The Study of Astronomy, adapted to the capacities of youth / In twelve familiar dialogues, between a tutor and his pupil: explaining the general phænomena of the heavenly bodies, the theory of the tides, &c.

by teacher of astronomy John Stedman

Pupil. Now I understand it perfectly: the half EHG rises above, and the other half EFG sinks below it, from the points EG, which I perceive are in a line with the orbit of the earth. But pray, Sir, have you any name for that dotted line?

Tutor. Yes, it is called the line of the nodes; and the points EG the nodes of the planet: the latter is called the ascending node, because, when the planet is in G, it is ascending or rising above the orbit of the earth; or, which is the same thing, above the ecliptic: and when in E, it is descending or sinking below it, whence it is called the descending node. But you must remember that the orbits of all the planets do not cross or intersect the ecliptic in the same points; but that their nodes or intersections are at different parts of it.

Pupil. How can the orbit of the earth and the ecliptic be the same?

Tutor. They are very different; but being in the same plane, if the orbit of any planet inclines to one it must incline equally to the other.

Pupil. You will, I fear, Sir, think me very stupid: but I must beg of you to inform me what you mean by a plane?

Tutor. Any flat surface is a plane. You may therefore suppose the edge of a round tea-table to represent the ecliptic, and a circle within it, drawn from the center of the table, the orbit of the earth: will they not be both in the same plane?

Pupil. Certainly.

Tutor. You must not imagine, when I am speaking to you of the plane of the ecliptic, or plane of the earth’s orbit, that it is a visible flat surface, or, in speaking of the orbits of the planets, I mean solid rings.—No. The planets perform their revolutions with the utmost regularity, in unbounded space; and, like a bird thro’ the air, leave no track behind them.

Pupil. How then are they retained in their orbits?

Tutor. The question, I confess, is natural, and is what I expected; but I must of necessity postpone it to another opportunity; and shall now fulfil the promise I made of shewing you in what manner the inferior planets may be seen when in their inferior conjunctions. Cast your eye again on the little design I gave you, and consider, if Venus were in her ascending node at G, when the earth is at b; or, in her descending node, at E, when the earth is at a, what the effect would be.