Precession of the Equinoxes

Last post, I mentioned how the constellations of the zodiac do not match what you see in your horoscope because of something called precession. As I explained then, precession is just saying that the Earth wobbles as it rotates on its axis, much like a top does when it slows down.

 



The amount of wobble for the Earth is not as extreme as it is for a slowing down top, i.e. we don't have to worry about rotation of the Earth ending any time soon.

Because of the wobble of the Earth, two things occur that can be measured.

1. The location of the North Celestial Pole will change. Right now, the north star is Polaris in Ursa Minor. In about 12,000 years, the north star will be Vega, in the constellation Lyra.

   

   http://people.timezone.com

2. The location of the equinoxes and solstices have changed over time. When the vernal equinox occurred 2000 years ago, the Sun was in Aries. The summer solstice occurred when the Sun was in Cancer (hence, the tropic of Cancer). The autumn solstice occurred when the Sun was in Virgo. And the winter solstice happened when the Sun was in Capricorn (and from where the name for the tropic of Capricorn). Now, the Sun is in Pisces when the northern hemisphere spring begins. This is where we get the term "Precession of the Equinoxes".

   

   Original artwork by me (yes, I know I can't draw)

 

The period of the "wobble" of Earth is about 26,000 years. So while Vega will become the north star in about 12,000 years, by the year 28,000, Polaris will be back at the celestial north pole.

Precession of the North Celestial Pole, (Note at +2000 the NCP is near Polaris. The bright star near +10000 is Deneb in Cygnus. At +14000, the NCP will be near the bright star Vega)

Image Credit:

"Precession N" by Tauʻolunga - self, 4 bit GIF.

 

 

If you are planning on using the stars to find north in the next 10000 years, you won't be able to use Polaris any longer in about 1000 years.

Celestial Sphere

The celestial sphere is the imaginary sphere covering the sky surrounding the Earth with the Earth at the center of the sphere. It is a way to map the location of stars, galaxies, and extra-solar objects in the sky, as well as track the movements of the Sun, Moon, and planets among the background stars.

There are a few important locations on the sphere that should be defined to help get a sense of what to look for in the sky.

Zenith: the point on the celestial sphere directly above your location on Earth. This point is dependent on where you are on Earth. To someone in Pittsburgh, PA, USA, their zenith is not that same as to someone in Sydney, New South Wales, Australia.

Nadir: the point on the celestrial sphere directly below your location on Earth. Like the zenith, this is location dependent.

The North (South) Celestial Pole: the point on the celestial sphere directly above the North (South) Pole on Earth. The North (South) Celestial Pole is the zenith for the North (South) Pole and the The South (North) Celestial Pole is the nadir for the South (North) Pole.
 
The Celestial Equator: similar to the equator on Earth. In fact, it is the imaginary line on the celestial sphere is directly above the Earth's equator. It is used to help explain the equinoxes as well as the solstices.

Celestial Prime Meridian: the line on the celestial sphere equally 0h Right Ascension. It is the line directly corresponding to 0° longitude on Earth, i.e. the prime meridian running through Greenwich, England.

Right Ascension: equivalent to longitude on Earth, this is an angle measured in hours, minutes, and seconds from 0h to 24h. The angle increases from east to west, from the Celestial Prime Meridian

Declination: equivalent to latitude on Earth, also measured in degrees, arcminutes, and arcseconds from 0° to 90° from the Celestial Equator to the Celestial Poles. Travelling north from the Celestial Equator to the North Celestial Pole, the angles are positive, and travelling south from the Celestial Equator to the South Celestial Pole, the angles are negative.

Ecliptic: this is defined in another page, located here. It is the imaginary path the Sun traces over a course of the year on the Celestial Sphere. It is tilted at 23.5° with respect to the Celestial Equator and when it crosses the Celestial Equator, the equinoxes occur. At the Sun's northernmost point on the ecliptic, we have the summer solstice in the Earth's northern hemisphere and the winter solstice in the southern hemiphere. At the Sun's southermost point, the solstices are switched.