Be patient, this parade lasts 25,800 years


The pyramids at Gizah in Egypt stand against the ancient sky when Thuban in Draco was the polestar. Our era’s polestar, Polaris, is the yellow star at left. Photo: Ricardo Liberato; background from Chris Marriott’s SkyMap.com.

Back in the good old days, Thuban (THU-ben) in the constellation Draco the Dragon was the polestar. It occupied the spot where Polaris the North Star sits now. Ah, but that was almost 5000 years ago. Much has changed since then.

Keeping track of the stars is bad enough. Earth’s rotation and revolution around the sun are infamous for sowing confusion among beginning skywatchers about what’s up when. Over a longer span of time we also have to contend with precession.

While our axis maintains its tip of 23 1/2 degrees, over a period of 25,800 years it changes the direction of its rotation like the gyroscope does in the animation. Since the "north star" is the star that Earth’s north polar axis happens to point to in space, that means we see a series of different north stars during a full cycle. Think of the Earth’s axis as describing a slow circle in the sky like a children’s top running down. Whatever star lies on that circle will eventually be a pole star.

Right now, Polaris in the Little Dipper holds that title but not forever. The role will slide to the star Gamma in Cepheus the King around the year 4000 A.D. and finally return to Thuban in 23,000 A.D. That’s so much time to think about it hurts my head.


This map shows the sky around 9 o’clock in late March as you face north. Thuban is easy to find between the two Dippers. Created with Stellarium.

Thuban, while not bright, is very easy to find. Just look midway between the two end stars of the Little Dipper and the star Mizar, in the bend of the Big Dipper’s Handle. In the days of the Great Pyramids, all the other stars in the northern sky circled around Thuban, hub star of the north, just like they do around Polaris in our era. Since Thuban’s is on the dim side, you wonder if anyone really took notice of it. I mean, it’s not like our North Star, which is as bright as one of the Dipper stars.


This is the circle described in the sky by Earth’s wobbling axis. Thuban was the polestar around 3000 B.C. and closest to the pole in the year 2787 B.C. Polaris has replaced it in our era. The minus numbers refer to B.C. years, the plus to A.D. Credit: Tao’lunga

If you’ve studied pyramids at all, you’ll come across frequent references to the astronomical orientation of Khufu’s Great Pyramid at Ghizah, constructed around 2560 B.C. There’s a shaft built into one face that some say is aligned with the Thuban. Others think it has more to do with building construction than astronomy but no one knows for sure. One thing’s for certain: the shaft does point north, nearly in the star’s direction. Perhaps it was the escape route for the pharoah’s soul. Whatever the truth is, who can deny the poetry of a star shining all night down a stoney hole into the heart of an ancient tomb?

Precession (left) is caused by the gravitational attraction of the sun and moon on the Earth’s slightly bulgy equator, which makes our axis wobble in rhythm. Didn’t know Earth was a little heavy around the middle? Photos may imply otherwise, but the Earth’s not a perfect sphere. Our planet’s rotation makes it bulge slightly at the equator, increasing the diameter there by 27 miles compared to the poles. Good thing my equatorial bulge is considerably smaller. It keeps me out of trouble.
 


Two views of the northern sky on a March evening separated by nearly 5000 years. Illustration: Bob King with Chris Marriott’s SkyMap.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

6 thoughts on “Be patient, this parade lasts 25,800 years

  1. Dear Mr. King,

    I don’t know exactly how to formulate this question, its related to this Thuban post and Polaris, I guess it’s something like this:

    If a city in the Asian Pacific is located in the world, just north of 25° 2′ 21″ N. / 121° 31′ 30″ E. and your body is facing “geographic” North, in the direction of Polaris, how many degrees of separation is Thuban (both from the star Polaris and if possible from the earth’s celestial pole, at the junction point of the equatorial line)? And (here is where I am just simply ignorant) are the degrees the same, say as your body is facing North in another city, such as Los Angeles 34° 3′ 8″ N / 118° 14′ 34″ W? And finally, as we perceive the star with our naked eye, is Thuban Northeast or Northwest of Polaris? I mean in which direction is this gyroscope rotating? errrgh… sorry, I am completely perplexed.

    Any precise number and direction you suggest would be very much appreciated.

    Thank you for your wonderful blog.
    Best,
    Sol

    • Hi Sol, good basic questions about astronomy. Thuban and Polaris are about 25 degrees apart no matter where you are in the world. Depending on the time of night or day, Thuban can be variety of different directions from Polaris. Around 10 p.m. local time in early June, it’s due south ABOVE Polaris, at 4 a.m. due west, at 10 a.m. due south again but BELOW Polaris and at 4 p.m. due east. It revolves around Polaris in a circle which is why it changes direction.
      As far as fixed coordinates on the celestial sphere, ie. it right ascension and declination, Thuban is south and east of Polaris.
      Does this help?

  2. I am a little confused. Is the precession circle the whole tilt of the earth:23.5 degrees or is it a smaller precession of say a few degrees about the 23.5 degree tilt? For instance, does the earth change its orientation from 22 Degrees to 25 Degrees over 26,000 years? Or the whole 23.5 +23.5 = 47 degrees?

    Is seems that either is within the conversation and is not clear to me.
    From the drawings it seems that the circle is less than 23.5 + 23.5 or 47 Degrees total.

    Thank you for any explanation.

    • Roger,
      The Earth’s tilt remains the same. It’s merely wobbling about a point in the sky called the north ecliptic pole. Picture a toy top as it slows down after spinning awhile. Before it tips over its axis tilts over to one side and begins to wobble and describe a circle in the air above the top. With the Earth it takes 26,000 years for out axis to describe a circle like the top does in a second or two. Fortunately, we’re not tipping over anytime soon! Does this help?

      • Thanks.
        I thought perhaps there were two separate precessions; one caused by the loss of momentum of he spinning earth and one by the Suns and Moons gravitation on the bulge of at the Equator of Earth because of its shape.

        I am not a physicist, but it seems with a top the precession increases with the loss of angular momentum of the spinning body. It would seem that the loss of the earth’s momentum is very small throughout time.

        So I thought that there might be two precessions eombined to provide a small variation in the pointing direction of the earth, say 22 to 24 degrees which is the small precession and the 47 degree precession. So could that be true?

        Thanks for the quick response.
        Roger

        • Roger,
          There’s only one precession – the one with the 26,000 year period – however if you like to delve into details, there’s also a much smaller variation on the precession circle called nutation. That amounts to second of arc. See here for details: http://www.ualberta.ca/~dumberry/nutation.html
          But back to the big stuff. Separately, Earth’s tilt varies from 22.1 to 24.5 degrees and back again with a period of 41,000 years due to the gravitational attraction of the moon and planets. Here’s a little explainer with nice visuals on that: http://www.sciencecourseware.org/eec/GlobalWarming/Tutorials/Milankovitch/
          Earth loses angular momentum over the long haul because of tides raised by the orbiting moon. It causes Earth to very gradually slow down and the moon to move farther away.

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