Astronomy: More about the winter night sky
In your observation of the winter sky (these nights) you will keep in mind a few things to make your night-long diligence worthwhile.
Firstly, as the sky rotates (in fact, it is the earth which rotates, the sky remains there and any perceptible, visible change in it takes place over hundreds, often many thousands of years) new constellations and stars keep appearing in the east, as ‘older’ stars that had appeared earlier in the night, keep disappearing in the west. So, you are confronted with new stars/constellations every two hours or less.
Secondly, keep reminding yourself about the ecliptic; it is the apparent path, or highway of the sun (and planets) on the celestial sphere (sky) caused superficially by the tilt of the earth which is about 23 1/2 degrees. All planets orbit within the sun’s path are ecliptic. They do not veer away from the ecliptic. Even the moon traces the same path. Further, sun’s path over a period of one year, with stars in the background, is called zodiac (circle of animals, in Greek). Whenever you learn to identify a planet, you know that it is on nearly the same path as the sun’s as well as that of the moon.
Thirdly, think about the circumpolar constellations and stars — those that do not disappear from view in summer, winter, spring or fall, and remain at the same place in the sky. Only their shape (sometimes called their aspect in astronomy) changes slightly, but that is because of our changing position across the sky. It is only apparent, not real. The interesting thing about circumpolar stars is that these are those stars that lie in ‘close proximity’’ of the Pole Star. Throughout the night, the sky seems to rotate around the Pole Star, which stays in the same place. Stars nearby make a barely perceptible movement.
Fourthly, what is Pole Star after all? Why it is called so? Will it remain in its current place, and hold the honoured position always? What have the stars got to do with the angle of earth’s tilt to stay in the same place in the sky for a long period of time?
Earth’s tilt with relation to the sun gives us our seasons. Now, if we draw an imaginary line from the South Pole straight to the North Pole and extend it all the way into the sky, the one star our imaginary line will touch is the Pole Star. Its real name is Polaris, the major star of constellation Small Dipper (Dubbe Asghar to Arab star gazers). There is a Big Dipper too; before the Americans named it so, it was and still is known as Dubbe Akber, or the Great Bear, so named by Arabs. As you must have noticed from names of stars and constellations (besides other less utilised jargon), these names are almost invariably in Arabic, given to them by Arab astronomers of 7th-13th centuries.
Now, it should be easy for you to imagine that while earth rotates on its axis, the axis itself does not do so. It points straight to the Pole Star. All “nearby” stars are called circumpolar. They do not move much but slowly go in circles, wide and incomplete, semicircles around the Pole Star.
Like earth moves on its axis (causing day and night), as well as around the sun (causing months and years, and change of seasons); there is yet another movement called precession. As the two abovementioned movements (axial and orbital) perform their jobs relentlessly, and inexorably, the top portion of the globe does its own. Its movement makes a little circle, just as a top does. It shakes gingerly rotating in a small circle. That full circle is called precession. In the course of 26,000 years it takes to complete one precessional circle (also called cycle), the axis of the earth points to one of the stars in that little circle. For the last several thousand years, that star happens to be Polaris in the constellation Small Dipper. Hence the Pole Star.
But Polaris will not be the Pole Star forever. As the wobble continues, other stars will keep taking its place until the circle is complete in 26,000 or so years. Indications are that when the biggest pyramid, Giza was built over 4,000 years back; Polaris was still the Pole Star. Next to be the Pole Star is a star of not much significance called, Thuban.
Try to draw the orbit of earth on paper in January. No matter if it is not accurate. Then draw the earth’s position as it would be in February, again by guess work! Two positions will give you roughly the distance travelled by the earth in one month. Now, in the same manner, imagine the sky. Need I tell you that position of the stars (aspect of the sky) will have changed considerably in the course of one month? With a little practice you will become a perfect sky watcher!
The writer is a professional astronomer and a former head of PIA Planetaria. He can be reached at firstname.lastname@example.org