Page 324 - Third Book of Reading Lessons

READING LESSONS. 323
which case the numher of secondary planets would be nineteen. The time in which a planet per rms its revolution round the sun, is alled i year: and the time of its motion round its axis, its day.
5. The earth, like the other planets, is spherical, but not an exact sphere. Its diameter is about 7,912 miles, and its circumference nearly 25,000. It has an inclined position, its axis making with a perpen­ dicular to the plane of its orbit an angle of twenty­ three degrees twenty-eight minutes (23° 28'); and as it always points to the same direction of the
heavens, the northern half of its axis is turned to­ wards the sun during one half the year, and the southern half during the other. Vhen, there re, it is summer in the northern hemisphere, it is winter in the southern. At two periods of the year, the axis of the earth does not incline to the sun, nor de­ cline om it. They are called equinoxes, that is, equal night; the night, and consequen y the day, being then equal in every part of the world. Both hemispheres at this period enjoy an equal degree of light and heat.·
6. The moon is 240,000 miles distant om the earth, and moves in its orbit round that planet at the rate of 38 miles per minute. It has three motions; one round the earth in about twenty-nine days and a half; another round its own axis in the same space of time; and a third round the sun with the earth ma year.
7. mets rm part of the solar system, and ap­ pear to be thin, lmy bodies, with long, transparent trains, issuing from that side which is turned away
om the sun. They di er om· all the planets in their gure, motion, and orbit, and move round the