# Thread: 23.5 degrees - Earth's axial tilt

1. I have a question... I'm not a scientist or mathematician, but I've run across some differing numbers and was wondering if anyone can give a straight answer...

The accepted average for the Earth's axial tilt is 23.5 degrees. Yet I've seen other astrological reports that say it "used" to be 23.5, now it's 23.4... 23.43... and still others that state the axial tilt is currently moving towards an upright position, decreasing the tilt degree.

Is any of this true, and what exactly is the current tilt degree... and how much is it moving per year, if anything. What effect does any of this have (or would it have) on seasons, weather, etc.

Thanks!!

Paul  2.

3.  4. thanx SkinWalker!

I've been to that page before, thus the post here, lol.

Like I said, I'm not a mathematician... so how would I take information like this and make sense of it:

Values

The Earth's axial tilt varies between 22.1° and 24.5° (but see below), with a 42,000 year period, and at present, the tilt is decreasing. In addition to this steady decrease there are much smaller short term (18.6 years) variations, known as nutation, mainly due to the changing plane of the moon's orbit. This can shift the Earth's axial tilt by plus or minus 0.005 degree.

Simon Newcomb's calculation at the end of the nineteenth century for the obliquity of the ecliptic gave a value of 23° 27 8.26 (epoch of 1900), and this was generally accepted until improved telescopes allowed more accurate observations, and electronic computers permitted more elaborate models to be calculated. Lieske came with an updated model in 1976 with ε equal to 23° 26 21.448 (epoch of 2000), which is part of the approximation formula recommended by the International Astronomical Union in 2000:

ε = 84381.448 − 46.84024T − (59 × 10−5)T2 + (1.813 × 10−3)T3, measured in seconds of arc, with T being the time in Julian centuries (that is, 365,25 days) since the ephemeris epoch of 2000 (which occurred on Julian day 2,451,545.0). A straight application of this formula to 1900 (T=-1) returns 23° 27 8.29, which is very close to Newcomb's value.

With the linear term in T being negative, at present the obliquity is slowly decreasing. It is implicit that this expression gives only an approximate value for ε and is only valid for a certain range of values of T. If not, ε would approach infinity as T approaches infinity. Computations based on a numerical model of solar system show that ε has a period of about 41,000 years, the same as the constants of the precession p of the equinoxes (although not of the precession itself).

Other theoretical models may come with values for ε expressed with higher powers of T, but since no (finite) polynomial can ever represent a periodic function, they all go to either positive or negative infinity for large enough T. In that respect one can understand the decision of the International Astronomical Union to choose the simplest equation which agrees with most models. For up to 5,000 years in the past and the future all formulas agree, and up to 9,000 years in the past and the future, most agree to reasonable accuracy. For eras farther out discrepancies get too large.

How can I make this understandable to a layman. If according to this wiki the Axis "IS" decreasing... at what rate? As in points of a degree per year or thousand years... something stupid people can understand, lol!

Thanks again!

Paul  5. A wobbly spinning top nutates (ie, "to nutate" means "to wobble"). Think of a spinning top with its spin axis tipped at an angle from the vertical, and with this "wobble" angle decreasing (ie, the spin axis is moving toward the vertical).  6. For layman you can break it down in simple terms.

The tilt varies about 2.4 deg over a 41,000 years. A back of the envelop calculations that about 240km (each deg is roughly 100km) divided by half the period is about a 12 m/year average change. The tropics are getting

Effects on seasons? The seasons are less extreme at the minimum tilt angle.

In person the whole concept is really fun and easy to explain by holding a globe of the earth and a flashlight.  7. In person the whole concept is really fun and easy to explain by holding a globe of the earth and a flashlight.
*Get's image of a toy box with a smiling kid holding a torch and a globe printed on the front. *  8. Thank you so much Lynx!

So... just to be sure my simple brain got it...

1 degree = 100km

1km = 1000m.

12 meters (0.012km) per year

basically we're looking at 0.012 degrees of movement per year... give or take, yes?   9. 12m/yr * 1km/1,000m * 1°/100km = 12°/100,000yr = 0.00012°/yr  10. LOL - Thanks!!!  Bookmarks
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