Astronomical Occurrences

In JULY 1819.

THE Sun enters Leo at 48 m. after 6 in the evening of the 23d of this month, and he rises and sets during the same period as in the following

TABLE

Of the Sun's Rising and Setting for every fifth Day.

July 1st, Sun rises 46 m. after 3. Sets 14 m. after 8

3

Equation of Time.

8

8

8

7

The numbers in the following table must be added to the time as shown by a good sun-dial, in order to have the mean time at the instant as shown by a well regulated clock.

Thursday, July 1st, to the time by the dial add 3 16

Tuesday,

Full Moon,
Last Quarter, 14th
New Moon, -22d

First Quarter, 30th

Moon's Passage over the first Meridian.

Should the weather prove favourable, the Moon may be seen to pass the first meridian, adopted by the astronomers of this country, at the following times during this month: viz,

Eclipses of Jupiter's Satellites.

There will be four eclipses of Jupiter's first satellite visible at the Royal Observatory this month, and two of his second satellite: the immersions will take place within a few seconds of the following times:

1st Satellite, 3d day, at 59 m. after 11 at night.

11th
19th

53 16

1 morning. 10 evening.

24th

21

O morning.
11 night.
2 morning.

Form of Saturn's Ring.

July 1st {Conjugate axis

Transverse axis

Other Phenomena.

1.000

-0.098

He will

Mercury will be in superior conjunction at past 3 in the afternoon of the 1st of this month. also be in conjunction with a in Leo on the 27th, when the star will be 7' south of the planet. The Moon will be in conjunction with a in Scorpio at 34 m. after 3 in the morning of the 5th; with Saturn at 59 m. after 6 in the evening of the 12th; with ẞ in Taurus, at 30 m. past 1 in the morning of the 19th; and with Pollux at 2 in the afternoon of the 21st.

α

On TIME and its APPLICATION.

[Continued from p. 151.]

One of the most important applications of time, is that which constitutes the science of chronology, or

the arrangement of historical events according to the order of the epochs at which they happened. One of the chief bases upon which this science rests, is astronomy. Accordingly, we find that chronology, comparatively speaking, is but a modern science; for until astronomy had made great progress, and the doctrine and calculation of eclipses become well understood, their application to the purpose of determining the epochs of ancient events could not take place. Besides the natural and obvious divisions of time, as years, months, weeks and days, which result from the immediate revolutions of the heavenly bodies, there are others which result from the less obvious consequences of these revolutions, and which are denominated cycles and periods. These cycles have chiefly arisen from the incommensurability of the revolutions of the heavenly bodies with each other. The apparent revolution of the Sun about the Earth being arbitrarily divided into 24 h., becomes the basis of all our mensuration of time. But neither the annual motion of the Sun nor that of the other heavenly bodies can be measured exactly by hours and their multiples. The Earth, for instance, occupies nearly 365 d. 5 h. and 49 m. in making one revolution in her orbit, which period constitutes the length of our year; and the Moon requires 29 d. 12 h. and 44 m. to complete one revolution about the Earth: it was therefore to express a number of complete revolutions in years and days only that cycles were introduced. A cycle is, therefore, such a period of time, which, after several revolutions of the same body, bring it to the same place in the heavens, or to the same division of time.

The cycle of the Sun is a period of 28 years, in which the days of the month return again to the same days of the week; the Sun's place to the same signs and degrees of the ecliptic on the same months and days; or at least so nearly so as to deviate only about one degree in 100 years. The leap years also begin

the same course over again with respect to the days of the week on which the days of the month fall.

The cycle of the Moon is a period of 19 years, after which period the conjunctions, oppositions, and other aspects of the Moon return to the same state as they were at the commencement of this period, and happen within about an hour and a half of the same time. The Roman indiction is a period of 15 years, established for the purpose of indicating the time of certain payments due to the republic. This indiction was established in the 312th year of the christian

era.

If

The christian era commenced in the 9th year of the solar and the 1st of the lunar cycle; therefore to find the year of the solar cycle at any time, add 9 to the given year of the christian era, and divide the sum by 28; the quotient will be the number of cycles, and the remainder the cycle for the given year. there be not any remainder, the cycle is 28. Thus, if the cycle were required for 1819, we should have 1819 +9 = 1828, which divided by 28, gives 65 for the number of complete cycles, and 8 for the year of the present cycle. To find the lunar cycle, add 1 to the year of Christ, and divide by 19; the quotient will be the number of cycles elapsed in the interval, and the remainder will be the state of the cycle at the given time. Hence for the present year we have 1819 +1=1820, which divided by 19, gives 95 for the number of complete cycles, and 15 for the year of the present cycle. Again, if the Roman indiction were required, 312 must be subtracted from the given year of the christian era, and the remainder divided by 15. For the present year we have 1819-3121507; and 1507 15 100, with 7 ÷ = for the remainder, which is therefore the indiction for this year. The cycle of Easter, or the Dionysian period, is a space of 532 years, produced by multiplying the two former cycles of 28 and 19 years together. The Julian period is another division of time, which

was anciently much used, and is now occasionally referred to, especially in some works on astronomy. This period embraces a duration of 7980 years, and is equal to the continued product of the three preceding periods; for 28 × 19 × 15 7980. As the christian era commenced in the 4713th year of this period, whenever the Julian year is required, it will be sufficient to add this number to the current year. For example, if it were required to find the Julian year answering to 1819, we have 4718 + 1819 = 6532, the year required.

The principal application, however, of modern astronomy to the purposes of chronology is in ascertaining the uncertain dates of ancient events; and in this respect the powers of Newton conferred the greatest service on that science. The following specimen will show the manner in which his astronomical knowledge was employed for this purpose.

The sphere was formed by Chiron and Museus at the time, and for the purpose of the Argonautic expedition, as is evident from the reference which several of the asterisms have to that event. At this time the cardinal points of the equinoxes and solstices were placed in the middle of the constellations Aries, Cancer, Libra, and Capricorn. Newton establishes this point by a consideration of the ancient Greek calendar, which consisted of 12 lunar months, and each month of 30 days, and therefore required an intercalary month, began sometimes a week or a fortnight before or after the equinox or solstice; and hence the first astronomers were led to the preceding disposition of the equinoxes and solstices; which is also confirmed by the direct testimonies of Eudoxus, Aratus, and Hipparchus. This being established as a basis, Sir Isaac reasons in the following manner.

At the end of the year 1689, the equinoctial colure cut the ecliptic in 6° 44′ of Taurus, and by this reckoning the equinox was then gone back 36° 44'