Tales From The Great Precession


You can use the moon to find Venus or the other way around — whatever works for you. Just be sure to face west about 30-45 minutes after sunset or around 7:45-8 p.m. for the region tonight. Venus and the moon will be only about five degrees apart. Maps created with Stellarium

We have a real nice pairing up of the moon and Venus in the western sky about a half-hour to hour after sundown tonight. The moon will be a fresh-faced thin crescent about 28 hours old for Midwestern viewers. You’ll need a wide open view to the west to make sure you see it. Tomorrow night the moon will have shot up above Venus and be much easier to see if a little less fragile.


Thuban, the old pole star, is located in midway between Mizar in the Big Dipper and Kochab in the Little.

Once the sky’s dark around 8:45-9 p.m. pivot on your feet 90 degrees to the right and you’ll face due north. Look high up in the northeastern sky to find the Big Dipper standing on end of its handle during early March evenings. The Little Dipper’s brightest star is Polaris the North Star and equal in brightness to those that compose the Big Dipper. Finding Polaris is easy to find by using the two end stars of the Dipper’s bucket — draw a line through them and it points right to it. You can fit your outstretched fist with room to spare between Polaris and Kochab (KO-cab), the Little Dipper’s next brightest star. Directly between it and Mizar, the star at the bend of the Big Dipper’s handle, lies a fainter star with a hallowed history. This is Thuban (THOO-bahn), the North Star at the time the Egyptian pyramids and Stonehenge were built.

Earth’s axis experiences a periodic wobble like a top slowly losing speed (left). Called precession, it takes 26,000 years to complete one cycle. Since the axis is like a finger pointing to the North Star, it follows that as the axis wobbles the direction in which it points changes. Right now the north end of the Earth’s axis is aimed at Polaris but between about 3900 – 1900 B.C. it pointed at Thuban in the constellation Draco the Dragon.  Around the year 2700 B.C. Thuban lay almost exactly off the end of the Earth’s axis and made an excellent if somewhat faint Pole Star.

After 1900 B.C. the closest bright star to the sky’s north pole was Kochab, followed around 500 A.D. by Polaris. Since Polaris assumed the throne, the Earth’s axis has been steadily sharpening its aim so that the star is now less than a degree from due celestial north. Closest approach of just under a 1/2 degree (one full moon diameter) will happen in 2102 after which the axis, like a lover seeking a new partner, will cast its gaze longingly in the direction of Gamma in the constellation Cepheus. Round and round the axis goes until it returns to Thuban again in 20,346 A.D. and the cycle begins anew.

The tippy-top effect is caused by the twist-pull of the combined gravity of the sun and the moon on the Earth’s equatorial bulge. Our planet’s not a perfect sphere but instead is slightly wider around the equator than around the poles. The moon and sun’s attraction on the bulge is enough to cause the axis to precess or gyrate.


As the Earth rotates, its axis steadily points toward the pole star which is why Polaris appears stationary in the sky (above, left) as all the other stars move in circles around it. A time exposure photo shows the effect well. Photo: Bob King

12 Responses

  1. Larry

    How will precession affect the seasons? Does that mean 13,000 years from now our ancestors will be roasting marshmallows by a campfire on a long warm December night to celebrate Christmas?

  2. astrobob

    Hi Larry — precession doesn’t affect the seasons at all even though roasting marshmallows in December sounds delightful. The tip of the axis is still 23.5 degrees throughout. It just rolls through a circle (describes a circle in the sky) over 26,000 years. If you look at the gyroscope/top animation you’ll see that the tip remains constant but the direction changes.

  3. Larry

    Ahh, I get it. I was associating the top of the gyroscope animation with North. If I go out and notice Procyon is all of a sudden the North Star I will start stocking up on marshmallows around Thanksgiving 😉

  4. astrobob

    Sounds good. I’ll be keeping an eye on Procyon! By the way, it’s the invention of the leap day (leap years) that keeps the calendar aligned with the seasons.

  5. astrobob

    Good question Lakshmi — it might be possible but it’s not in any future scenario. The moon helps maintain Earth’s axial tip within a narrow range. If the equator was in the same plane as our orbit, the axis inclination would be zero degrees and there would be no seasons.

  6. Lakshmi

    Thanks Bob!

    Is ice age a result of the axis inclination being zero degrees?

    Also, why are Tuesdays associated with planet Mars??

  7. Lakshmi

    Thanks Bob!

    Is ice age a result of the axis inclination being zero degrees?

    Also, why are Tuesdays associated with planet Mars??

  8. astrobob

    The cause of the ice ages is not well understood but it isn’t brought on by a zero degree inclination. It may have to do with long-term Earth-sun distance variations and some modest changes to Earth’s orbital tilt but just as likely with climate changes brought on volcanism, the movement of tectonic plates, the amount of carbon dioxide in the atmosphere and the like. The Romans dedicated the third day of the week to Mars. In some languages like French and Spanish Tuesday is named Martes (Spanish) and Mardi (French) which still recall the old Roman tradition.

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