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.

The Zodiac Constellations

The Zodiac Constellations on the Ecliptic

Image Credit:

One Minute Astronomer 

 

Depsite the name of this post, this is not about astrology, which is not a real science. Hopefully, those of you reading this, have read your horoscope, but only for entertainment purposes. I'm guessing you all know that the Signs of the Zodiac came from constellations in the nighttime sky. What I'm going to tell you, is that the signs you were born under are actually incorrect. The Zodiac Signs are based on the sky 2000 years ago.

The Astrological Signs of the Zodiac and their date ranges:

• Aries (March 21 to April 19)
• Taurus (April 20 to May 20)
• Gemini (May 21 to June 20)
• Cancer (June 21 to July 22)
• Leo (July 23 to August 22)
• Virgo (August 23 to September 22)
• Libra (September 23 to October 22)
• Scorpio (October 23 to November 21)
• Sagittarius (November 22 to December 21)
• Capricorn (December 22 to January 19)
• Aquarius (January 20 to February 18)
• Pisces (February 19 to March 20)

What these dates refer to was the constellations the Sun was in on that day. For example, 2000 years ago, on November 9th, when the Sun rose, Scorpius (not Scorpio) rose with the Sun, and the Sun was located in that constellation. However, that is not true today. Due to precession*, the zodiac constellations are not in the same location during the year that they were 2000 years ago.

*In simple terms, precession is the Earth wobbling on its axis much like a spinning top does when it begins to slow down.

Today, the zodiac constellations are more like this:

• Aries (April 19 to May 13)
• Taurus (May 14 to June 19)
• Gemini (June 20 to July 20)
• Cancer (July 21 to August 9)
• Leo (August 10 to September 15)
• Virgo (September 16 to October 30)
• Libra (October 31 to November 22)
• Scorpius (November 23 to November 29)
• Ophiuchus (November 30 to December 17)
• Sagittarius (December 18 to January 18)
• Capricorn (January 19 to February 15)
• Aquarius (February 16 to March 11)
• Pisces (March 12 to April 18)

Notice that there is a 13th constellation in the zodiac, Ophiuchus (serpent-bearer), the Sun is in Virgo for the longest time, and the Sun is in Scorpius for the shortest amount of time. This is based on the official constellation boundaries given by the IAU. These constellations all lie on the ecliptic and therefore, the Sun goes through them.

Image Credit:

The Ohio State Univeristy, Dept of Astronomy