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Violeta, published in honour of those who pretend to great knowledge with little study,) that the common object of all the sciences, and their utility to man, may be divided into two parts: one of which is, to obtain a less imperfect knowledge of the Supreme Being, by which the heart of man is disposed to render bim more profound and sincere worship; the other is, to render men more social; for, by imparting to each other the productions of their understanding, they become united, as we may say, in spite of ocean and distance.
I'he word time, though of frequent recurrence in conversation, is by no means easy to define. Perhaps we may call it the measure of existence or the regular flow of duration.
Duration, if unlimited, is called eternity: time is therefore a limited duration.
We have already observed (4) that time is measured by the revolution of the earth about its axis, which is a sublimely regular motion. By this revolution, which is from west to east, the sun, the fixed stars, and other celestial objects, are made, apparently, to perform a revolution round the earth from east to west.
Besides the motion of the earth about its axis, it continually revolves about the sun from west to east, in an elliptical* orbit, at the mean or average distance of about 95 millions of miles. The period of this revolution is called a tropical year.
The mean diameter of the earth’s orbit is therefore nearly 190, or, as we say, in round numbers, nearly 200 millions of miles.
This distance, however, compared with that of the nearest of the fixed stars, is a mere point, so very insignificant, that they are seen from any part of our orbit, exactly in the same point of the heavens.
From the observations of the most skilful astronomers, made with the finest instruments, it is concluded that Sirius, or the Dog Star, which, from its superior magnitude and brilliancy, has been considered the nearest of them all, cannot be at a less distance from the sun than ten millions of millions of miles ; some have supposed that it is not nearer than 300 millions of millions; from which the student will infer, that with respect to the distance of these splendid orbs from each other, man, with all his boasted science, is reduced to mere conjecture; yet he must by no means imagine that the observations made upon
* Elliptical, somewhat oval.
the starry heavens with powerful instruments, have been made in vain. They have, on the contrary, been of incalculable advantage in establishing many important and useful facts, upon which are founded the regulations of the calculation of time; and, more especially, in perfecting the science of navigation. They have opened to our view, in an awe-inspiring manner, the immensity, beauty, and harmony of the works of the Almighty, profusely disseminated in the fearful depths of illimitable space, and thus, by showing man how limited is his sphere of action in this vast creation, assured him of the insignificance and folly of human pride.
In a clear frosty night, we think we see with the naked eye an innumerable multitude of stars; but the fact is that we cannot, under the most favorable circumstances, see more than two or three thousand above the horizon at the same time; but we are informed that Dr. Herschel saw 588 stars in the field of his telescope at once, and computed, in a very small portion of the celestial hemisphere 258000 stars.
To give the student a clearer idea of the distance of these orbs from each other, let us suppose the earth stationary at the nearest probable distance, and no doubt we might say possible distance, from Sirius, namely 10 millions of millions of miles; that the creation took place 6000 years ago, (the year consisting of 3651 days;) and that a cannon ball, shot from Sirius at that time, had continued to move, with its initial velocity of 1200 feet per second, in a direct line towards the earth : we find that it would have travelled 430330909091 miles, or not quite the two hundred and thirtieth part of the distance; and hence, that it would require, to perform the whole of its flight, upwards of one million three hundred and eighty thousand years.
Let us close this article with the sublime words of the Prophet Isaiah, c. 40, v. 26 : “Lift up your eyes on high, and behold who hath created these things, that bringeth out their host by number: he calleth them all by names, by the greatness of his might, for that he is strong in power; not one faileth.”
295. The extremities of the earth's axis—which points, with respect to the diurnal revolution, appear stationary—are called the North and South poles.
The equator is an imaginary line which passes round the earth and is equally distant from either pole.
A line drawn directly over the head of an observer, and
pointing north and south, is called a meridian line, or the meridian of the place where he stands. This line, in its extension round the earth, passes through the poles, and cuts the equator at right angles.
When a star, in its apparent revolution, passes the meridian of any place, it is said to culminate, and the time which elapses between any two culminations of a fixed star, or one entire revolution of the earth about its axis, is called a sidereal day, from the Latin sidus, a star.
296. From the time that the sun's centre passes the meridian till it returns to the meridian again, is called a solar day.
As the sun is not in the centre of the earth's orbit,* the motion of the earth in its orbit is somewhat accelerated by a nearer approach to, and retarded by a recession from that luminary. From this, as well as other causes, the solar day is not of uniform length; it is, however, the medium length of a solar day, or average length of all the solar days in a year, which is the time shown by a well-regulated clock; that is to say, we divide the time of a mean solar day into 24 hours, such as shown by the clock. This is the time in common use,
called by astronomers
24 hours ...
Mean Solar, or Common Time.
..1 hour, h.
.1 day, d. 7 days ...
1 week, w. 2 weeks..
1 fortnight. 4 weeks....
.1 month, mo. 1
1 common or or 365 days 6 hours....... A labourer's month consists of 26 working days, and a mercantile month of 30 days.
The odd 6 hours of the Julian year are not reckoned till they amount to a day: a common year, therefore, consists of 365 days, and every fourth year, cailed bissextile or leap-year,
The year is also divided into 12 calendar months, which, in number of days, are irregular, as follows:
Julian year, y.
* Orbit, the figure circumscribed by the revolution of a planet.
1st month... January......... 31 days.
31 6th....... June
30 7th... July....
December... 31 When the year is exactly divisible by 4, it is leap-year, in which the second month (February) has 29 days.
297. The solar or tropical year is found, by observation, to consist of 365 d. 5 h. 48 m. 51 sec.
Now, as the motion of the earth in its orbit and that on its axis are both in the same direction, it is evident that the earth must perform rather more than one entire revolution on its axis, while the sun, at a mean rate, appears to pass from any meridian to the same meridian again, and that the successive increments* will, in one year, amount to one entire revolution of the earth about its axis ; that is, the earth will, in one year,
perform one more revolution on its axis than the number of apparent revolutions made by the sun in the same time. The ratio, therefore, of the sidereal day to the solar, being inversely as the number of each which composes the solar year, will be nearly as 365 to 366, or el, and, therefore, if we multiply 24 hours by 365 and divide by 366, the result will be the length of a sidereal day, in mean solar time, very nearly.
365 X 24 Thus :
= 23 h. 56 m. 4 sec. the length of a side
366 real day, or the time in which the earth performs one revolution on its axis.
If we subtract the length of the sidereal from that of the solar day, we find that the difference is 3 m. 56 sec., or very nearly 4 minutes. Hence, a fixed star will culminate on any day nearly 4 minutes, by the clock, sooner than on the preceeding day. If, therefore, you stretch a meridian line over head, and with a plumb line set a post directly underneath, you may
* Increments, portions of increase.
fix in the post a piece of tin with a very small hole in it, through which you may observe the culmination of any fixed star. A står of the first magnitude, and having a good elevation, is best; such as Aldebaran in Taurus, Bellatrix in Orion, or any other fixed star, which you can easily recognize. Then, if, by your clock or watch, the star culminates about 4 minutes sooner each night than on the preceding, the clock or watch is well regulated.
If, by the clock, the star culminates more than 4 minutes sooner, the clock goes too slow, and the excess of time above 4 minutes is the time the clock has lost. If, on the contrary, the star does not culminate till the same hour as on the preceeding evening, the clock has gained 4 minutes ; and if it does not culminate till past the hour by the clock, 4 minutes added to the time past the hour, will show how much the clock has gained.
298. For the exact time of day, observe, through a screen* of some kind, to protect the eye, the time when the
sun's centre passes the meridian. This will be the mean solar noon, if the observation is taken on or about the 15th of April, the 15th of June, the 1st of September, or the 24th of December. If the observation is taken between those times, look at the Almanack for the equation of time: then; if the sun is fast of the clock, subtract from the apparent noon ; if slow, add to the apparent noon, the equation of time, which will give the true time by the clock.
299. The true solar year not being equal to the year of 365 days 6 hours upon which Julius Cæsar established the leapyear, (the difference, 11 m. 9 sec., amounting, in about 130 years, to a whole day,) Pope Gregory XIII. ordered that every 100th year, which, according to the Julian method, would be bissextile, should be a common year of 365 days, except those centurial years, which, when we cut off two ciphers on the right, are divisible by 4, which were to remain bissextile. This is called the Gregorian method, or new style.
300. If the fixed stars could be seen in the daytime, the sun would, by the earth's daily motion in its orbit, from west to east, appear to progress among them in the same direction. The great circle or path described by the sun among the fixed
* Very important. The omission would certainly injure the sight of the observer.