GEOGRAPHY OF THE HEAVENS FOR DECEMBER.

BY PROF. M. B. GOFF, Western University of Pennsylvania.

THE TELESCOPE.

Since much that we know about the heavenly bodies, has been revealed to us through the medium of the telescope, it may be advisable to give some slight account of this instrument. As early as 1608 it appears to have been invented in Holland by a professor of mathematics named James Metius, though the honor is claimed by their friends for several other parties, among them Lipperhey and Jansen, spectacle makers in the town of Middleburg. The claims of Jansen were supported by Peter Borelli, in a small volume published in 1655, entitled “De Vero Telescopii Inventore,” and he was for a long time regarded as the inventor. The story runs that Jansen had shown a telescope sixteen inches long to Prince Maurice and Archduke Albert, who realizing the importance of such an invention in war, induced him to keep it a secret. But the narrative given by Borelli rested on such a slight foundation, that it obtained but little credence. Later evidence shows that Hans Lipperhey, on the 2d of November, 1608, made application to the states-general of Holland for a patent for “an instrument to see with at a distance,” but was refused on the ground that the invention was already known. While there is little doubt but that the discovery was, as claimed, made in Holland, it is also highly probable that great efforts were put forth for some time to keep the matter a profound secret. At least, no results were published to the world until made known by Galileo in the manner thus related by Professor Newcomb: “About six months after the petitions (for patents to the states-general of Holland) of Lipperhey and Metius, Galileo was in Venice on a visit, and there received a letter from Paris, in which the invention was mentioned. He at once set himself to the re-invention of the instrument, and was so successful that in a few days he exhibited to the astonished authorities of the city a telescope magnifying three times. Returning to his home in Florence, he made other and larger ones which revealed to him spots on the sun, the phases of Venus, the mountains of the moon, the satellites of Jupiter, the seeming handles of Saturn, and some of the myriads of stars, separately invisible to the naked eye, but whose combined light forms the milky way. But the largest of these instruments magnified only about thirty times, and was so imperfect in construction as to be far from showing as much as can be seen with a modern telescope of the same power.”

The telescope has been aptly compared to an eye. In the eye nearly parallel rays of light fall on a lens, and this lens throws an image. In the telescope, nearly parallel rays of light fall on a lens, and this lens throws an image, and then another lens enables the eye to form an image of that image by again rendering the rays parallel; these parallel rays entering the eye just as rays do in ordinary vision. The efficiency of the telescope depends on its power of illuminating and magnifying. If the object glass (in a “refractor,” as an ordinary spy-glass, the lens next the object viewed) be twenty times greater than the pupil of the eye, it receives twenty times more light, and forms an image theoretically twenty times as bright (though practically much of the light is lost by reflection from and passage through the object glass). The magnifying power depends on the relative focal length (the distance from the lens to the image) of the object glass and the eye-glass (the lens next the eye). For example, if the focal length of the object glass be twenty-five inches, and that of the eye-glass one-half inch, then the magnifying power is represented by the quotient of twenty-five by one-half, which is fifty. In order to obtain a good image the illuminating power must be good and the magnifying glass (eye-piece) perform its work well.

Since the time of Galileo, refracting telescopes have been of course much improved. Their size also has increased until they are now constructed with object glasses twenty-six inches in diameter; and it is reported that one is projected for the Lick Observatory in California, which is to have an objective thirty-six inches in diameter. Reflecting telescopes, so called because instead of refracting or bending the rays of light, they reflect them from a concave mirror, have been constructed with circular mirrors six feet in diameter, and it is believed by some makers that they can be successfully operated when the mirrors are as much as seven or eight feet in diameter.

It must not be supposed, however, that in general the greater the telescope, the more successful the observations. Both calculation and experience indicate a limit beyond which increase in size, even if it affords greater power, diminishes in clearness. And it is of little value to us that we bring the moon apparently within forty miles, if we can not distinguish its features—if the face of the “man in the moon” should be a mere blur, like a blot on a piece of paper. It is, in fact, exceedingly doubtful whether the moon has ever been seen through the telescope so well as it would be seen with the naked eye at a distance of 500 miles.

To afford an idea of what has been done in the manufacture of these instruments we mention the following: Of refractors, the one in the United States Naval Observatory at Washington has an object glass twenty-six inches in diameter; that of Mr. R. S. Newall, Gateshead, England, an object glass of twenty-five inches; Observatory of Harvard College, Cambridge, Mass., fifteen inches; Allegheny Observatory, connected with the Western University of Pennsylvania, at Allegheny, Pa., thirteen inches; Michigan University, at Ann Arbor, Mich., twelve and five-tenths inches; Middletown University, Connecticut, eleven inches. Of reflectors, the one constructed by the Earl of Rosse, at Parsontown, Ireland, has a mirror six feet in diameter; the Observatory of Melbourne, Australia, four feet; and Mr. S. Lassell, Marblehead, England, two feet in diameter.

THE SUN

During this month makes us his shortest visit. This he does on the 21st, making his stay about 9h. 16m. in length. Winter begins theoretically the same day at 4:51 a. m., when the sun stands still before he starts on his northern journey, and the astronomers say that the sun enters Libra. On the 24th at 2:00 p. m., sun is 90° east of Uranus, that is, is quartile with this planet; on the 12th, at 2:00 a. m., he is 180° from Jupiter, that is, in opposition; on the 31st, at 11:00 p. m., nearest the earth; on the 1st, 16th and 30th he rises at 7:05, 7:18 and 7:24 a. m., and sets at 4:34, 4:35 and 4:44 p. m., respectively. Twilight ends on the 16th at 6:17 p. m.

THE MOON

Presents the ordinary phenomena of its changes as follows: Full on the 2d, at 1:51 p. m.; last quarter on the 9th, at 6:22 a. m.; new on the 17th, at 8:16 a. m.; first quarter on the 25th, at 8:13 a. m. It rises on the 15th, at 6:08 a. m., and sets on the 1st and 30th, at 5:21 a. m. and 5:13 a. m. respectively. Is nearest the earth on the 2d, at 9:30 p. m., and again on the 31st, at 10:48 a. m. Is farthest away from the earth on the 16th, at 11:06 p. m. Runs highest on the 4th, on which date its altitude equals 68° 1′ 33″; and on the 31st, when its altitude amounts to 68° 1′ 56″. It runs lowest on the 17th, when the elevation is 30° 56′ 19″ in latitude 41° 30′ north.

MERCURY

Presents us this month with an unusual number of phenomena, none of which, however, are of a striking character. He rises on the 1st at 8:28 a. m.; on the 16th, at 8:55 a. m.; on the 31st, at 7:48 a. m.; sets on the corresponding days at 5:16, 5:53 and 5:20 p. m.; that is, during the entire month setting from one-half hour to one and one-third hours later than the sun; and thus being visible to a careful observer for perhaps ten or twelve days both before and after the 17th, the day on which he reaches his greatest distance (20° 12′) east of the sun. His motion during the first seventeen days is 30° 33′ 44″ direct, and for the remaining fourteen 5° 9′ 55.5″ retrograde. Diameter increases from 5.2″ to 9.6″. At 3:00 p. m. on the 4th he is 1° 26′ south of Mars; on the 19th, at 6:34 a. m., 6° 27′ south of the moon; on the 25th, at 10:00 a. m., stationary; at midnight on the 29th, 2° 25′ north of Mars; and on the 30th, at 5:00 a. m., reaches its nearest point to the sun.

VENUS

Is now on the wane, decreasing from 14.2″ to 12.4″ in diameter, though still an object of beauty in the morning sky. She rises at 4:03 a. m. on the 1st; at 4:36 a. m. on the 16th, and at 5:10 a. m. on the 31st. Her motion is direct and equals 38° 30′ of arc. On the 14th, at 4:37 a. m., she is 1° 15′ south of the moon.

MARS

Makes a very poor showing, his diameter being small, only about 4.2″, and his rising and setting nearly the same as that of the sun. At 8:35 a. m. he rises and at 5:33 p. m. sets on the 1st; on the 16th, rises at 8:25 a. m., and sets at 5:25 p. m.; and on the 31st, rises at 8:11 a. m., and sets at 5:21 p. m., remaining above the horizon on an average of about nine hours each day. His motion is direct and amounts to 25° 57′ of arc. On the 18th, at 1:35 p. m., he is 5° 59′ south of the moon. His declination on the 1st is 24° 17′ south, and on the 31st, 22° 58′ south.

JUPITER

Is morning star throughout the month, rising November 30th at 11:07 p. m., and setting December 1st, at 12:19 p. m.; on the 15th, rising at 10:10 p. m.; setting next day at 11:22 a. m.; and on the 30th rising at 9:10 p. m., and setting on the 31st at 10:23 a. m. Direct motion 31′ 39″; retrograde motion 12′ 22″. Diameter increases from 36.2″ to 39.6″. On the 8th, at 10:52 a. m., 4° 10′ north of the moon; about midnight on the 20th, stationary.

SATURN

During the former part of the month will be a morning star, but during the latter and greater part an evening star, though shining during most of the night time. He rises on the 1st at 5:19 p. m., and sets on the 2d at 7:57 a. m.; rises on the 16th at 4:14 p. m., and sets on the 17th at 6:52 a. m.; rises on the 31st at 3:11 p. m., and sets on the first day of the new year at 5:47 a. m. His motion, 2° 39′ 15″ of arc, is retrograde; and his diameter diminishes about two-tenths of a second of arc. On the 3d, at 5:48 a. m., 3° 15′ north of the moon; on the 12th, at 2:00 a. m., 180° west of the sun; and on the 30th, at 1:48 p. m., 3° 16′ north of the moon. As mentioned in a former number of The Chautauquan, from this time for several months will be the most favorable for seeing the rings (the “handles” of the seventeenth century) of this planet.

URANUS

Has a direct motion of 35′ 1″ of arc during the month, and its diameter increases two-tenths of a second. On the 10th it will be found, at 11:08 a. m., 1° 37′ north of the moon; and on the 24th, at about 2:00 p. m., 90° west of the sun. It will also appear as an evening star, rising at 1:28 a. m., and setting at 1:26 p. m. on the 1st; rising at 12:30 a. m., and setting at 12:28 p. m. on the 16th; and rising at 11:33 p. m. on 30th, and setting at 11:39 a. m. on 31st.

NEPTUNE.

“Distance lends enchantment to the view.” Our imagination may readily picture a huge ball having a diameter of 34,500 miles, and a density of 1.15 times that of water, at a distance of 2,775 millions of miles from the sun, and making its way around it in a period of about 165 years, at the rate of about 3.36 miles per second; but methinks most of us would prefer to remain where we are rather than migrate to a “land of liquids” and spend our lives in swimming through oceans of liquefaction. Neptune rises on the 1st at 3:34 p. m., and sets on the 2d at 5:30 a. m.; rises on the 16th at 2:35 p. m., sets on the 17th at 4:29 a. m.; rises on the 31st at 1:35 p. m., sets on January 1st at 3:29 a. m. Retrogrades 40′ 35″ of arc. Diameter, 2.6″.