropical year.

If the sidereal year were the true year, calculations for both objects inside and outside the solar system would be made relative to it. But that is not the case. Instead, the locations of objects outside the solar system are calculated with respect to the sidereal year, and objects inside the solar system are calculated with respect to the tropical year.

The obvious implication is that the difference between the tropical and sidereal years is a result of the motion of the solar system.

Angular momentum

The sun contains 99% of the mass in the solar system, but only 1% of the angular momentum, if the sun is not moving. This is a major problem with the lunisolar model, because the proportion of angular momentum to mass should be constant among objects in the system. The binary model solves this problem. If the sun is moving at the proper speed and angle to provide for the 24,000 year cycle rather than being stationary, then its ratio of angular momentum to mass is right on target.


Implications include:

  • The existence of some force or massive stellar object nearby causing the sun to move in a curve;
  • Some unknown mechanism, naturalistic or designed, which causes the sun to move in a 24,000 year cycle
  • Some explanation for how the ancients became aware of these motions without the trappings of "modern science;"
  • Some explanation for why scientists today are unable or unwilling to accept the demonstrable fact that the sun is moving, preferring a lunisolar model of precession which has been unambiguously falsified;

Related References