Before I proceed to explain what I promised you, it is necessary you should be informed that the earth as seen from the sun, in its periodical revolutions, will describe a circle among the stars which astronomers call the ecliptic, and sometimes the sun’s annual path, because the sun, as seen from the earth, always appears in that line.

Pupil. Do not all the planets move in the ecliptic?

Tutor. No.—On account of the obliquity of their orbits, they are, in every revolution, one half of their periods above the ecliptic, and the other half below it.

Pupil. I think I comprehend your meaning; but shall be obliged to you, Sir, if you can make it clearer to me.

Tutor. I have here a little design, (Plate II. Fig. 1.) which will answer our purpose: where S represents the sun; ABCD, the orbit of the earth; and EFGH, the orbit of one of the inferior planets, suppose Venus.

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.

Pupil. She would be in a line with the sun.

Tutor. And, on the sun’s disc, she would appear a dark round spot, passing over it. These appearances, which are called transits, happen very seldom: because she is very seldom in or near her nodes at her inferior conjunctions. There was one in June 1761, one in June 1769; and the next will be in the year 1874. And as Mercury is seen in the same manner, it is a proof that their orbits must be within that of the earth.

Pupil. I thank you, Sir, and shall be obliged to you to inform me how many constellations the earth pastes over in every revolution?

Tutor. Twelve, which correspond with the months of the year, and are called the twelve signs of the zodiac.

Pupil. What is the zodiac?

Tutor. That part of the heavens which contains the twelve signs, and which you may conceive to be a zone or belt extending eight degrees on each side the ecliptic, in which the planets constantly revolve: so that no planet is ever seen more than eight degrees either north or south, that is above or below the ecliptic.

Pupil. What am I to understand by a degree?

Tutor. All circles, whether great or small, are supposed to be divided into 360 equal parts, called degrees, and each degree into 60 equal parts, called minutes: therefore, if I speak of a circle in the heavens, the circumference of the earth, or any other circle, by a degree is meant the 360th part of that circle; and a minute the 60th part of a degree.

Pupil. What are the names of the twelve signs?

Tutor. The first is called Aries, which you know signifies a Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capricorn, the Goat; Aquarius, the Water-bearer; and Pisces, the Fishes.

Pupil. Do you wish me to commit these to memory, Sir?

Tutor. It is very requisite; but as I know you are fond of verse, you shall hear what Doctor Watts says—

Pupil. I like it much, as it will assist my memory.

Tutor. As the twelve signs correspond with the months of the year, the earth must pass over nearly one degree every day, one sign every month, and in twelve months complete a whole circle, or 360 degrees; therefore every sign must contain 30 degrees, because 30 multiplied by 12 is equal to 360.

Pupil. It must be so.

Tutor. You must remember, that when the earth is in any sign, as seen from the sun, the sun will be in the opposite sign, as seen from the earth: for instance, if the earth be in Aries, the sun will be in Libra; if in Taurus, the sun will be in Scorpio, &c. therefore, as by the earth’s annual motion, the sun appears to move, we always speak of the sun’s, not the earth’s place, in the ecliptic.—You do not seem to understand me?

Pupil. Not perfectly, Sir.

Tutor. Take this orange, and put it in the middle of the round table before us, and place an apple on the opposite side next the window: the orange may represent the sun, the apple the earth, and the window the sign Aries. Now go round the table to the apple; look at the orange, and tell me to what part of the room the eye will be directed.

Pupil. To the part opposite to the window, Sir.

Tutor. If then you suppose the door, which is opposite to the window, to be the sign Libra, the sun will be in Libra when the earth is in Aries—will it not?

Pupil. It is very plain.

Tutor. I shall now give you a table of the signs, their characters, the corresponding months, and the days of the month the sun enters each sign, by means of which, if you reckon a degree for a day, you may find the sun’s place, nearly, for any day in the year.

Pupil. This will give me much pleasure, and I shall be happy to have it.

Pupil. Why do you write northern and southern signs, Sir?

Tutor. Because they are situated north and south of a circle in the heavens, called the equinoctial, which circle crosses the ecliptic in the points Aries and Libra, and extends 23-1/2 degrees on each side of it; and which I shall have occasion to mention to you another time.

Pupil. When you think proper, Sir, I shall be glad to have it explained to me.

Tutor. Look at your table, and tell me what sign and what degree the sun is in the 30th of March, and 20th of October.

Pupil. The sun enters Aries the 20th of March, of course he must be 10 degrees in that sign the 30th; and, as he does not enter Scorpio till the 23d of October, he must want three degrees of completing the sign Libra; he must therefore, on the 20th of October, be in 27 degrees of Libra.

Tutor. Very well.—Do you learn the table, as you will have a farther use for it.