Friday, June 26, 2009

The Star-Formerly-Known-as-Pole

Snaking along the long body of Draco the Dragon from his head, which we examined in last week’s post, we eventually come to a fairly faint naked-eye star opposite the bowl of the Little Dipper. This is the notable star Thuban.

The name Thuban (THOO-bahn) comes from the Arabic for serpent. Because of the star’s importance to ancient cultures, however, it has born many other names: “Judge of Heaven,” “Proclaimer of Light,” “Crown of Heaven,” and “High One of the Enclosure of Life,” to name a few.

Looking north to Draco and the Dippers
Star maps created with
Your Sky

Thuban’s ancient significance stemmed from the fact that it was once the North Star, marking the position of the North Celestial Pole. The North Celestial Pole is the imaginary fixed point in the sky that the Earth's axis would intersect, were it extended from the North Pole into space. The North Star, because it marks geographic north, has been important throughout the ages for navigation.

We are accustomed to the star Polaris, which marks the end of the handle of the Little Dipper, being our North Star. It seems odd at first to think that another star might have been the North Star or Pole Star long ago. In fact, Polaris won’t continue to be our North Star forever. This phenomenon is caused by precession, the change in the alignment of Earth’s axis.

Because the spinning Earth wobbles slowly over time, our axis doesn’t always point at the same spot in the sky. This wobble is caused by gravitational tugging by both the Sun and the Moon. The result is that the position of the North Celestial Pole--where Earth’s axis points--moves over time against the backdrop of the stars, completing a circle in about 26,000 years. The North Celestial Pole will pass closest to Polaris around the year 2100, after which it will begin to move away from it.

The good news is, if we wait just 26,000 years, Polaris will be our North Star once more. What goes around, comes around.

Astronomy Essential: Space is extremely cold.

Intergalactic space, the regions of space that lie between galaxies, is an inhospitable minus 455 degrees Fahrenheit.

Compare that to absolute zero, the coldest theoretical temperature, which is minus 459.67 degrees Fahrenheit, not much colder.