"When the sun is eclipsed, it depends on the situations of the earth and moon in their orbits, whether the sun or moon subtends the greatest angle as seen from the earth; since at their mean distances their apparent diameters are each about half a degree. If the sun's apparent diameter is the greater, the eclipse, when the centres coincide, must be annular, the margin of the sun's disc being still visible in the form of a ring: when the moon's apparent diameter is greater than the sun's, the eclipse, if central, becomes total: but still a ring of pale light is seen round the disc, which has been attributed to the effect of the sun's atmosphere, since that of the moon is probably too inconsiderable to produce the appearance: a red streak is also sometimes observed at the margin, before the actual emersion of the sun. The degree of darkness depends on the situation of the place of observation within the shadow, on account of the greater or less illumination of the atmosphere within view; sometimes a considerable number of stars may be seen during a total eclipse of the sun. "It is obvious that, since the earth is much larger than the moon, the whole shadow of the moon will only pass over a part of the earth's surface; and that no solar eclipse can be visible in the whole of the hemisphere turned to the sun: while lunar eclipses, on the contrary, present the same appearance wherever the moon is visible. In the same manner, to a spectator on the moon, an eclipse of the earth, or a transit of the moon's shadow over the earth's disc, would have nearly the same appearance wherever he might be stationed; but an eclipse of the sun by the earth would be total to that part of the moon's surface only, which to us appears dark at the same time. "The moon's nodes arrive very nearly at the same situation with respect to the earth after 223 lunations, or revolutions of the moon, which are performed in 18 years of 365 days each, 15 days, 7 hours, and 43 minutes; so that after a period of about 18 years, the series of eclipses recommences nearly in the same order. This circumstance was observed by the ancients, and is mentioned by Ptolemy and by Pliny. When the full moon happens within 72° of the node, there must be a lunar eclipse, and there may be an eclipse at the distance of 13° from the node. An eclipse of the sun may happen when the moon changes, or comes into conjunction with the sun, at any distance within 174° of the node. The mean number of eclipses which occur in a year is about 4; and there are sometimes as many as 7; there must necessarily be two solar eclipses, but it is possible that there may not be even one lunar. In speaking of the magnitude of the part of the sun or moon eclipsed, it is usual to consider the whole diameter as divided into twelve parts, called digits, each of which contains thirty minutes; thus if one-fifth part of the diameter were dark, the extent of the eclipse would be called 2 digits 12 minutes. "The moon travels through the heavens with a motion contrary to their apparent diurnal revolution. Hence she rises and sets, on an average, about three quarters of an hour later every day. The least possible difference between the times of the moon's rising ou two successive days, is, in London, 17 minutes; and this circum, stance occurs once in about 19 years, which is nearly the period of the moon's nodes with respect to the heavens; the greatest possible difference is 1 hour 17 minutes. But it happens every month that the difference becomes greater and less by turns, and when the least difference is at the time of the full moon, it is usually called the harvest moon. In parts nearer to the poles, the moon often rises at the same hour on two succeeding days. "The eclipses of the satellites of Jupiter exhibit appearances extremely interesting for their utility in identifying the same instant of time in different places. On account of the small inclination of their orbits to the plane of Jupiter's orbit, the first three never pass the shadow without being plunged into it, and the fourth but sel dom; while those of Saturn are much less frequently liable to be eclipsed, on account of their greater deviation from the plane of his ecliptic. These satellites are also frequently hidden behind the body of the planet, and this circumstance constitutes an occultation; hence it happens that we can never see both the immersion of the first satellite into the shadow of Jupiter, and its emersion from it; but both the immersion and emersion of the three outer satel lites are sometimes observable. The ring of Saturn exhibits a variety of forms according to its angular position; it disappears to common observation when either its edge or its dark side is presented to us; but to Dr. Herschel's telescopes it never becomes invisible; the light reflected from the planet being probably sufficient for illuminating in some measure the side not exposed to the sun's direct rays. Nat. Phil. Vol. I. p. 527. In a total eclipse of the sun, 12 May, 1706, a streak of light was observed 6′′ or 7" before the sun's disc; hence Flamstead infers a lunar atmosphere th of the moon's diameter in height; but this might have been from oblique reflection.-Phil. Trans. 1706. During a total eclipse of the moon, Ulloa observed that there was a great appearance of light round the moon, which seemed to be agitated, and emitted rays to the distance of a diameter; it was reddish next the moon, then yellowish. Stars of the first and second magnitude were seen, those of the first for about 4 minutes. A minute and a quarter before the emersion, a small point was visible near the disc of the moon. From the ruddy colour of the light, the ring is referred to the moon's atmosphere; the spot to a fissure in the moon's substance. Such a fissure must have been above 40 miles in depth.-Phil. Trans. 1779. The Egyptians reckoned by years of 365 days; Hipparchus and Ptolemy employ the same method. In A.D. 940, the first day of the Egyptian year, was the first of January; another Egyptian year began 31 December. In the new stile, 10 days were omitted in 1582; before this time, each century contained 36,525 days. Robinson. To find the prime number, sometimes called the solar cycle, add 9 and divide by 28; the indiction, add 3 and divide by 15. Add Ito the year and divide by 19, the remainder is the golden number, take 1 from the golden number, multiply by 11, and divide by 30, the remainder is the epact, or the moon's age, on the first of January.-Lalande. In astronomical language, 1 Jan. 1805, 6 o'clock A.M. is 1804, Dec. 31d. 18h.-Lichtenberg. CHAP. XXIII; STARS VISIBLE IN LONDON, INCLUDING ALL OF THE FIRST AND SECOND MAGNITUDE. WHEN a spherical surface has been projected on a plane, it has been usual to consider it as viewed from a particular point, either infinitely remote, as in the orthographical projection, or situated in the opposite surface of the sphere, as in the stereographical. The latter method produces the least distortion, and is the most commonly used, but even here, at the extremities of the hemisphere, the scale is twice as great as in the middle. Sometimes another principle is employed, and the hemisphere is divided into segments, by omitting portions in the directions of their radii, as if the paper were intended to be fixed on a globe; and in the same form as if a spherical surface were cut in the direction of its meridians, and spread on a plane. If the number of these divisions be increased without limit, the result will be the projection, which is employed in the circular part of this diagram, and in the same manner the zone on each side the equinoctial, being cut open by innumerable divisions, so as to be spread on a plane, will coincide with the two remaining portions. By these means the distortion becomes inconsiderable. In the common stereographical projection indeed, the distortion would be of no consequence, if it represented always those stars only, which are at once above the horizon of a given place, for we actually imagine the stars in the zenith to be much nearer together, than when they are nearer the horizon, and the picture would appear to agree very well with the original; but their positions being continually changing, the inconvenience remains. It is not however, necessary, in projections of the stars, to refer them in any instance to a spherical surface. Among Dopplemayer's charts, published at Nuremberg, there are six which represent the sides of a cube, on which the various parts of the constellations are represented; the eye being probably supposed to be situated in the centre. Funck and others have represented the stars as projected on the inside of two flat cones. But the most convenient representation of this kind, and which would approach very near to the projection here employed, would be to consider the eye as placed in the centre of a hollow cylinder, so proportioned that all the cir cumpolar stars should be represented on one of its flat ends, and all those which rise and set on its concave surface; or if it were desired to have a division without referring to any particular latitude, the circular part might extend to the limits of the zodiac, and the parallelogram, into which the cylinder unfolds, might comprehend all |