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October 27, 2012

You can’t feel it, but Earth is wobbling on its axis like a spinning gyroscope that’s running down. It’s a very slow motion though — it takes about 26,000 years to complete a single wobble.

We can see the effects of the wobble in the changing tableau of the night sky — an effect called precession. It’s not visible from night to night, but it adds up. It causes Earth’s axis to point in different directions, giving us different “pole stars.” It also causes the Sun to appear against different sets of stars at the same time of year.

An example of the latter effect is seen in Aries, the ram.

More than 2,000 years ago, the Sun passed across the constellation at the time of the March equinox, which marked the start of the year. So Aries took on special significance — it was “first” among the constellations.

The point of the equinox was known as the First Point of Aries. And it marked the “zero” point for the celestial coordinate system — a system of measuring positions in the sky that’s the equivalent of longitude and latitude here on Earth.

But thanks to precession, over the centuries the equinox has moved westward, into the adjoining constellation Pisces. Yet it’s still known as the First Point of Aries — the central marker for mapping the entire sky.

Aries is in the east this evening, well to the lower left of the Moon. It’s not all that much to look at, although its brightest star does stand out. More about that tomorrow.


Script by Damond Benningfield, Copyright 2012

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