Polar motion with Earth's gravitational influences
One of the most easily recognizable star patterns, at least for those of us in the northern hemisphere, is the group of seven bright stars known as the Big Dipper.
It is not a formally recognized constellation, but rather is incorporated within a larger constellation, Ursa Major, the Great Bear. This time of year the Big Dipper is located low in the northwestern sky during the evening hours, and skirts the northern horizon around midnight before starting to climb into the northeastern sky before dawn.
The two stars at the front of the Big Dipper’s bowl are informally known as the pointers, and if one extends an imaginary line from the southern of these through the northern and then continues on northward, one comes to another fairly bright star in the far northern part of the sky.
This is the star Polaris, often referred to as the Pole Star or the North Star, so called because the Earth’s rotational axis is aimed at it, and throughout the course of a night it remains essentially stationary, while all the other stars in the nighttime sky appear to rotate around it.
There are a couple of popular misconceptions about Polaris. While it is fairly bright as stars go, it is not the brightest star in the nighttime sky – it is only about the 50th brightest. Furthermore, the Earth’s rotational axis does not point directly at Polaris; the star is displaced about 2/3 of a degree – slightly over the apparent diameter of the full moon – from the sky’s true north pole. If one were to make careful measurements, one would see that Polaris itself makes a tiny rotational circle around the sky’s north pole.
The Earth’s equator is tilted some 231Ž2 degrees with respect to the Earth’s orbital plane, and since the Earth rotates, the gravitational influences of the sun, the moon, and the other objects in the solar system causes the Earth to wobble, or precess. As a result, the Earth’s rotational axis doesn’t stay put against the background stars, but rather slowly shifts position. It so happens that the sky’s north pole is approaching Polaris; it will be closest in the year 2102, when it will be just under half a degree, i.e., the apparent diameter of the full moon, away from it. After that, the north pole will start pulling away from Polaris.
Over time, other stars have been the North Pole Star in the past, and other stars will be the North Pole Star in the future. About 4,000 years ago the star known as Thuban in the constellation Draco, located between the Big and Little Dippers, was the North Pole Star, and about 2,000 years from now the star known as Gamma Cephe--in the constellation Cepheus, now high in the northern sky during the evening hours--will fill that role. About 12,000 years from now the North Pole Star will be the brilliant star Vega, which is currently high in the northwest during the evening hours.
The same thing happens in the southern hemisphere. At present there really is no bright South Pole Star; the closest thing is a rather dim naked-eye star known as Sigma Octantis. A somewhat brighter star, Gamma Chamaeleontis, will be the South Pole Star in about 2,000 years, the bright star Delta Velorum will fill that role in about 7,000 years, and the brilliant star Canopus will be somewhat of a South Pole Star in about 12,000 years--at the same time Vega is fulfilling the role as the northern counterpart.
Other effects also happen as a result of the Earth’s precessional motion. The vernal equinox, i.e., the point at which the sun’s path, or ecliptic, crosses the celestial equator going north, was in the constellation Aries about 3,000 years ago, and in fact this point in the sky is still often called the First Point of Aries. It is now in the constellation Pisces, however. In about 600 years it will shift into the next constellation to the west, beginning the much-ballyhooed, and, from an astronomical standpoint, essentially meaningless, Age of Aquarius.
It takes the Earth approximately 25,800 years to complete one precessional wobble. There are other small cycles and other factors at work, however, so one precessional cycle is not exactly identical to the previous one. The angle that the Earth’s equator makes to the Earth’s orbital plane, or obliquity, does not remain constant, but rather undergoes both long-term and short-term cyclical variations of its own which generally go under the term nutation; these motions also need to be included. Over time, the Earth’s rotation is, very gradually, slowing down, and the shifting of continental land masses also makes small, but nevertheless accumulative, contributions to this process.
Finally, the stars themselves do not remain motionless, but have their own motions through the galaxy. Polaris, for example, is slowly traveling towards the east-southeast, and, even with all other things being equal, by the time the sky’s north pole returns to its present location 25,800 years from now Polaris will have traveled half a degree farther away from the polar location, although it would still be close enough that it could justifiably be considered as the North Pole Star.
While Earth will almost certainly still be here in that distant era, spinning around its axis and orbiting the sun just like it does now, one can only imagine what the state of humanity will be like then – or even if there will still be humans around to witness that newer incarnation of Polaris as the Pole Star.
Alan Hale is a professional astronomer who resides in Cloudcroft. Hale is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is http://earthriseinstitute.org