Precession of the Equinoxes

  • The Earth has three motions, a daily rotation, an annual rotation and the Precession of the Equinoxes, which was discovered by Hipparchus (c.190-120 BCE).
  • The changing position of the Equinoxes along the Plane of the Ecliptic is known as the Precession of the Equinoxes, and makes the Tropical Year 20 minutes shorter than the Sidereal Year.

Why the Precession of the Equinoxes occurs

Ecliptic
  • The Earth takes one year to circle the Sun, and the Sun’s path in the Celestial Sphere is known as the Plane of the Ecliptic.
Celestial Equator
  • The Earth’s Equator when projected onto the Celestial Sphere is known as the Celestial Equator.
Obliquity of the Ecliptic
  • The Plane of the Ecliptic intersects the Plane of the Equator (the Celestial Equator) at 23.5 Degrees and this angle is known as the Obliquity of the Ecliptic and does not change.
The Equinoxes
  • The two points where the planes intersect each other are known as the Vernal Equinox on March 20th-21st, and the Autumnal Equinox on September 21st-22nd.

The Precession of the Equinoxes

  • But what does change is the position where the Equinoxes intersect on the Plane of the Ecliptic.
  • The changing position of the Equinoxes along the Plane of the Ecliptic is known as the Precession of the Equinoxes.
  • As a result the Tropical Year is shorter than the Sidereal Year by 20 minutes.

One complete Revolution every 25,868 years

  • Because the Plane of the Ecliptic and the Plane of the Equator rotate in opposite directions at the rate of 20 minutes a year, they perform one complete revolution every 25,868 years.

Tropical Year

  • The Tropical Year is also known as the Solar Year.
  • This is the time taken for the Sun to return to the same position after the cycle of the Seasons is complete.
  • For example, from the Summer Solstice until the next Summer Solstice.
  • The Tropical Year is 20 minutes shorter than the Sidereal Year.

Sidereal Year

  • This is the time taken for the Earth to complete one full orbit around the Sun, in relation to the fixed stars.
  • The Sidereal Year is 20 minutes longer than the Tropical Year.
  • Because the Plane of the Ecliptic and the Plane of the Equator rotate in opposite directions at the rate of 20 minutes a year, they perform one complete revolution every 25,868 years.

 

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