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line, suppose the star to be upon it. As the earth turns on its axis, the star is left behind, and after a complete revolution, the meridian again arrives to it, this interval is called a siderial day, or day as determined by the stars, and to ascertain this day, or its length, we must have some means of determining with the utmost exactness when the star is on the meridian. This is accomplished by means of the transit instrument, invented by Huygens, and shown in the engraving below.
The ordinary transit instrument consists of a telescope, A B, of any convenient length, fixed firmly at right angles to a conical hollow axis, E F, the extremities of this axis are truly turned, and rest in two angular bearings which are called Y’s, since they are not unlike this letter, the instrument can be lifted out of these bearings, and reversed, so that the ends E and F may change places. The end of the axis F, is furnished with a small graduated circle C, for the purpose of reading the elevation, or altitude of the body observed, and at D, is a small lamp, the light of which shining into the hollow arm E, is reflected by a reflector inside the tube, down to the eye. The object of this illumination is to make a system of fine lines, usually raw silk, or spiders-web, visible at night, at the same time with the star. In looking into the transit telescope, five of these lines are usually seen, shown in the engraving. A B is, by means we cannot now describe, located
Transit INSTRUMENT. 51
as exactly in the meridian as possible. It will be seen that when the axis of the transit telescope, E F, is placed due east and west, and also made perfectly horizontal by means of the spirit level H, the telescope A B, will move in the meridian, i. e., it will, if
directed to the heavens, mark the exact situation of the meridian, at the time, of the particular place where the instrument is located. We are thus furnished with the means of determining, with the greatest exactness, the precise time of a siderial revolution of the earth, and as the apparent time of moon, or twelve o'clock, is precisely the instant when the sun’s centre is on the meridian, we are also enabled to determine, with considerable precision, the local time, or clock time at the place. The transit instrument and the astronomical clock, are the two chief instruments of the observatory, and by their means, the positions of celestial objects can be ascertained with the utmost micety. It would be out of place for us to describe more minutely these invaluable aids to the astronomer, and we pass to consider in the next chapter, the “Calendar,” or the division of the year into months, weeks, and days, and at the same time we shall give an historical sketch of its gradual progress to the present state of perfection. It is a difficult thing to comprehend fully, or even partially, the relative dimensions, situation, and movement of our globe. We are so accustomed to look around us and behold the solid foundations of the earth, to see plains and oceans, extending as far as the eye can reach, and man is so small, when compared with the immensity of creation around him, that we are wont to look upon the hills as everlasting; and the ground whereon we tread, and in the utmost confidence build houses, and proud works of art, as unchangeable. We are so accustomed to behold the grand luminary which gives light and warmth to the world, and cheers myriads with its bright rays, rising and marking out the length of a day; we are so accustomed to plan ahead, and to contrive for years yet to come, as though there was no possibility of a change ; we are so accustomed to behold the fair orb of night, as she illumines a quiet and sleeping earth, and so wont to gaze upon the evertwinkling and bright stars, that we long ago have ceased to think of our earth as a minute orb, smaller by far than many of those upon which we turn such careless eyes now. We rarely, if ever, imagine that its present surface was once the bed of a vast ocean; that its present crust has been caused to heave and swell like a sheet spread out upon the waves, uplifted by internal fires, until the strained surface has cracked open, and the flames, and molten rock found egress. Careless from a thousand causes, we deem ourselves, like the conceited wise men of old, as the only important beings of the universe, and our habitation, as eternal, and unchangeable. It is the peculiar province of Astronomy and Geology, to free the mind from such superstitions, and to elevate and ennoble it by loftier contemplations. The younger Herschel, has truly remarked, “Geology, in the magnitude and sublimity of the objects of which it treats, undoubtedly ranks next to Astronomy in the scale of the seiences.”
We have, in the present volume, associated the two, as was necessary in giving such a sketch of the earth as was planned, and shall strive to interest as well as instruct the reader. — Of one thing we are most certainly convinced, and that is, there is not a more interesting subject, to which we may devote our attention.
“Change of days
THE revolution of the earth on its axis, being adopted as the standard of measure, it was natural that the number of days to the year should be a subject of early investigation. We have already alluded to the helical rising of the stars, and it is apparent that upon ascertaining the distance of the sun from any particular star, and after a certain interval, determining when his distance from the same star, is the same as before, the early astronomers could determine the length of the year, or time occupied by the sun in his apparent revolution around the earth. As it was difficult to observe any stars at the same time with the sun, its place in the heavens, or position in the ecliptic, was determined by measuring its distance from Venus, and then the distance of Venus from some known star. Or, we may imagine the time of sunset to be carefully observed, and afterwards the time of setting of some particular star, then, upon making due allowance for the time elapsed, the sun's position among the stars could be ascertained. The rising and setting of certain stars, or constellations, was early adopted as the precursor of the return of certain seasons of the year. We find continual allusions to this among the early poets, and even in the Book of Job, we have, “Canst thou bind the sweet influence of the Pleiades, or loose the bands of Orion? The Pleiades were also called Vergillae, i. e., daughters of the spring. The Egyptians watched in like manner the rising of the dog star, which gave notice of the approaching season of inundation by the Nile. The length of the year was soon ascertained to be about 365 days; and as the moon, apparently, made near 12 revolutions around the earth in that time the year was subdivided into 12 months, which, in reference to the phases of the moon, were again subdivided into weeks, of seven days each. — The time occupied by the sun in the departure from any particular meridian, until its return to that meridian again, is called a Solar day, and a similar revolution, a star being the object, is called a Siderial day. We have already shown that the Solar day was longer than the Siderial day, on account of the apparent backward motion of the sun among the stars; but it is obvious, that the Siderial day, is the true measure of the time of revolution of the earth on its axis. Now if the earth made an exact number of revolutions on its axis, during the time in which it moves from a particular part of the heavens, back to that particular position again, it is evident we would have an exact number of siderial days to a year. It is found, however, that the siderial year does not consist of an exact number of days, but contains, also, a fractional part of a day. When a long interval of time elapses between different observations, so that the earth makes a great number of revolutions around the sun, the length of the year may be very correctly ascertained. Thus — On the 1st day of April, 1669, at 0h. 3m. 47s., mean solar time, (which we shall explain presently,) Picard observed the distance of the sun from the star Procyon, measured on a parallel of latitude, to be 98° 59' 36". In 1745, which was 76 years after, La Caille observed the sun, to determine exactly the time when his difference of longitude should be the same from the star, as in Picard's observation. Now the day of the month in which La Caille observed, had been reckoned on from Picard's time, just as if the year had consisted of exactly 365 days, except every leap year, when a day had been added, for a reason that will appear presently. It was not until April 2d, at 11h. 10m. 45s., mean solar time, that the difference of longitude was the same as when Picard observed. Now here it was obvious that the earth had in reality, made just exactly 76 revolutions. The number of days however, was as follows, viz: 58 years, of 365 days each, and 18 leap years, of 366 days each,