If the Moon moved in the same plane or level with the Earth, we should have an eclipse every full and change; but as she travels 5¾ degrees to the North of it, and the same to the South of it, every lunation, she only crosses the plane of the Earth’s orbit in two places, which points of intersection (called the Moon’s nodes) though in a trackless path, move 19¾ degrees towards the West every year, and therefore pass round the Heavens in 18 years and 225 days; which is the golden number of our calendar. Hence, when one of these nodes is between the Earth and the Sun at the change, the Moon’s shadow is thrown on the Earth, and she eclipses the Sun; and if she comes to the full when either node is opposite to the Sun, she falls into the Earth’s shadow, and loses for a short time her borrowed light: hence, as she mostly passes above or below the Earth’s shadow, we have Eclipses very seldom. These phænomena are produced in the Eidouranion as they are in nature, and perfectly evident on inspection.
Scene 3.
THEORY OF TIDES,
ILLUSTRATED.
This Scene also consists of the Sun, the Earth, and the Moon. But the intention is to shew how the Earth and Moon agitate each other round their common centre of gravity, causing two tides every 25 hours. It is a circumstance truly astonishing, to see in calm weather, and under a serene sky, the violent agitation of the great fluid mass of the ocean, whose waves roll against the shore with so grand impetuosity.—This spectacle invites to reflection, and rouses a desire to penetrate into its cause: hence the Earth’s three-fold motion appears in this scene:—that, on its axis, to produce day and night: that, round the Sun, to produce the year and our seasons; and that, round the centre of gravity with the Moon, to produce spring and neap tides, by their combined and opposite influences. The Moon is so near the Earth (240,000 miles at a medium) in comparison of the Sun (near 100 millions of miles) that the Moon’s attraction on the waters of the ocean and on the air of our atmosphere (for there are tides in both) is to that of the Sun as ten is to three. So at the change of the Moon, the attraction of the Sun and Moon being in the same direction, a power of thirteen influences the sea, and we have spring tides; but at the quarters of the Moon, the two luminaries counteract the attractions of each other, so the Sun’s power of three being taken from the Moon’s of ten, leaves only seven operating upon the sea, and neap tides take place.
A tumbler filled with water, may be whirled by a string vertically round the head, without any danger of the water falling out of it. Those parts of the Earth that come successively opposite to the Moon, perform a much larger circle round the centre of gravity, than the parts immediately under the Moon: hence the waters opposite the Moon are thrown off, as it were, by their centrifugal motion, and rise above the common level, as well as the waters exposed to the Moon’s immediate attraction; thus two tides are produced in 25 hours, opposite to each other; and by the Earth turning through those protuberances, its waters rise and fall.
The Sun would produce two small, but similar tides, if the Earth had no Moon; therefore at the full of the Moon the Sun’s centrifugal tide being reinforced by the Moon’s attraction, and the Moon’s centrifugal tide being also assisted by the Sun’s attraction, spring tides take place at the full, as well as at the change, of the Moon.
☞ This Scene also receives collateral assistance, exhibiting and explaining the setting of the tides in all the large oceans and the principal seas of the Earth.