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ed by astronomers as the true solar system. Some of the ancient Egyptians discovered the revolution of Mercury and Venus round the Sun. The general principles of the system were afterwards taught privately by Pythagoras to his disciples, five hundred years before the Christian era. But, being af terwards rejected, it was nearly lost, till revived by Copernicus, a native of Thorn, in Polish Prussia. In the centre of this system is placed the Sun, around which the primary planets revolve from west to east. The Earth turns on its axis. The Moon revolves round the Earth. The other secondary planets perform their revolutions around their primaries from west to east, at different distances and at different times, the satellites of Herschel only excepted. Beyond these, at an immense distance, are the fixed stars, as centres to other systems. (Plate 1. Fig. 3.*)

Some authors inform us, that Copernicus finished his great work in 1530; but did not venture it in print till near the time of his death, which happened on the twenty-second of May, 1543. He died suddenly, by the rupture of a blood vessel, soon after completing his seventieth year, and a few days after revising the first proof of his work.

Copernicus was an accurate mathematician, and applied his useful knowledge to the improvement of astronomy. Perplexed with the epicycles and ec

* In this figure, the orbits are delineated, nearly as possible, according to their elliptical form, and in due proportion. The stars of the zodiac are placed according to their true situation in the celestial zone.

In the diagrams of some works, the orbits of the inferior planets are represented greatly enlarged beyond their proper dimensions.

verse.

centrics, by which Ptolemy attempted to account for the irregular motion of the heavenly bodies, he searched the lore of antiquity. “ He tried to find among the ancient philosophers a more simple arrangement of the universe. He found, that many of them had supposed Venus and Mercury to move round the Sun; that Nicetas, according to Cicero, made the Earth revolve on its axis, and by this means freed the celestial sphere from that inconceivable velocity, which must have been attributed to it to accomplish its diurnal revolution. He learned from Aristotle and Plutarch, that the Pythagoreans had made the Earth and planets move round the Sun, which they placed in the centre of the uni

These luminous ideas struck him. He applied them to the astronomical observations, which time had multiplied, and had the satisfaction to see them yield, without difficulty, to the theory of the motion of the Earth. The diurnal revolution of the heavens was only an illusion due to the rotation of the Earth, and the precession of the equinoxes is reduced to a slight motion of the terrestrial axis. The circles, imagined by Ptolemy, to explain the alternate, direct and retrograde motions of the planets, disappeared. Copernicus only saw in these singular phenomena the appearances produced by the motion of the Earth round the Sun with that of the planets; and he determined, hence, the respective dimensions of their orbits, which till then were unknown. Finally, every thing in this system announced that beautiful simplicity in the operations of nature, which delights so much when we are fortunate enough to discover it. Copernicus pub

lished it in his work On the Celestial Revolutions. Not to shock received prejudices, he presented it under the form of an hypothesis. “ Astronomers," said he, in his dedication to Paul III, “being permitted to imagine circles, to explain the motion of the stars, I thought myself equally entitled to examine, if the supposition of the motion of the Earth would render the theory of these appearances more exact and simple."

From what is the term astronomy derived? Of what is it compounded? How long has the science of astronomy been cultivated? What nations claim the honor of being the first astronomers ? At what time aid the astronomer Zoroaster live? What was in the monument of Osymandyas ? Why were both Chaldea and Egypt peculiarly favorable for astronomical observations ? What other nations may claim a high antiquity in the science of astronomy? Where was the land of Uz? For what was it distinguished? Why are we to suppose from the book of Job that the Arabians were astronomers ? What does Josephus say concerning the knowledge of astronomy before the deluge? What were the pillars of Seth ? By whom was the astronomical cycle of 600 years invented ? On what account is astronomy useful? Why is navigation dependent on astronomy? Why geography? Why does it afford a sublime view of the Creator's works? 'What noted systems of astronomy have there been? From what does the Ptolemaic take its name? What was it? What shows the absurdity of this hypothesis ? Who was Tycho Brahe ? Why did he reject the Copernican system? What was the Brahean system? What cannot be explained by the Brahean system? Which is the true solar system? By whom and when was this first taught? Who was Copernicus? How are the heavenly bodies arranged in his system? When did Copernicus finish his work? What led him to consider other systems absurd? What did he discover by investigation ? Under what form did he present his system?

A GLOSSARY

OF ASTRONOMICAL TERMS.

Altitude is an arch of a vertical circle intercepted between the centre of a heavenly body and the horizon.

Amplitude is the distance of a heavenly body from the east or west point of the horizon, measured on an arch of that circle, the body being in it, or referred to it by a verticle.

Antipodes, inhabitants living at opposite points of the Earth's surface, under opposite meridians and in opposite parallels.

Anteci, inhabitants living under the same meridian, but in opposite parallels, north and south.

Aphelion, the point in the orbit of a planet farthest distant from the Sun.

Apogee, the point in the Moon's orbit most distant from the Earth. The term is sometimes applied to the Sun's place when farthest from the Earth.

Apsis, the aphelion or perihelion point. The line connecting these is called the line of the apsides. Arch of a circle, a part or portion of the circumference. Asteroids, four small planets between Mars and Jupiter.

Axis, an imaginary line on which the Sun or a planet revolves.

Azimuth, the distance of a heavenly body from the north or south point of the horizon, when the body is in that circle, or referred to it by a verticle.

Centrifugal force, that by which a revolving body endeavors to recede from the centre of its motion.

Centripetal force, that which attracts a revolving body to the centre.

Comet, a celestial body moving round the Sun in an orbit very eccentric.

Conjunction, the meeting of heavenly bodies in the same longitude, on the same side of the Earth, though they may not be in the same latitude.

Constellation, a number of stars contained in an assumed figure.

Cosines, cotangents, and cosecants, are sines, tangents, and secants of the complement of an arch.

Cycle, a period of time.

Declination, the angular distance of a heavenly body from the equator.

Dichotomized, divided into two parts.

Disk of the Sun or a planet, the hemisphere presented to an observer, appearing like a plain circle.

Eccentricity, the distance in a planet's orbit between one of the foci and the centre.

Eclipse, a partial or total obscuration of a heavenly body.

Ecliptic, a great circle in which the Earth performs its annual revolution round the Sun; or in which the Sun appears to revolve round the Earth.

Elongation, the angular distance of a heavenly body from the centre of its motion; as a planet from the Sun, or a secondary from its primary.

Epact, the excess of the solar above the lunar year of 354 days, or 12 mean lunations.

Equator, a great circle of the Earth drawn round the centre from east to west.

Equinox, a point in the ecliptic, where it is cut by the equator. There are two equinoxes, the vernal and autumnal.

Focus, a point in the elliptical orbit of a planet, round which it revolves.

Foci, the plural of focus, two points in the transverse axis of a planet's orbit.

Galaxy, the milky way.

Geocentric motion, the apparent motion of a planet as seen from the Earth.

Gibbous, convex, protuberant; applied to the Moon between the first quarter and the full, or between the full and last quarter; also applied to some of the planets.

Globe, a sphere representing the Earth or visible heavens. Golden number, a period of 19 years; the cycle of the Moon.

Heliocentric motion, the motion of a planet as seen from the Sun.

Hemisphere, half of a sphere or globe.

Horizon, a great circle of the Earth, 90° from the zenith of a place, the plane of which divides the Earth into upper and lower hemispheres. This is denominated the rational horizon. The sensible horizon is the circle which bounds our sight.

Inclination, the angular distance between the orbit of a planet and the ecliptic.

Latitude of a heavenly body, its distance north or south from the ecliptic.

Latitude, on the Earth, the distance north or south from the equator.

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