NICETY REQUIRED IN ASTRONOMICAL CALCULATIONS.
In the Edinburgh Review, 1850, we find the following illustrations of the enormous propagation of minute errors:
The rod used in measuring a base-line is commonly about ten feet long; and the astronomer may be said truly to apply that very rod to mete the distance of the stars. An error in placing a fine dot which fixes the length of the rod, amounting to one-five-thousandth of an inch (the thickness of a single silken fibre), will amount to an error of 70 feet in the earth’s diameter, of 316 miles in the sun’s distance, and to 65,200,000 miles in that of the nearest fixed star. Secondly, as the astronomer in his observatory has nothing further to do with ascertaining lengths or distances, except by calculation, his whole skill and artifice are exhausted in the measurement of angles; for by these alone spaces inaccessible can be compared. Happily, a ray of light is straight: were it not so (in celestial spaces at least), there would be an end of our astronomy. Now an angle of a second (3600 to a degree) is a subtle thing. It has an apparent breadth utterly invisible to the unassisted eye, unless accompanied with so intense a splendour (e. g. in the case of a fixed star) as actually to raise by its effect on the nerve of sight a spurious image having a sensible breadth. A silkworm’s fibre, such as we have mentioned above, subtends an angle of a second at 3½ feet distance; a cricket-ball, 2½ inches diameter, must be removed, in order to subtend a second, to 43,000 feet, or about 8 miles, where it would be utterly invisible to the sharpest sight aided even by a telescope of some power. Yet it is on the measure of one single second that the ascertainment of a sensible parallax in any fixed star depends; and an error of one-thousandth of that amount (a quantity still unmeasurable by the most perfect of our instruments) would place the star too far or too near by 200,000,000,000 miles; a space which light requires 118 days to travel.
CAN STARS BE SEEN BY DAYLIGHT?
Aristotle maintains that Stars may occasionally be seen in the Daylight, from caverns and cisterns, as through tubes. Pliny alludes to the same circumstance, and mentions that stars have been most distinctly recognised during solar eclipses. Sir John Herschel has heard it stated by a celebrated optician, that his attention was first drawn to astronomy by the regular appearance, at a certain hour, for several successive days, of a considerable star through the shaft of a chimney. The chimney-sweepers who have been questioned upon this subject agree tolerably well in stating that “they have never seen stars by day, but that when observed at night through deep shafts, the sky appeared quite near, and the stars larger.” Saussure states that stars have been seen with the naked eye in broad daylight, on the declivity of Mont Blanc, at an elevation of 12,757 feet, as he was assured by several of the alpine guides. The observer must be placed entirely in the shade, and have a thick and massive shade above his head, else the stronger light of the air will disperse the faint image of the stars; these conditions resembling those presented by the cisterns of the ancients, and the chimneys above referred to. Humboldt, however, questions the accuracy of these evidences, adding that in the Cordilleras of Mexico, Quito, and Peru, at elevations of 15,000 or 16,000 feet above the sea-level, he never could distinguish stars by daylight. Yet, under the ethereally pure sky of Cumana, in the plains near the sea-shore, Humboldt has frequently been able, after observing an eclipse of Jupiter’s satellites, to find the planet again with the naked eye, and has most distinctly seen it when the sun’s disc was from 18° to 20° above the horizon.
LOST HEAT OF THE SUN.
By the nature of our atmosphere, we are protected from the influence of the full flood of solar heat. The absorption of caloric by the air has been calculated at about one-fifth of the whole in passing through a column of 6000 feet, estimated near the earth’s surface. And we are enabled, knowing the increasing rarity of the upper regions of our gaseous envelope, in which the absorption is constantly diminishing, to prove that about one-third of the solar heat is lost by vertical transmission through the whole extent of our atmosphere.—J. D. Forbes, F.R.S.; Bakerian Lecture, 1842.
THE LONDON MONUMENT USED AS AN OBSERVATORY.
Soon after the completion of the Monument on Fish Street Hill, by Wren, in 1677, it was used by Hooke and other members of the Royal Society for astronomical purposes, but abandoned on account of the vibrations being too great for the nicety required in their observations. Hence arose the report that the Monument was unsafe, which has been revived in our time; “but,” says Elmes, “its scientific construction may bid defiance to the attacks of all but earthquakes for centuries to come.” This vibration in lofty columns is not uncommon. Captain Smythe, in his Cycle of Celestial Objects, tells us, that when taking observations on the summit of Pompey’s Pillar, near Alexandria, the mercury was sensibly affected by tremor, although the pillar is a solid.