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Montude. Ty fixed star, tarius, so iscovered as found to
1665, it appeared again, and disappeared in 1681. In 1715 it appeared, as it does at present, and is of the sixth magnitude. In 1686, Kircher observed x in the Swan to be a changeable star in the neck of that constellation ; and, from 20 years' observations, the period of the return of the same phases was found to be 405 days. In 1604, Kepler discovered a new star near the heel of Serpentarius, so very brilliant that it exceeded every fixed star, and even Jupiter, in apparent magnitude.
-. Montanari discovered two stars in the ship marked B and y by Bayer to be wanting : he saw them in 1664, but lost them in 1668. Mr. Goodricke has discovered the periodical variations of Algol, or B, in Medusa's head to be about 2 d. 22 h. Its greatest brightness is of the second magnitude, and least of the fourth.
The meridian height of the Sun begins now to decline; of course the day will become shorter, and the Sun's rising and setting will be later in the one case and earlier in the other. Friday, 1st, Sun rises 45 m. after 3. Sun sets 15 m. after 8 Monday, 11th, - - - 52 - - - 3. - - - 8 - - 8 Thursday, 21st, - . - 3. - - 4. . - - 57 - - 7 Sunday, - 31st, - - - 17 - - - 4. • • • 43 - - 7
Equation of Time. The following table will show what is to be added to the apparent time as exhibited on the dial, in order to set the chronometer to equal or true time, for each fifth day of the month :
m. s. Friday, - - - 1st, to the time shown on the dial ADD 3 16
ednesday, - 6th, - - - - - - - - - - - 4 11 Monday, - - 11th, - - - - - - - - - -. 4 57 Saturday, - - 16th, - -
. - - 5 33 Thursday, - - 21st, - -
• • • 5 57 Tuesday, . - 26th, - -
- - - 6 7 Sunday, - - 31st, - - - - - - - - - - - 6 1
The Sun will enter the sign Leo on the 23d day of July, at 57 m. after 1 o'clock at noon,
The Moon will be at the full on the 2d day, at 34 m.
after 4 in the afternoon; it enters its last quarter on the 10th, at 54 m. past 2 in the afternoon: the new Moon, or change, occurs at 26 m. past 6 in the morning of the 17th, and it enters its first quarter at 3 m. past 4 in the morning of the 24th.
The time of the Moon's rising for the first five days after it is full is as follows, viz.
July 3 . - : 57 m, past 8!
4 -,. 35 - 9.
5 - - - 7 - - 10 . . 6 - - - . 33 - - 10
7 - - - 56 - - 10 There will be a solar eclipse on the 17th at the time of new Moon or change, but it will be invisible in these parts. :. On the 8th, Venus will eclipse the star marked € 8, the star being 23 m. south of the planet's centre; and on the 16th it will eclipse the star 17, this star being 43' north of the planet's centre. On the 30th, the same planet will eclipse y ; and on the 31st, u II; and in both cases the stars will be 22 m. north of the planet's centre. Mercury is stationary on the 27th, and its greatest elongation will be on the 12th. Şaturn will be in opposition to the Sun at past 10 o'clock on the 20th. There will be no visible eclipse of any of Jupiter's satellites this month.
VIEW OF THE SOLAR SYSTEM.
Of the Moon. Next to the Sun, the most conspicuous of all the heavenly bodies is the Moon. The changes which it undergoes are more striking and more frequent than those of the Sun, and its apparent motions - much more rapid : hence they were attended to, even before those of the Sun were known; a fact which explains why the first inhabitants of the earth reckoned their time by the Moon's motions, and of course followed the lunar instead of the solar year.
The Moon has a peculiar motion from east to west, which is known from observation ; for if we attend to her any evening when she is situated near a given fixed star, we shall find her, in 24 hours, about 13 east of that star, and her distance continually increases, till at last, after a certain number of days, she returns again to the same star from the west, having performed a complete revolution in the heavens.From a long series of observations, it has been ascertained that the Moon makes a complete revolution in 24 d. 7 h. 43 m.; this is called the periodical month; but, if we refer to the time passed from new Moon to new Moon again, the month consists of 29 d. 12 h. 44 m., which is called the synodical month. This difference is occasioned by the earth's annual motion in its orbit. Thus, if the earth had no motion, the Moon would make a complete round in 27 d. 7 h. 4 m.; but while the Moon is describing her journey, the earth has passed through nearly a twelfth part of its orbit which the Moon must also describe before the two bodies come again into the same position that they before held with respect to the Sun: this takes up so much more time, as to make her synodical month equal to 29 d. 12 h. 44 m. The Moon's mo.
tion in her orbit is more unequal than the apparent - motion of the Sun : in one part of her orbit she moves faster, in another slower. By knowing the time of a complete revolution, we can easily calculate the mean motion for a day, or any given time, and the mean motion is called the mean anomaly. The true' motion is called the true anomaly; the difference between the two is called the equation. Now the Moon's equation sometimes amounts to 6° 18' 32". Her apparent diameter varies with the velocity of her angular motion. When she moves the fastest, her diameter is largest; it is smallest when her angular motion is slowest. Hence it follows that the distance of the Moon from the earth varies.Kepler was the first person who ascertained that the orbit of the Moon is an ellipse, having the earth in one of its foci. Her imaginary radius-vector describes equal areas in equal times, and her angular motion is inversely proportional to the squares of her distances from the earth.
The point of the Moon's orbit which is nearest the earth is called the perigee; the opposite point is the apogee. The line which joins these opposite points, is called the line of the Moon's apsides. It moves slowly eastward, completing a sidereal revolution in about 9 years.
The Moon's orbit is inclined to the ecliptic at an angle of 50, and the points where it intersects the ecliptic are called the nodes. Their position is not fixed in the heavens. They have a retrograde motion, that is to say, a motion contrary to that of the Sun; and they make a complete revolution in the heavens in a little less than 19 years. The ascending node is that in which the Moon rises above the ecliptic towards the north pole ; the descending node, that in which she sinks below the equator towards the south pole.
The mean distance of the Moon from the earth is 240,000 miles, and, as their respective diameters are as about 11 to 3, the bulk of the earth is to that of the Moon as 113 : 33; or as 1331 : 27 ; or as 49:1. That is, the earth is 49 times as large as the Moon.
The different appearances or phases of the Moon constitute some of the most striking phenomena of the heavens. When she emerges from the rays of the Sun in an evening, she appears after sun-set as a small crescent just visible. The size of this crescent increases continually, as she separates to a greater distance from the Sun; and when she is exactly in opposition to that luminary, she appears under the form of a complete circle. This circle now declines, changing into a crescent as she approaches nearer that luminary, exactly in the same manner as it had increased, till at length she disappears alto
gether, plunging into the Sun's rays in' the morning at sun-rise.
The Moon is an opaque globe, like the earth, and shines only by reflecting the light of the Sun ; therefore, while that half of her which is towards the Sun is enlightened, the other half must be dark and invisible: hence she disappears when she comes between us and the Sun, because her dark side is then towards us. When she has gone a little way forward, we see a little of her enlightened side; which still increases lo our view as she advances forward, until she comes to be opposite to the Sun, and then her whole enlightened side is towards the earth, and she appears with a round illuminated orb, which we call the full Moon, her dark side being then turned away from the earth. From the full she seems to decrease gradually, as she goes through the other half of her course, showing less and less of her enlightened side every day, till her next change or conjunction with the Sun, and then she disappears as before. .
The axis of the Moon being nearly perpendicular to the ecliptic, it has scarcely any difference of seasons. One half of the Moon's surface has no darkness at any time, the earth constantly affording it a strong light in the absence of the Sun; while the other half has a fortnight's darkness and a fortnight's light by turns. Our earth, unquestionably, performs the office of a Moon to the Moon, waxing and waning regularly, but appearing thirteen times as large, and, of course, affording thirteen times as much light as she does to us. When she changes to us, the earth appears full to her; and when she is in her first quarter to us, the earth is in the third quarter to her, and vice versa. To the Moon the earth seems to be the largest body in the universe, and must indeed be a most magnificent sight. . ;
We come now to speak of Eclipses, which formerly were subjects of dread and terror, but which