By Tim Trott, 5th November 2008 in Astronomy

An equinox is the moment in which the plane of Earth's equator passes through the center of the Sun, a time when both hemispheres of the Earth are illuminated equally and day and night are the same length.

Because of the geometry involved, the equinoxes are the only times when the solar terminator is perpendicular to the equator, and at the equator the Sun is exactly overhead at midday. The Sun appears to travel the line of the equator.

In the northern hemisphere, before the Spring Equinox the North pole is tipped away from the Sun, the days are shorter than nights. After the equinox, the north pole is angled toward the Sun, so that days start to become longer than nights, and moving the Northern hemisphere into Spring and Summer. When the Autumnal Equinox occurs, the days start to become shorter and the nights longer. For the Southern hemisphere these effects are reversed.

First Point of Aries

The First Point of Aries is a position in the sky where the Ecliptic and the Celestial Equator cross each another. The First Point of Aries, which is actually in Pisces, defines the zero-point for Right Ascension.

First Point of Aries
First Point of Aries

Due to the effect of precession, the First Point of Aries crossed into the neighbouring constellation of Pisces in about 70 BCE. It has taken about 2,000 years to cross Pisces, and it will cross into the next zodiacal constellation, Aquarius, in about the year 2600. Following its journey along the Ecliptic, it will return to Aries once again in about 23,000 years.

Because the First Point of Aries is relative to the Ecliptic and the Celestial Equator, the First Point of Aries is fixed relative to the Earth. It acts as the zero-point for calculating coordinates on the Celestial Sphere, its own coordinates are always fixed, regardless of its motion. They are, of course, zero hours Right Ascension and zero degrees Declination.

Due to the floating nature of the First point of Aries, coordinates for Right Ascension and Declination need to be adjusted over time to compensate. These adjustments are referred to as Epochs. When referring to celestial coordinates, an Epoch is often quoted along with, for example J2000 referrs to the instant of 12pm (midday) on 1 January 2000, and J1900 refers to the instant of 12pm on 1 January 1900.

Precession of the Equinoxes

We all know that the North Star (or Pole Star) is Polaris, but the true North Celestial Pole is slightly above Polaris and it is moving.

It was Hipparchus who first discovered that the celestial pole was moving, and it was noted in his observations from between 147 BC to 127 BC. Back then the celestial pole was closer to Thurban than it was to Polaris.

Precession of the Equinoxes

The precession of the equinoxes refers to the change of the Earth's rotational axis with respect to the stars in the galaxy. The precession of the equinoxes is caused by the rotational axis of the Earth changing over a period of about 25,765 years, centred around the ecliptic north pole, with an angular radius of about 23.4° the angle known as the obliquity of the ecliptic.

Precession of the Equinoxes over 26,800 year cycle

Currently this annual motion is about 50.3 arcseconds per year (1 degree every 71.6 years). The process is very slow, but cumulative. A complete precession cycle covers a period of approximately 25,765 years, the so called great Platonic year, during which time the equinox covers a full 360°.

Even in the last 50 years the North Celestial Pole has moved, albeit a very small amount.
Even in the last 50 years the North Celestial Pole has moved, albeit a very small amount.

The North Celestial Pole for the year 2000 is marked with a large capital X, while for the year 1950 it is marked with a small x, showing the effect of precession over a 50 year period. The ragged circle of faint stars down and to the left of Polaris is called the "Engagement Ring," with of course Polaris as its shining stone.


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