My Dark Sky

In space, our Earth not only feels the gravitational pull of the mighty Sun, it too can feel the gravitational effect of the Moon acting on it. Our Moon may be much much smaller than our Sun, but remember, it is much much closer.

Since our Earth is not completely rigid, as it rotates on its axis, centrifugal force causes the equator to bulge. This is the same force that causes you to fling to the side of a giant spinning wheel in a theme park. The non-uniform gravitational force of the Sun and the Moon will pull on this bulge and causes the Earth to wobble as it spin around it axis, just like a spinning top that is almost falling off.

Technically, this wobble is called precession. Precession of Earth’s rotational axis takes approximately 26,000 years to make one complete revolution. Through each 26,000-year cycle, the direction in the sky to which the Earth’s axis points goes around a big circle. In other words, precession changes the “North Star” as seen from Earth.

Precession – the slow and gradual shift of Earth’s axis through a 26,000-year cycle.
When viewed from outside and looking down onto the Earth from the north, the direction of precession is clockwise. When standing on Earth looking outward, the axis appears to move counter-clockwise across the sky.

Currently, the Earth’s axis is pointing somewhere near Polaris in Ursa Minor, hence north star today is Polaris. However, almost 5,000 years ago, Thuban in the constellation of Draco was the north star, and was used by the ancient Egyptians. In 13,000 years from now, precession will bring Earth’s rotational axis near Vega in Lyra. That time, Vega will become our north star.

Precession was discovered by Greek astronomer Hipparchus, who had access to several centuries of Babylonian and Greek records. He compared these ancient records to his own and concluded that the axis which the heavens seemed to rotate shifted gradually, though very slowly.

~ by thChieh on October 14, 2008.

Posted in General Astro Talk