Dreidel Earth

As a brief follow up to the last post, which discussed the Earth’s axial tilt, I thought I’d add another brief point about how the Earth is like a dreidel – in one respect, at least. This won’t involve much actual Torah, but dreidels are, at least, a very traditional part of Chanukah customs.

As mentioned, the Earth’s axis of rotation points in the same direction throughout each orbit around the Sun. Nowadays, its orientation points the North Pole towards the star Polaris, a.k.a. the North Star.

Celestial Poles, Celestial Equator and the Ecliptic (from Cosmic Perspective, from Pearson)
Celestial Poles, Celestial Equator and the Ecliptic
Celestial Sphere model of sky, showing constellations and Celestial Poles (from Cosmic Perspective, from Pearson)
Celestial Sphere model of sky, showing constellations and Celestial Poles

However, this was not always the case. Like a dreidel – or any other spinning top – the Earth’s axis precesses over time.

Earth's precession is like a spinning top (from Cosmic Perspective, from Pearson)
Earth’s precession is like a spinning top

Precession is the circular motion traced by an axis of a rotating object, in addition to the object’s rotation around its axis. You have presumably seen this recently when spinning a dreidel. When you spin the dreidel, its axis of rotation is along the handle used to spin it. As the dreidel slows, the tip of the handle begins to circle around as well while the dreidel is still spinning around its axis. This is precession.

Why does it happen? A spinning object, such as a dreidel, maintains its uprightness while spinning due to conservation of angular momentum, which resists gravity pulling it down immediately (which is what would happen if you tried to stand it up without spinning it, of course). As it spins, unless it is a perfectly balanced sphere, gravity affects its angular momentum by pulling its axis of spinning downwards more, even as it continues to spin. This pulls the axis away from the vertical direction of spin and widens the circles traced by the axis, thus causing the precession to increase. Friction slows down the spin around the axis, reducing the angular momentum of the spin and the precession gets greater, until the top falls.

The Earth spins on its axis similar to a top. (As you probably know, Earth makes one rotation in about 24 hours and this causes the cycle of day and night that we experience on Earth.) But the Earth is not a perfect sphere; it bulges at the equator. The extra pull of the Sun’s and Moon’s gravity acts analogously to the Earth’s gravity pulling on a spinning dreidel, and causes the Earth’s axis of rotation to precess – but very slowly; one circle takes about 26,000 years. The 23.5 degree angle of axial tilt (which stays roughly the same) rotates around in a circle.

So over the course of 26,000 years the projection of the north pole on the sky – known as the Celestial North Pole – traces a circle along the sky. About 5,000 years ago, around the time the great pyramids were built in Egypt (long before the Israelites were slaves…), the North Pole pointed to a star named Thuban, which was their “North Star”. In the future, in about 13,000 years,  it will point to the star Vega. And so on…

So that’s how the Earth is like a dreidel. That Hashem made out of clay…

Happy Chanukah!

 

Plus: Check out this video from NASA & YouTube of the first dreidel in space! (It gets boring after about minute 3…)

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