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

THE SUN,

While affording such accurate methods of determining time, does not directly furnish that by which we are accustomed to be guided, so that astronomers are wont to speak of solar time, as mean solar time, and apparent solar time, the former being that kept by our clocks and the latter that from which the former is estimated. If the earth moved around the sun at an unvarying rate, then each time the sun threw the shadow of a vertical rod directly north (in the northern hemisphere) would indicate noon at the point where the rod was located; and the interval between two such successive shadows would be exactly one day, and we could divide the time into twenty-four equal spaces and call each space one hour. Now, this is what we imagine the earth to do; or rather, for the sake of convenience, and because the results are the same in either case, we conceive the sun to move around the earth, and make a mean or mock sun, move around the earth in a day of twenty-four hours of equal unvarying length, and call this a mean solar day. But the true or apparent solar day is considerably different from this, being sometimes less and sometimes more than twenty-four hours in length, the true sun reaching the meridian as much as 16¼ minutes before or after the mean sun, both reaching it together only four times each year, viz.: On April 1, June 15, September 1, and December 24. Of course it would be impossible to construct a time-piece that would keep pace with the true sun. Indeed, it is difficult enough to construct one that will keep with the mean sun. But all difficulty is obviated by making clocks whose rate of error can be determined. This rate being known it is easy to estimate the correction, and thus obtain exact time. For example, suppose a clock to gain 0.24 of a second per day, then in two days it will gain two times 0.24 of a second, and in three days three times 0.24 of a second, etc. These amounts subtracted from the noon-time of the clock would give the correct noon. For any other hour, a part of the 0.24 depending on the number of hours after noon must be subtracted. If the clock loses, then in a similar manner the proportioned loss must be added. In actual practice, we may say that even the best chronometers do not keep exact time; and every one has to be “corrected” in the manner indicated.

To obtain mean time from apparent time we apply to apparent time a correction called the equation of time. Thus, at noon, on October 1st, in Washington, the equation of time is minus 10 minutes 34.58s., which means that if from 12 hours we subtract 10m. 34.58s. we shall have 11h. 49m. 25.42s., the mean time of apparent noon, or noon as indicated by the north shadow of the vertical rod. Or, if at 10m. 34.58s. after the sun crosses the meridian we set our watch at 12 o’clock, we shall have exact mean time. On the 31st of October the real sun will be 16m. 17.56s. ahead of the mean sun, while on the 31st of December it will be 3m. 38.11s. behind the mean; that is, will reach the meridian at 12h. 3m. 38.11s. p. m. mean time.

Beside the ordinary clocks, the chronometers used by navigators keep mean solar time, and family almanacs usually in some form give the “equation of time” under the headings, “clock slow” or “fast,” or “sun south.” Astronomers use also another kind of time called “sidereal,” of which we may have something to say in the future.

On the 1st, 16th and 31st of this month the sun rises at 5:57, 6:13 and 6:30 a. m., and sets on the same days at 5:42, 5:18 and 4:58 p. m. respectively, showing a decrease of one hour and seventeen minutes in thirty days, or at an average rate of 2m. 34s. per day. On the 18th and 19th the sun will be eclipsed, entering the moon’s shadow at 10:20 p. m. on the 18th in longitude 132° 0.6' east, and in latitude 63° 30.5' north; and leaving it at 2:15 a. m. on the 19th in longitude 134° 22.7' west, and in latitude 33° 25' north. Greatest eclipse occurs at 12:18 a. m. on the 19th. Magnitude, 0.638. As the entire eclipse is during our night, it will of course be invisible to us, but can be seen by the inhabitants of the northeastern part of Asia and the northwestern portion of North America.

THE MOON

Will be eclipsed on the 4th, beginning at 3:07 p. m. and ending at 6:41 p. m., Washington mean time. The beginning of totality will be at 4:07 p. m., and the ending at 5:40 p. m.; the middle at 4:53 p. m. Magnitude, 1.533. As the moon in this section does not rise till about 5:20, only the latter half of the eclipse will be visible. On the 16th, the moon rises at 3:45 a. m.; on the 1st, sets at 2:21 a. m., and on the 31st at 3:11 a. m. On the 4th, at 4:52 p. m., full; on the 11th, at 9:21 a. m., enters upon its last quarter; on the 18th, at 7:23 p. m., new moon; and on the 26th, at 11:46 p. m., enters upon its first quarter. Is nearest to the earth on the 7th, at 8:48 a. m., and farthest away on the 23d, at 7:48 a. m.

MERCURY

Presents himself as an object of interest for a few mornings before and after the 5th, the day on which he reaches his greatest western elongation, 17° 58' from the sun; and as he rises nearly an hour and a half earlier than the latter body, may with a little care be clearly distinguished. His times of rising are as follows: On the 1st at 4:33; 16th, at 5:12; and 31st, at 6:22 a. m. His motion is direct and amounts to 43° 46'. As he moves away from the sun, after 6:00 a. m. on the 3d, his diameter diminishes from 7.4 to 4.6. On the 17th, at 7:58 p. m., he is 2° 1' north of the moon.

VENUS

Remains a morning star during this month, shining, toward its close, with somewhat decreased brilliancy. Her motion is altogether eastwardly, and amounts to 33° 12' 46, diameter changing from 21.6 on the 1st to 17.6 on the 31st. On the 1st, she will rise at 2:13; on the 16th, at 2:35; and on the 31st, at 3:01 a. m. On the morning of the 7th the trio, Jupiter, Venus and Alpha Leonis (Regulus) will give an exhibition worth much more than all the trouble it costs to obtain the view. Jupiter rises first at 2:30; a few minutes later, and to the south, Venus appears, and almost at the same time a little northward, Regulus: the three presenting a combination rarely witnessed. On the 6th, at 11:00 a. m., Venus will be 1° 15' south of Jupiter; on the 7th, at 7:00 p. m., 55' south of Regulus; and on the 15th, at 2:48 a. m., 3° 35' north of the moon.

MARS,

Though accounted an evening star, will be above the horizon in the day time most of the month, on the 1st rising at 8:55 a. m. and setting at 7:01 p. m.; on the 16th, rising at 8:50 a. m. and setting at 6:32 p. m.; and on the 31st, rising at 8:46 a. m., and setting at 6:08 p. m. Its declination on the 31st is 21° 34' south. Motion for month direct and amounting to 22° 28'. Diameter decreases from 4.6 to 4.4. On the 21st, at 6:15 a. m., 4° 10' south of the moon.

JUPITER

(Together with Venus) makes this month’s mornings brilliant. Rising on the 1st at 2:26, on the 16th at 1:44, and on the 31st at 12:57 a. m.; and his diameter increasing from 31 to 32.2, makes each stay appear longer and each return brighter. His motion is direct and equals 4° 56' 51. On the 6th, at 11:00 a. m. he is 1° 15' north of Venus; and on the 14th, at 11:57 a. m., 4° 42' north of the moon.

SATURN

Rises on the 1st at 9:29 p. m., and sets on the 2nd at 12:09 p. m.; rises on the 16th at 8:29 p. m., and sets on the 17th at 11:09 a. m.; rises on the 31st at 7:29 p. m., sets 10:02 a. m. on November 1st. His motion, 42' 26, is retrograde. Diameter increases from 17.8 to 18.8. On the 5th, at 8:00 a. m. is stationary; and on the 9th, at 3:50 p. m., 3° 30' north of the moon.

URANUS

Will be morning star throughout the month, moving eastwardly 1° 40' 45; diameter increasing about one-tenth of a second. On the 8th, at 3:00 a. m. will be 1° 10' south of Mercury; on the 16th, at 7:19 p. m., 2° 5' north of the moon. Its times of rising are as follows: On the 1st, 5:13 a. m.; on the 16th, 4:19 a. m.; on the 31st, 3:24 a. m.

NEPTUNE’S

Motion for the month is 44' 22 retrograde; diameter, 2.6. On the 7th, at 11:44 a. m., he is 1° 33' north of the moon. His night ascension on the 31st is 3h. 21m. 23.5s., and declination 16° 36' north, about 1h. 8m. west of Aldebaran in the constellation Taurus. He rises on the 1st, at 7:38; on the 16th, at 6:39, and on the 31st, at 5:39 p. m.