FOOTNOTES:
[1] Numbers are purposely omitted here, because abstract numbers convey little or no ideas to young pupils. Those who wish these relations expressed in numbers will find them in Table I at the end of the book.
LESSON III.
EXPLAINING THE MOTION OF THE PLANETS AND COMETS ROUND THE SUN.
§ 14. You have learned, in the preceding lesson, that the Earth and the other Planets are regularly moving round the Sun; but you must know that the time which each requires to complete a whole revolution, that is, the time which each needs to move once round, cannot be the same with all; as you may easily judge yourself: for the Planets which are next to the Sun will, of course, have a much smaller journey to perform, than those which are further from it. Thus Mercury, which is the first Planet in order from the Sun, will naturally come round much sooner than Jupiter, which is placed at a much greater distance from it.
That you may the easier understand this, the following Plate, No. VI, will represent to you our Solar System:
The Sun from which proceeds all light and heat, is placed in the centre. Then come the Planets Mercury and Venus; Third in order is our Earth with its Satellite the Moon; and so on. The rest of the Planets in the same order in which they are represented on Plate [II]. You will also perceive there the orbs of two Comets, distinguished as you were told, by a tail of light. The Planets Jupiter, Saturn and Herschel are each represented with their Moons, and the Planet Saturn with its luminous ring.
No. VI.
Now you will easily understand that Ceres, Vesta, Juno, Pallas, Mars, Jupiter, Saturn and Herschel, have each a much greater distance to travel, than our Earth; but that, on the contrary, Venus and Mercury can complete their revolution in a much shorter time.[2]