northern hemisphere. But at the time of our winter, the rays of the Sun fall obliquely; in high latitudes, very obliquely in winter. Of course a much less number light upon any given space, as a square mile, or any number of square miles, than when they fall directly upon such space. (Plate v. Fig. 6.) The short continuance of the Sun above the horizon in the contracted days of winter, does not give time for the heat to accumulate, while the long days of summer give opportunity for redoubled force. It may be added, that 3,000,000 miles, though a vast distance, are very small in proportion to the immense distance of the Sun.

Spring and summer to us are coincident with autumn and winter to the inhabitants of the southern hemisphere; autumn and winter to us, with spring and summer to such inhabitants.

The mean distance of the Earth from the Sun has been found to be about 95,000,000 miles.* This was ascertained by observations made on the transit of Venus, in the year 1761. Prior to these observations, the distance was considered much less. But their accuracy, confirmed by those on the transit of 1769, seems now to command the full assent of the philosophic world. Taking the distance as now reckoned, it makes the diameter of the Earth's orbit 190,000,000 miles, and the circumference 569,902,100 miles, about equal to the elliptical orbit. The Earth, moving this immense distance in a year, must travel more than 68,000 miles every hour. All the inhabitants of the Earth are carried at this inconceivable velocity, 140 times greater than that of a cannon ball, in their perpetual movement round the Sun. Even this velocity is increased, on a part of each day, by the

* We can form but very inadequate ideas of the immense distance of the planets from the Sun. Could a celestial courier travel the expanse of heaven at the rate of 40 miles in a day, he would not be able to pass from the Earth to the Sun in 6,500 years. Had he commenced such a journey at the creation, he would yet be far short of its completion, and must spend hundreds of years more before his arrival at that luminary.

motion of the Earth on its axis. It may shock the credulity of those who are unaccustomed to philosophical observation, that a motion of such velocity should be imperceptible. But we must take notice, that terrestrial objects around the observer, even the atmosphere, move with him in the same direction; so that with the heavenly bodies only can he compare his motion. By observation on those bodies, the motion of the Earth is ascertained beyond the slightest doubt of the astronomical student. But this motion, if wonderful, is not altogether singular. The passing of a vessel on still water is imperceptible, except from meeting the air, and the apparent motion backward of surrounding objects, till it strikes the shore or other obstruction. No motion on the stillest water is so uniform and even as that of the Earth in its orbit.

The retrograde motion of the axis of the Earth round the poles of the ecliptic causes the difference between the tropical and sidereal years. The equinoxes are annually carried backward, from east to west, 50.118 in a year. Thus, in every year, they meet the Sun 20 minutes 24.4 seconds before the Earth arrives at the point in the heavens whence it started at the commencement of the year. This retrograde motion is called the precession of the equinoxes. With the equinoctial points move all the signs of the ecliptic. "It follows, that those stars which, in the infancy of astronomy, were in Aries, are now in Taurus; those of Taurus in Gemini. Hence, likewise, it is, that the stars, which rose or set, at any particular season of the year, in the times of Hesiod, Eudoxus, Virgil, or Pliny, by no means answer at this time, to their descriptions." An example of the change may be seen on our celestial globes. The constellations are placed 30° from the signs to which they originally belonged. This change of place shows the motion of the equinoxes for 2,154 years. A complete revolution of the signs requires a period of 25,858 years. Hence the pole star, or the north pole, as it is called, will not

always be the point to which the pole of the Earth will be directed; but in something more than 12,000 years, will be about 47° from the pole of the Earth, and, when on the meridian, will be in the zenith of some parts of New England.

How should the contemplation of these celestial motions and long periods constrain us to improve the short, fleeting moments of time assigned to us; and lead us to admire and adore the wisdom and power of Him who formed and still governs the universe with infinite ease; to whom'a thousand years are as one day!'

Why is the Earth more worthy of our attention than the other planets? What is the form of the Earth? How may any person satisfy himself of the Earth's rotundity? Who first circumnavigated the Earth? Who next? How was the true form of the Earth discovered? What is the mean diameter of the Earth ? What is the difference between the equatorial and polar diameters of the Earth? What is the Earth's circumference? What its superficial contents? How many great circles are considered encompassing the Earth? What are they? How do you describe each? What are the smaller circles? What is latitude? What is longitude? Into how many zones is the surface of the Earth divided? How many motions has the Earth? In what time does the Earth turn on its axis? In the rotation of the Earth on its axis do different parts of its surface move with equal velocity? Where are objects lightest, at the equator or at the poles? How do the heavenly bodies appear to rise and set in a direct sphere? How in an oblique sphere? How in a parallel sphere? How high can the Sun appear at the poles? How high the Moon? How is the Earth's orbit formed? What is it? Were the ancients acquainted with the true form of the Earth's orbit? Who first assigned to the orbit its true elliptical figure? What is the inclination of the Earth's axis to the plane of its orbit? To what are we indebted for the inequality of the days and nights, and the variety of the seasons? How may a fit representation of these be made? In what do a candle and a globe fail in giving an exact representation of the Sun and Earth? How long is a sidereal year? How long is a tropical? How long is an anomalistic year? Why is the Earth longer in passing the six northern signs than the six southern? How much longer is it? How far forward does the aphelion of the Earth move in a year? In what time does it complete a revolution? When did the northern and southern hemispheres enjoy equal lengths of summers and winters? What may corroborate the Mosaic account of the creation? When will the seasons in different hemispheres again become equal? How much farther from the Sun is the Earth in the aphelion than in the perihelion of its orbit? At what time of the year is the Earth in the

aphelion, and at what time in the perihelion of its orbit? Why have we not warmer weather in winter than in summer? In spring and summer to us, what seasons have the inhabitants of the southern hemisphere? How far is the Sun distant from the Earth? At the rate of 40 miles a day, how long would a celestial courier be in travelling from the Earth to the Sun? What is the diameter and what the circumference of the Earth's orbit? How far must the Earth move in an hour? Why is a motion of such velocity imperceptible? What causes the difference between the tropical and sidereal year? At what rate are the equinoxes carried backward? What is meant by the precession of the equinoxes? Why are the stars on a celestial globe placed differently from their ancient situation? In what time do the signs of the ecliptic perform a complete revolution? Where will the north pole star appear 12,000 years hence?

SECTION VIII. The Moon.

This con

The Earth has one satellite, the Moon. stant attendant is distant from the Earth 240,000 miles.* The Moon, though inferior to most of the heavenly bodies, next to the Sun, is to us by far the most interesting. (By dispelling the gloom of night, she is the solace of the weary traveller and by constantly changing her countenance, she gives variety and beauty to the nightly canopy.

The Moon performs a revolution round the Earth, from a point in the ecliptic to the same again, in 27 d. 7 h. 43 m. 5 s.) from a star to the same again, in 27 d. 7 h. 43 m. 12 s. It revolves from the Sun to the Sun again in 29 d. 12 h. 44 m. 3 s. This is called a mean lunation, and is a synodical revolution. The Moon always presents the same face to the Earth. Hence in the same time that it performs a revolution, it must revolve on its axis; unless the different sides of the Moon present the same prospect. That there should be a uniformity of appearance in the different sides of the Moon seems very improbable. Astronomers seem agreed in the coincidence of its revolutions; or that it revolves on its axis in the same time that it performs a revolution

*This is its mean distance.

round the Earth. If this opinion be correct, it must be considered, that the side of the Moon next to the Earth is composed of matter more dense than that of the opposite side; and that the powerful attraction of the Earth causes it to revolve on its axis.

Several authors have asserted, that the Moon performs a revolution in (29 days); and, in immediate connection, that it turns on its axis m the same time that it performs a revolution. The latter assertion is true; but it is in the time of the sidereal revolution, 27 d. 7 h. 43 m. 12 s. and not in the synodical, or a lunation, about 29 days, as will appear by an inspection of Plate v. Fig. 2. Let E be the Earth; A B C D the Moon's orbit; a a mountain on the side of the Moon next to the Earth. As the Moon passes in her orbit from A to B, 90°, it is evident she must turn on her axis 90°, in order that the same side may be towards the Earth: the mountain will then be at b. When the Moon is at C, having passed 180°, half her revolution, the mountain must be at c. The Moon at D presents the mountain at d. When the Moon returns to A, the mountain must come round to a again. Thus, in a sidereal revolution, 27 d. 7 h. 43 m. 12 s. the Moon must have revolved once on its axis, or the same side cannot be presented to the Earth. Dr. Brewster must have been strangely inattentive, when he went into the common error respecting the time of the Moon's revolution on its axis.

The diameter of the Moon is 2,180 miles. But it can be but 2,173 miles, if its apparent diameter be 31' 8", as stated by De la Lande.

The Earth must appear like a moon to the lunarians → but thirteen times as large as the Moon does to us. It must exhibit all the phases of the Moon, but at opposite times. When the Moon appears new to us, the Earth must be full to them; and when the Moon is full to us, the Earth must be new to them.

It is remarkable, that one hemisphere of the Moon enjoys continued light, the Earth, in the absence of the