317. If the Moon’s Orbit were coincident with the Plane of the Ecliptic, in which the Earth always moves and the Sun appears to move, the Moon’s shadow would fall upon the Earth at every Change, and eclipse the Sun to some parts of the Earth. In like manner the Moon would go through the middle of the Earth’s shadow, and be eclipsed at every Full; but with this difference, that she would be totally darkened for above an hour and half; whereas the Sun never was above four minutes totally eclipsed by the interposition of the Moon. But one half of the Moon’s Orbit, is elevated 5-1/3 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_, as has been already taken notice of § 288. When these points are in a right line with the center of the Sun at New or Full Moon, the Sun, Moon, and Earth are all in a right 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 Sun and Moon are more than 17 degrees from either of the Nodes at the time of Conjunction, the Moon is then too high or too low in her Orbit to cast any part of her shadow upon the Earth. And when the Sun is more than 12 degrees from either of the Nodes at the time of Full Moon, the Moon is too high or too low in her Orbit to go through any part of the Earth’s shadow: and in both these cases there will be no Eclipse. But when the Moon is less than 17 degrees from either Node at the time of Conjunction, her shadow or Penumbra falls more or less upon the Earth, as she is more or less within this limit. And when she is less than 12 degrees from either Node at the time of opposition, she goes through a greater or less portion of the Earth’s shadow, as she is more or less within this limit. Her Orbit contains 360 degrees; of which 17, the limit of solar Eclipses on either side of the Nodes, and 12 the limit of lunar Eclipses, are but small portions: and as the Sun commonly passes by the Nodes but twice in a year, it is no wonder that we have so many New and Full Moons without Eclipses. [Sidenote: Fig. I. PLATE X. Line of the Nodes.] To illustrate this, let _ABCD_ be the _Ecliptic_, _RSTU_ a Circle lying in the same Plane with the Ecliptic, and _VWXY_ the _Moon’s Orbit_, all thrown into an oblique view, which gives them an elliptical shape to the eye. One half of the Moon’s Orbit, as _VWX_, is always below the Ecliptic, and the other half _XYV_ above it. The points _V_ and _X_, where the Moon’s Orbit intersects the Circle _RSTU_, which lies even with the Ecliptic, are the _Moon’s Nodes_; and a right line as _XEV_ drawn from one to the other, through the Earth’s center, is the _Line of the Nodes_, which is carried almost parallel to itself round the Sun in a year.