A Field Book of the Stars/Motions of the Stars
THE MOTIONS OF THE STARS.
IT may be that the student desires to proceed in this conquest of the sky at a more rapid pace than the scheme of study permits. To assist such, it should be borne in mind that the circumpolar constellations, as Ursa Major, Ursa Minor, Draco, Cepheus, and Cassiopeia, are designated,—are visible in our latitude in the northern sky every night.
A reference to their diagrams, and a glance at any of the large plates showing the entire group in their respective positions, will suffice for the student to identify them.
The hours of darkness alone limit the speed with which a knowledge of the constellations can be acquired.
Let us suppose that the student begins his search for the constellations on the night of April 1st, at nine p.m. He has for his guide the large plate, and the spring group of eleven constellations set forth in the diagrams. The remaining three constellations of the circumpolar group are, as we have seen before, visible in the north.
If he faces the western sky, he will see Andromeda just setting, and Perseus, Taurus, Orion, Lepus, and Canis Major but a short distance above the horizon. If he is so fortunate as to be able to identify these, and the spring group, he may turn his attention wholly to the eastern sky, where new constellations await him.
In the southeast he may see Virgo. In the east well up blazes Arcturus, the gem of Boötes, below which is the beautiful Northern Crown, with the diamond in the head of Serpens beneath it. Hercules is rising, and Vega in the Lyre should be seen just flashing on the view in the northeast.
This completes the list of wonders visible at this precise time, but the stars apparently are never still, and doubtless, while the student has been passing from one constellation to another in the western and southern skies, others have been rising in the east and northeast.
At ten p.m. the Lyre is well up, and Ophiuchus and Libra can be discerned. At midnight Scorpio and Cygnus are ready to claim the attention. By two o'clock a.m., Aquila, Delphinus, and Sagittarius have risen, and at break of day Andromeda, Pegasus, and Capricornus can be seen if the student has had the courage to remain awake this length of time.
In no way can the seeming movement of the stars be better understood than by actual observation. The observer must bear in mind that the movement is an apparent one: that it is the earth that is moving and not the stars. He has only to think of the analogy of the moving train beside the one that is standing still, and the true state of affairs will at once be evident.
To further appreciate this apparent change in the situation of the constellations, the student should refer to the large plates successively. In each successive one he will note the advancement westward of the constellations mentioned above, rising in the east late at night.
The student can best get an idea of this westward apparent movement of the stars by noting the position of some bright first-magnitude star from night to night. He will soon be able to calculate the position of this star a month or more ahead, and this calculation applies to all the constellations and stars.
It is not within the scope of this work to go into this matter in detail. The author merely desires to mention this fact of apparent change of position in the stars, a fact that will be noticeable to the observer in a short time, and a fact that it is hoped he will be able to explain to his own satisfaction with the aid of the foregoing remarks.
It will be noticed that the stars on the diagrams are all numbered and lettered. The numbers refer to the magnitude of the star,—that is, the brightness of it, the first-magnitude stars being the brightest, the second-magnitude stars less bright, etc.
The letters are those of the Greek alphabet, and the student if not familiar with it is advised to consult a Greek grammar.
In the text, in referring to certain stars in the constellations, the genitive case of the Latin name of the constellation is given; for example, Vega is known as (α) Lyrae, meaning alpha of Lyra, Aldebaran as (α) Tauri, alpha of Taurus, etc.
The twilight hour affords an excellent opportunity of fixing the relative positions of the first-magnitude stars in the mind, for at that time they alone, save the planets, are visible.