Equinoxes and Solstices

The Earth’s orbital plane is known as the ecliptic plane.  The Earth’s revolution about the Sun makes the Sun appear to move eastward along the ecliptic about 1 degree per day. The circle labeled ecliptic in Figure 5.1 shows the Sun’s yearly path through the zodiacal constellations. The ecliptic is actually the Earth’s orbital plane extended out to the sky. The ecliptic is inclined to the celestial equator by 23.5 degrees because the Earth’s equator is tipped 23.5 degrees relative to its orbital plane.
 
 

Notice that there are four labeled points along the ecliptic. These points are called the vernal equinox (V.E.), the summer solstice (S.S.), the autumnal equinox (A.E.), and the winter solstice (W.S.). As the Sun moves eastward along the ecliptic, it is at the V.E. on March 21, the S.S. on June 21, the A.E. on September 21, and the W.S. on December 21.

Thus, while we often think of the equinoxes and solstices as dates, they are really points on the ecliptic. The dates refer to the time when the Sun is at each of the points.
 
 

Precession of the Equinoxes

The vernal equinox is presently in the zodiacal constellation of Pisces. At the beginning of spring the Sun is also in this constellation. But this has not always been the case. Hundreds of years ago the vernal equinox was in the constellation of Aries, the Ram, and even today this point is sometimes known as the First Point of Aries. In about 150 BC the Greek astronomer Hipparchus discovered that the vernal and autumnal equinoxes move slowly westward along the ecliptic. The rate of movement is about 50 seconds of arc per year. At this rate, the equinoxes complete one cycle around the ecliptic in about 25,800 years. This is known as the precession of the equinoxes.

Precession results from the Earth’s axis of rotation wobbling like a spinning top. One wobble takes about 25,800 years to complete. This motion is caused primarily by the gravitational pull of the Moon on the Earth’s equatorial bulge. Since the Earth’s axis points to the north (NCP) and south (SCP) celestial poles, these poles also precess. Over time this results in different stars becoming what is called the North Star (see Figure 5.2). Polaris is presently the North Star and lies within 0.5 degrees of the NCP, but back when the pyramids were built a star called Thuben, in the constellation of Draco the Dragon, was the North Star. A few thousand years from now Vega in the constellation of Lyra will be the North Star.

In the equatorial system of coordinates, right ascension is measured from the vernal equinox. Thus precession of the equinoxes causes right ascension to change slowly over time. Perhaps less obviously, an object’s declination also changes. Although small over the lifetime of an individual, this change accumulates over the years and must be taken into account not only in specifying the locations of objects but also in the design of solar calendars.
 
 

	StarryNight Movie: Precession of the Equinoxes
	Project 8: Precession of the Equinoxes

To correctly specify the location of an object on the celestial sphere, one must give not only its right ascension and declination, but also the year, known as the epoch, for which those coordinates apply. Star charts and star catalogs always indicate the epoch upon which they are based. For example, they may be for epoch 1950, epoch 2000, or indeed any year as long as it is noted.