m. s. May 1, from the time by the dial subtract 3.2 Friday, 6, 3 35 11, 3 54 Monday, 16, 3 58 Saturday, 21, 3 48 3 24 2 49 The Moon is full at 29 m. past 10 in the morning of the 4th it enters its last quarter at 41 m. past 2 in the afternoon; the change, or new Moon, is at 23 m. past 4 on the 19th, and it enters its first quarter at 31 m. after 7 in the morning of the 26th. The time of the Moon's rising for the first 4 days after it is full is as follows: Venus will be at her greatest elongation on the 21st day of the month. Jupiter will be in the quadrature at past 11 at night on the 22d, and the Georgian planet will be in opposition to the Sun, that is at the distance of six signs, or the two bodies will be in opposite parts of the heavens, at 6 in the morning of the 22d day of the month. There will be only two eclipses of Jupiter's first satellite visible at Greenwich this month; namely, on the 7th day, at 10 minutes past 12 at night. The earth on which we live is, unquestionably, a primary planet belonging to the solar system, and which like Mercury and Venus already described, and Mars, Jupiter, Saturn, and the Georgian planets, the subjects of future notice, depends on the Sun for its light and motion. In the darker ages of antiquity, this vast body, the abode of life and happiness to countless millions of creatures, was considered as a large circular plane spreading out indefinitely on all sides: and the surrounding heavens, in which the Sun, Moon, and stars, appear to move every 24 hours from east to west, were imagined to be at no great distance from it, and to have been created solely for the use and ornament of the inhabitants of the earth. It is now, and has for centuries, been completely ascertained that the earth is of a spherical figure, nearly resembling an artificial globe. The several proofs of the sphericity of the earth are as follow: (1) Several celebrated navigators, as Magellan, Sir Francis Drake, Lord Anson, and Captain Cook, have actually sailed round it, by setting out, for instance, at Plymouth or other places, and by continually steering their course in one direction, viz. westward, have, at length, arrived at the place from which they had departed; which could never have happened, had the earth been of any other than a spherical form. (2) This form is also evident, from the circular appearance of the sea, and the circumstances which attend large objects, when seen at a distance on its surface. For, as has been observed a thousand times, when a ship goes out to sea, the people on the surrounding shore first lose sight of the hull, or body of the vessel, afterwards of the rigging, and at last they only discern the topmast, which circumstances can only be explained by supposing the convexity of the water between the eye and the object; for if it were not so, the image of the largest and most conspicuous part of the vessel would remain longest on the eye. (3) Another proof is taken from the shadow of the earth upon the face of the Moon, in the time of a lunar eclipse. For the Moon, like the earth, having no light but what it receives from the Sun, and the earth being, at this time, interposed exactly between them, the Moon 1 must either wholly or in part become obscure and since in every eclipse of this kind, which is not total, the obscure part always appears to be bounded by a circular line, hence it is inferred the earth is spherical, it being certain that none but a spherical body can, in all situations, cast a circular shadow. There are three principal motions belonging to the earth, viz. a motion of rotation about its axis, a motion in an orbit round the Sun, and motion of its axis round the poles of the ecliptic. These three motions are derived from a single impulsion, supposed to be given to the earth at its origin, and combined with the perpetual action of the Sun. The rotation of the earth on its axis, called its diurnal motion, is the most uniform motion with which we are acquainted: it is performed in 23 h. 56 m. 4 sec. The earth, or, to speak with more accuracy, the centre of gravity of the earth and Moon, describes an orbit round the Sun, which orbit is not of a circular but elliptic form, the Sun being in one of the foci. If we suppose the plane of this orbit extended to the fixed stars, it will then trace the path in the heavens called the ecliptic. The earth does not move round this orbit in a perpendicular position, but inclined to the ecliptic in an angle of 23° 28'. The points in the earth's orbit, which are intersected by this plane, are called the equinoctial points. The motion of the earth in its orbit is not uniform; it depends on its distance from the Sun: when it is, nearest that body in winter, it moves the fastest, and it moves the slowest when it is farthest from the Sun in summer. It is, however, so far regular, that, with the exception of some trifling inequalities caused by the action of the Moon and planets, the radius-vector, as it is called, or line joining the centres of the earth and Sun, describes equal areas of the ellipse in equal times. The third motion belonging to the earth is that which produces the precession of the equinoxes. The rotatory motion of the earth about its axis produces a protuberant form in the equatorial regions of the earth; and the continued action of the Sun and Moon on this surrounding mass, or annulus, produces a rotatory motion in the axis of the earth round the imaginary axis of the ecliptic, the inclination of these supposed axes remaining perpetually the same. On this subject, which is rather difficult of apprehension, we shall not enlarge in the present volume; only observing that the revolution here referred to will not be accomplished in less than 27,000 years. The diurnal motion of the earth from west to east causes an apparent motion of the heavens from east to west. If the imaginary axis of the earth be prolonged each way till it intercepts the heavens, it will there form two points called the poles, round which the heavens appear to revolve. With us in the northern hemisphere, the north is the elevated pole, and the stars near it will describe small circles, increasing with their distance from the pole, till they arrive at a certain distance, when a portion of the circle will be intercepted by the horizon. It will be evident, by referring to an artificial celestial globe, that, as the polar distance of the stars increases, they will continue a longer time above the horizon; still, however, rising and setting north of the east and west points of the horizon, till they reach the equator. At this distance from the pole, they rise exactly in the east, and set exactly in the west; and at their greatest altitude, their distance from the zenith is observed to be equal to the elevation of the pole above the horizon. As the stars recede from the equator towards the south, they describe still smaller arcs, continue visible only for a few minutes, and no sooner appear on the meridian than they are carried down by the general motion of the heavens below the boundary of our sight. The Sun, Moon, and planets, appear to partake in a great measure of the diurnal motion of the heavens; they describe nearly the same course as a fixed star would do in the same situation. f The change from night to day is caused by the diurnal motion of the earth on its axis; and the vicissitudes of the seasons, by the combinations of this motion with its annual revolution round the Sun. The annual motion of the earth round the Sun is performed in an orbit, the plane of which is inclined to the equator at an angle of 23° 28', and the effect of this motion is to cause the apparent annual motion of the Sun. When the earth is in any point of its orbit, the apparent place of the Sun will be in the direction of the opposite point, and the apparent path of the Sun will be similar to that actually traced by the earth; and these two bodies will always be in opposite positions to each other. It was not till astronomy had arrived at a state of great perfection, that sufficient proofs could be collected to ascertain, with any degree of precision, which of these bodies were really in motion. The fact has, however, been long since demonstrated with as much certainty as any proposition in Euclid. The difference of seasons and of climates on the earth, and the inequality in the length of the days and nights, all arise from this circumstance, that the plane of the earth's orbit is inclined to that of the equator. If a straight line be supposed always to connect the centres of the earth and Sun, this straight line or radius-vector will always trace the ecliptic in the heavens; and the point where it intercepts the surface of the earth will be that point over which the Sun is vertical. Now it will be evident, with a little consideration, that when the earth is in the upper or north part of its orbit, this line, supposed to pass through the centre of the Sun, will intercept some point to the south of the equator; and, on the contrary, when the earth is in the lower or southern part of its orbit, this line will intercept some point to the north of the equator: the circle bounding light and darkness being |