« PreviousContinue »
rallax of the earth's annual orbit. And after repeated observations, they found this star, about the beginning of March, 1726, to be twenty seconds more southerly than at the time of the first observation. It now, indeed, seemed to have arrived at its utmost limit southward because in several trials made about this time, no sensible difference was observed in its situation.
By the middle of April it appeared to be returning back again towards the north; and about the beginning of June, it passed at the same distance from the zenith as it had done in December, when it was first observed. In September following, it appeared thirty-nine seconds more northerly than it was in March, just the contrary way to what it ought to appear by the annual parallax of the stars. This unexpected phenomenon perplexed the observers very much, and Mr. Molyneux died before the true cause of it was discovered.
"After this, Dr. Bradley, with another in strument more accurately adapted to his purpose, observed the same appearances, not only in that, but many other stars; and by the great regularity that appeared in a series of observations made at all times of the year, the Doctor was fully satisfied with regard to the general laws of the phenomena; and therefore endeavoured to find out the cause of them.
"He was already convinced that the apparent motion of the stars was not owing to the nutation of the earth's axis. The next thing
to post this
that offered itself was an alteration in the die rection of the plumb-line with which the instrument was constantly rectified, but this, upon trial, proved insufficient. Then he had recourse to what refraction might do; but here, also, nothing satisfactory occurred. At last, however, this acute astronomer found that the phenomenon in question proceeded from the progressive motion of light, and the earth's annual motion in its orbit: for he perceived that if the light were propagated in time, the apparent place of a fixed object would not be the same when the eye is at rest, as when it is moving in any other direction than that of the line passing through the eye and object; and that when the eye is moving in different directions, the apparent place of the object would be different."
From various observations made by Dr. Bradley upon the same stars, in the course of three years, he discovered a difference of about twenty seconds between their true and apparent places and hence, the velocity of light appears to be ten thousand times greater than that of the earth in her orbit; and therefore light will travel from the sun to the earth in eight minutes and about seven seconds.
This discovery of the aberration of light is an incontrovertible proof of the earth's motion in her orbit, and affords a most powerful confirmation of the correct principles of the solar or Copernican system; for Dr. Bradley has clearly ascertained that the motion of
light combined with the motion of our globe, occasions an apparent difference in the situa tions of the fixed stars; and as this motion evidently affects all the stars in a different manner, according to their places in the heavens, such a similarity of variations affords indisputable proof that the system of the universe as revived by Copernicus and improved by the immortal Newton, is strictly conformable with the true order of nature.
Of the Golden Number, Epact, Cycle of the Sun, Dominical Letter, &c. with easy methods of ascertaining them.
THE prime, or golden number, is, in other words, the cycle of the moon, or a circle or period of nineteen years, at the expiration of which, the changes of the moon will happen upon the same day (but upwards of an hour sooner) and she will make the same aspect with the sun as she did nineteen years before. It is said to have derived its appellation of Golden number from Julius Cæsar, who caused it to be inserted in the calendar in letters of gold. It is of great utility in ascertaining the age and changes of the moon; and may be easily found by any of our readers who are tolerably acquainted with common arithmetic.
For if they add 1 to the date of the year, and then divide by 19, the last remainder will show the golden number. It is possible, however, that nothing may remain after the division: and in that case, 19 itself is the number required.
The epact is the number of days that the solar exceeds the lunar year; the solar being vulgarly reckoned three hundred and fifty-five, and the lunar, three hundred and sixty-four days, the difference, viz. eleven days, is called the epact. Thus the epact of one year is eleven days; two years, twenty-two days, and so on; but as thirty are accounted a complete lunation, the pupil must subtract 30 when it exceeds that number, and the remainder will be the epact.
As a general rule for finding the epact, take 1 from the golden number, and divide the remainder by 3, when, if nothing remain, the epact will be the same as the dividend, viz. 1 less than the golden number: but if 1 remain, add 10 to the dividend, or if 2, add 20, and the sum will be the epact, for the Gregorian or new style.
To find the Gregorian epact for ever, it is only necessary to observe the following rule: divide the centuries of any Christian era by 4, rejecting the odd number, and multiply the remainder by 17; to this product add the quotient multiplied first by 43, to which add 86, and divide the last sum by 25: then multiply the golden number by 11, from which subtract
the last quotient, and rejecting the number 30 as often as you can, the remainder will be the epact.
The Cycle of the Sun is a revolution of twenty-eight years, at the expiration of which, the dominical, or Sunday letters, return again to the same order they were in twenty-eight years before, and the twenty-ninth year be gins a new cycle.
To find the cycle of the sun in the present century, add 25 to the date of the year, then divide by 28, and the remainder will give the sun's cycle for that year. After the year 1900, however, the rule will be as follows: To the number added in the nineteenth century, (viz. 25) the pupil must add 16 more, and from that sum subtraet 28, and the remainder will be the number, to be added to the date of the year from 1900 to 2000.
The Dominical Letter is so called because it is the lordly letter, or that which is placed in every calendar,against the first day of the week, or Sunday. There are seven Dominical letters answering to the days of the week, viz. A, B, C, D, E, F, and G: and these letters are also used for every month of the year, A being set for January first, B for the second, C for the third, &c. till they come to G, and then they commence again with A.
To account for the variation of these letters every year, we observe, that as the common computed year consists of three hundred and sixty-five days, and as seven days make a week,