Today, January 19, the Sun is in two places at one time! The banner for this page was made using an Astronomy program and on this date the program showed that the Sun is entering the Astronomical Zodiacal sign of Capricornus, while according to the pseudoscience, astrology, the Sun is just entering the boundaries of the next constellation to the east – Aquarius. How is this possible?
As the world turns, so does the sky, or so it appears to us from Earth. Our perspective of the sky is based in part on the Earth’s regular motions. Most of us are familiar with the concepts of rotation and revolution. Not everyone, though, is familiar with the less-perceptible Earth motion of precession. The Earth wobbles about its axis much the way a gyroscope does as it slows down. However, unlike a gyroscope, the Earth spins very slowly and will not stop spinning and topple over.
The Precession Circle
The Earth’s wobbling motion is referred to as precession, or the shifting of the Earth’s axis over time. The wobbling is caused by a combination of its 23.5° axial tilt from the plane of the solar system, and the gravitational pull of the Sun and the other planets back toward the plane of the solar system. Because of the Earth’s rotational spin, it, like a gyroscope, resists outside forces, and does not align itself with the solar system.
One way to visualize the changes caused by precession is to think about how the position of the Celestial Pole (the point in space directly over the Earth’s North Pole) moves with respect to the background stars. As the Earth precesses, the North Pole traces out a circle in the night skies, as does the South Pole. Presently, the Earth’s North Pole points almost directly towards the star Alpha Ursa Minor, commonly known as Polaris the North Star
. Polaris is less than one degree from the North Celestial Pole. However, Polaris has not always been our North Star, nor even our brightest polestar. Several thousands of years ago the Star Thuban
, in the constellation Draco the Dragon, was the Pole Star.
The changing skies Precession is a slow but steady motion. The completion of one cycle takes about 25,800 years. As the Earth precesses, our view of the sky slowly changes, so that after a long period of time, the stars and constellations shift their positions in the skies relative to the Sun.
Ancient cultures kept track of time by noting the Sun’s position with respect to the constellations. For example, the start of each season was identified by the Sun’s position within a constellation’s boundaries. However, due to precession of the Earth’s axis, this start position has shifted westward at over the centuries and in most instances is an entire constellation west from its position 3,000 years ago.
|Position of Sun for Start of Each Season
|Spring – March
|Summer – June
|Fall – September
|Winter – December
Click here to go to the Qué tal in the Current Skies web site for more observing information.