When you are tired of tracing the windings of the Dragon, turn to Ursa Minor and Polaris. The “Little Bear,” it has been remarked, has an even more preposterous tail than his greater brother. Polaris is at the end of the tail, or the end of the handle of the Little Dipper, and the bowl of the latter is on the bear’s flank.
If one knows nothing else of uranography, one should at least know Polaris, the “North Star.” To recognize that star is to be able to orient yourself wherever you may be in the northern hemisphere. A whole volume could be written on its connection with human affairs. For at least two thousand years it has been the cynosure of sailors, and of wanderers by land as well. You cannot be lost if you have Polaris to guide you. The magnetic compass varies and misleads, the sun and the moon change their places, all the other stars circle through the heavens, but Polaris is always there, shining over the pole of the earth, the image of steadfastness. Only the slow Precession of the Equinoxes affects it. At the present time it is within one degree and a quarter of the true pole of the heavens, and it is drawing nearer that point, so that in two hundred years it will be less than half a degree from it—less than the apparent diameter of the moon. The little circle that it daily describes in the sky may be disregarded, for it is hardly noticeable except with instruments; but it is easy to fix the star’s position with considerable accuracy by simple observation. Note that the Great Dipper and the “W”-shaped figure in Cassiopeia are on opposite sides of the pole. When one is above, the other is below; when one is on the east, the other is on the west. Draw an imaginary line from the star Mizar in the Great Dipper to the star Delta (δ) in Cassiopeia and it will pass almost directly through the pole. Polaris is on that line, a degree and a quarter from the pole in the direction of Delta Cassiopeiæ. If the observation is made when Delta is above the pole and Mizar below it, Polaris will be on the meridian, or north and south line, a degree and a quarter above the pole; when Delta is west of the pole and Mizar east of it, Polaris will be a degree and a quarter west of the meridian; when Delta is below the pole and Mizar above it, Polaris will be on the meridian a degree and a quarter below the pole; and, finally, when Delta is east of the pole and Mizar west of it, Polaris will be a degree and a quarter east of the meridian. The intermediate positions you can easily deduce for yourself.
But Polaris will not continue to be the unerring guide to the north that it now is. The Precession of the Equinoxes is carrying the pole progressively westward in right ascension, so that Polaris will eventually be left far behind. But the motion of the pole is in a circle about twenty-three and a half degrees in radius, and it requires about 25,800 years to complete a revolution round this circle. Consequently, at the end of that period, Polaris will have come back to reign again as the North Star for many centuries. In the interim other stars will have occupied its place. About 11,500 years from now the brilliant Vega, or Alpha Lyræ, will be the North Star, and in about 21,000 years Alpha Draconis (Thuban) will once more shine down the great northward-pointing passage in the pyramid of Cheops, if that pyramid shall still exist.
Polaris, unlike some of the others stars that we have been looking at, is running away into space instead of approaching us, at a speed which has been estimated at about 1,380,000 miles per day. Its present distance is not less than 200,000,000,000,000 miles. It has an invisible companion with which it circles in an orbit of a few million miles diameter in a period of about four days.
Polaris is also a celebrated visual double. With a telescope of two or three inches aperture you can see close by its flaming rays a minute blue star, a delicately beautiful sight. In the older days of telescopes, before they had attained the perfection which improvements in glass-making and lens-shaping have rendered possible, this little companion star of Polaris was a universal test of excellence. Its prestige was historical. The amateur owner of a telescope who could see that star clearly felt a joy that he could hardly express. The old makers of object-glasses, by rule of thumb, always tried them on the companion of the Polestar as a standard test for small apertures. The small star is of the ninth magnitude, and situated about 18″.6 from its primary.
The stars Beta (β), or Kochab (the “Star”), and Gamma (γ), in Ursa Minor, are called the Wardens, or Guards, of the Pole. In low northern latitudes, where these stars sweep the horizon at their lower culmination, Shakespeare’s description in Othello would be literally true during a great storm at sea:
“The wind-shak’d surge, with high and monstrous mane,
Seems to cast water on the burning Bear
And quench the guards of th’ ever-fixed pole.”
The constellations Cepheus, Cassiopeia, and Perseus, now low in the northwest and north, we leave for description to another chapter.
II
THE EVENING SKY AT THE SUMMER SOLSTICE
At 10 o’clock P.M. on the 21st of June, the longest day of the northern hemisphere, the aspect of the sky is that shown in [Chart II], accompanying this chapter. The same chart will answer for 11 P.M. on the 5th or 6th of June; 9 P.M. on the 7th of July, and 8 P.M. on the 22d or 23d of July. In fact, for any of the hours mentioned the date may be shifted several days forward or backward without seriously affecting the comparison of the chart with the sky, and the same may be said of each of the other circular charts. The stars simply rise about four minutes earlier each evening, and four minutes of time correspond to one degree of space measured on the face of the sky. So the whole sky shifts about one degree westward every twenty-four hours.