the retardation was observed to be the greatest when the earth was in that part of its orbit most remote from him. The diameter of the orbit of the earth being a hundred and ninety millions of miles, we are more remote from Jupiter, by the whole of that distance, at one time than at another; as, when the earth is in its orbit at a, its distance is greater from c than when at b by the interval between the two points. But notwithstanding this immense addition of space, or any conceivable increase, an eclipse would be observed to occur no later at the one than at the other, if light were propagated instantaneously. Roëmer found, however, a difference of eleven minutes to exist, which he afterward estimated at fourteen, but which the precision of modern astronomy has fixed at sixteen minutes and a quarter. This determines the progressive motion of light, and the rate of its velocity. It requires time for its transmission; and flying over the diameter of the earth’s orbit in sixteen and a quarter minutes gives it a velocity of twelve millions of miles a minute, or upward of a hundred and ninety thousand miles a second. Thus, in the eighth part of a second, it accomplishes the passage of a space equal to the equatorial circumference of our globe: yet so vast is the system to which we belong, that this swift-winged messenger, which requires no more than two hours to travel from the central sun to the farthest planet, could not dart through the intervening solitudes between us and the nearest of the stars under a period of five years. Notwithstanding the velocity of the rays of light, which travel more than fifteen hundred thousand times faster than a cannon ball, experiment has not yet been able to detect that they have any impulsive power. The surmise has, however, been thrown out—and it is not improbable—that the attrition of the solar beams with the terrestrial surface may have some connection with the phenomena of heat.
The national observatory of England—the noblest institution in the world for the extent and exactitude of its astronomical tables, and their practical value in the art of navigation—was originated by the spread of foreign commerce. The growth of colonies across the Atlantic, together with the establishment of relations with India, rendered it of the first importance to have an easy and accurate method of finding the longitude at sea. A plan was proposed, founded upon the principle now in use, of observing the lunar motions and distances during a voyage, and comparing them with a previous home calculation, thus ascertaining the difference between home time and time at sea, from whence the difference of longitude is readily deduced. A reward being sought by the proposer from the government of Charles II. it was referred to a commission to report upon the merits of the scheme. Flamstead, one of the commissioners, at once decided against its practical utility, on the ground of the inaccuracy both of the lunar tables and of the positions of the stars in existing catalogues, which only a lengthened course of observation could rectify. The king, declaring that his pilots and sailors should not want such assistance, immediately instituted the office of astronomer royal, and determined upon founding an observatory. The site—selected by Wren—was a commanding eminence in Greenwich Park, in former times the seat of Duke Humphrey’s tower, within view of all vessels passing along the Thames; a spot which Piazzi was accustomed to call the “paradise” for an observer; being free from a fluctuating atmospheric refraction which annoyed him in the climate of Sicily. The foundation-stone was laid August 10th, 1675. An original inscription, still existing, states the design of the building—the benefit of astronomy and navigation. The observatory has been successively under the superintendence of Flamstead, Halley, Bradley, Bliss, Maskelyne, Pond, and Airy, its present head, with assistants for its proper management. It is not a spot devoted to star-gazing, and the general observance of celestial phenomena, but essentially a place of business, carrying on by day and by night, when the weather permits, those observations of the sun, moon, planets, and principal stars, passing the meridian, from which the nautical almanac derives its information. This has been done with admirable regularity for a long series of years, nor has Europe any data comparable with the Greenwich tables. During the interval in which the office of astronomer royal is necessarily vacant, the business of the observatory proceeds; and that interval is now less than formerly. Thirty-three days elapsed between Bradley’s last observation and Bliss’s first; fifty-three between Bliss’s last and Maskelyne’s first; four between Maskelyne’s last and Pond’s first; and two between Pond’s last and Airy’s first. It has been asserted by Baron Zach, that, if the other observatories had never existed, our astronomical tables would be equally perfect; and Delambre, when delivering an éloge on Maskelyne before the Institute of France, remarked, that if by some grand revolution in the moral or physical world, the whole of the monuments of existing science should be swept away, leaving only the Greenwich observations and some methods of computation, it would be possible to reconstruct from these materials the entire edifice of modern astronomy.
A few years ago it was resolved by the Lords of the Admiralty, that the time should be shown at Greenwich once in every day of the year. This is done by means of a large black ball which surmounts the north-western turret of the observatory. The ball, seen in the vignette, is elevated by machinery to the index, showing the four cardinal points; and, the instant it begins to descend, marks the mean solar time to be 1 P.M. Being plainly observable from the Thames, the arrangement affords a convenient opportunity for seamen to regulate their chronometers and clocks.
Greenwich Observatory.
The fame of Flamstead, the first astronomer royal, does not rest upon any brilliant discovery, but upon an enlightened view of the importance of accurate observation, and the unwearied zeal and industry with which he pursued it. A better representation of him cannot be given than by supposing Tycho Brahe in possession of a telescope, and the adaptation of it to other instruments. Laplace calls him “one of the greatest observers that has ever appeared,” and Delambre remarks, “his name will be eternally cited like those of Hipparchus and Tycho, both of whom, as an observer, he surpassed.” Born in the neighborhood of Derby, and brought up in limited circumstances in that town, he wrought his way to a station at the head of practical astronomy, and established a continental reputation by dint of strong natural genius and unremitting application, in the face of great discouragements. Bad health was a frequent attendant upon him all his days. The patronage of the crown did not screen him from the want of adequate resources, while from several of his scientific contemporaries he encountered dishonorable treatment. The salary attached to his office, then a hundred a year, was often in arrears. Instruments were promised him by the government, but he had to find his own, commencing his duties in 1676 with an iron sextant of seven feet radius, two clocks, and a quadrant of three feet radius, with two telescopes, which he brought with him from Derby. With these instruments he could only measure the relative positions of the stars, and it was not until 1689 that he succeeded in constructing at his own expense a mural arc to determine their absolute places. From this period, through an interval of thirty years, his time was spent in valuable labors, the fruit of which appears in the formation of a catalogue of three thousand stars, and a vast collection of lunar and planetary observations, from which Newton derived material assistance in forming his lunar theory. Yet, as if some annoyance must follow him to the grave, upon his death in 1719, the government of the day attempted to claim his instruments as public property, because found in the national observatory. The name of Flamstead, lost in a great measure to public recollection, or only dimly recognized as one of those who, with “lamp at midnight hour
in some high, lonely tower,
——may oft outwatch the Bear,
With thrice great Hermes”—