RETROGRADE MOTION OF THE PLANETS.

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Scene No. LIV.— The Sun ’s Place in the Ecliptic .

This scene illustrates the meaning of the expression of the sun’s place inthe ecliptic. It has been already explained, that by the ecliptic is meant theimaginary circle in the heavens, in which the sun appears to move as seen fromthe earth, or the circle in which the earth appears to move as seen from thesun. The zodiac is that portion of the heavens through the middle of whichthe ecliptic appears to run, and is portioned out into twelve parts, or divisions,each of which is termed a constellation ; as Aries, Taurus, Gemini, &c. Thesun, as seen from the earth, always appears in the ecliptic, and, consequently, insome one of these constellations. That portion of the zodiac in which the sunappears, is termed its place in the ecliptic, the ecliptic being itself within thelimits of the zodiac.

The scene shews the sun in the centre, and the earth in several points of itsorbit round it; and beyond it is a circle, representing the constellations of thezodiac in their order. From the lower figure of the aarth, the sun is seen in theconstellation Cancer ; the sun is then said to be in Cancer. In the next follow-ing figure, to the right, the sun appears, from the earth, to be in Taurus ; fromthe next above it, in Pisces ; from the upper figure of the earth, the sun appearsin Sagittarius; and from the next, or left-hand figure, it is seen in Libra. Ineach of these positions, the earth, as seen from the sun, of course, appears inthe opposite constellation. For instance, in the lower figure of the earth, thesun, from our planet, appears in Cancer ; but, from the sun, the earth appearsin the opposite constellation, Capricornus .

Scene No. LV.— The Apparent Retrograde Motion of the Planets .

This scene illustrates the apparent retrograde movements of the planetsMercury and Venus . The outer circle of the scene is the circle of the zodiac,or stars, among which the apparent paths of those planets seem to be. In thecentre is the sun ; the circle of small globes, next beyond the sun, representsMercury in his orbit ; on the border of the scene, to the right, is a green globe,representing the earth. Now, if we imagine the balls, representing Mercury , tomove round in their orbit, beginning at the point opposite to the earth, and tomove from the right towards the left, the lines which are drawn from the earththrough the planet at each stage of its progress, and extended as far as the zodiac,will shew to what parts of that circle the observer on the earth refers the planet;or, which is the same thing, in what parts of it the planet appears to be from time totime. At its first position, at a, it appears, by the direction of the line, to be in theconstellation Leo ; when it has moved on to b, it appears to have moved forwardto the next constellation, Cancer; when at c, it appears still to go forward, andto be at the beginning of Gemini; when at d, to have advanced to the end of