Why Solar and Lunar Eclipses Don't Happen Every Month

The moon orbits the earth once per month, which means the moon is on the sun side of the earth every month. So. "why aren't there eclipses every month?" is a question answered eloquently in a 1757 astronomy book by James Ferguson. It's the 18th century equivalent of "astronomy for dummies," complete with amazing illustrations. Here's Ferguson's surprisingly good 250-year-old explanation for why there aren't eclipses every month, illustrated by me. "Every Planet and Satellite is illuminated by the Sun; and casts a shadow towards that point

of the Heavens which is opposite to the Sun. When the Sun's light is so intercepted by the Moon, that to any place of the Earth the Sun appears partly or wholly covered, he is said to undergo an Eclipse; though properly speaking, 'tis only an Eclipse of that part of the Earth where the Moon's shadow falls. When the Sun is eclipsed to us, the Moon's Inhabitants on the side next the Earth (if any such there be) see her shadow like a dark spot travelling over the Earth, about twice as fast as its equatoreal parts move, and the same way as they move. If the Moon's Orbit were coincident with the Plane of the Ecliptic,

in which the Earth always moves, the Moon's shadow would fall upon the Earth at every New Moon, and eclipse the Sun to some parts of the Earth. But one half of the Moon's Orbit is elevated 5 degrees above the Ecliptic, and the other half as much depressed below it: consequently, the Moon's Orbit intersects the Ecliptic in two opposite points called the Moon's Nodes. When these points are in a line with the Sun at New or Full Moon, the Sun, Moon, and Earth are all in a line; and if the Moon be then New, her shadow falls upon the Earth; if Full the Earth's shadow falls upon her. When the Moon [is] more than 17 degrees from either of the Nodes at [a New Moon], the Moon is then too high or too low in her

Orbit to cast any part of her shadow upon the Earth. But when the Moon is less than 17 degrees from either Node at the time of Conjunction, her shadow falls upon the Earth. [The moon's] orbit contains 360 degrees; of which 17 [degrees], the limit of solar Eclipses on either side of the Nodes, [is but a small portion;] and as the Sun passes by the Nodes but twice in a year, it is no wonder that we have so many New Moons without Eclipses."