the mean distance of thirty seven millions of miles, and completes his revolution in about three months. According to Sir Isaac Newton, the heat and light of the sun on the surface of Mercury, are almost seven times as intense as on the surface of the earth in the middle of summer; which, as he found by experiments made for that purpose with a thermometer, is sufficient to make water fly off in steam and vapour. Such a degree of heat, therefore, must render Mercury uninhabitable to creatures of our constitution; and if bodies on its surface be not inflamed and set on fire, it must be because their degree of density is proportionably greater than that of such bodies is with us. When Mercury passes over the sun's face, or is between us and the sun, this is called his transit, and the planet appears like a black spot in the sun's disk. The light emitted by Mercury is a very bright white. Fair Venus next fulfils her larger round, With softer beams, and milder glory crowned; Venus is computed to be sixty-eight millions of miles from the sun, and completes her annual rotation in about seven and a half months, turning on her axis in a little less than twenty four hours. The light, which this planet reflects, is very brilliant, and often renders her visible to the naked eye in the day-time. When Venus is to the west of the sun, she rises before the sun, and is called the morning star; when she appears to the east of the sun, she shines in the evening, and is then called the evening star. She is in each situation alternately, for about two hundred and ninety days; and, during the whole of her revolution, she appears, through a telescope, to have all the various shapes and ap pearances of the moon. As the orbit of Venus is within that of the earth, like Mercury, she sometimes passes over the sun's face, and her transits have been applied to one of the most important problems in astronomy,—that of determining the true distances of the planets from the sun, The atmo sphere of Venus has been calculated to be fifty miles high; this has been learned from observing her transits, when her atmosphere was seen to throw a shade on the sun's disk about five seconds before the more opaque part touched his edge. When the elongation of Venus is about forty degrees, her lustre far exceeds that of the moon, at the same apparent distance from the sun. For though the moon reflects more light to us than Venus does, yet this light is dull, and has none of the briskness which attends the beams of Venus, This difference is supposed to arise from the circumstance of Venus having an atmosphere far more dense than that of the moon. seen? QUESTIONS. 1. What is the appearance of Mercury? 2. What is the length of his year?-Distance from the sun? 3. Why is it seldom 4. What is its greatest elongation? 5. What calculation did Newton make with respect to the light and heat of Mercury? 6. What must be the consequence of such a degree of heat? 7. What is called a transit of Mercury? 8. What is the distance of Venus from the sun?—Length of her year?-Day? 9. When is Venus evening and when morning star?-How long in each situation? 10. To what purpose have her transits been applied? 11. What is said of her atmosphere? 12. When is the lustre of Venus greatest, and to what is it attributed? LESSON 44. The Earth. Meridian, a great circle passing through the poles of the world, and also through both zenith and nadir; it crosses the equator at right angles, and divides the sphere into two hemispheres, the eastern and the western; it has its poles at the east and west points of the horizon. It re THE planet which we inhabit is called the earth. volves about the sun at the mean distance of ninety-five, or, as some state, of ninety-three millions of miles. It completes this revolution in a year, and turns on its axis in a day, or twenty-four hours. If the earth were seen from the sun, it would appear to describe, while revolving in its orbit, a circle among the stars. But to us on the earth, the sun appears to describe precisely the same circle, only beginning at the opposite point. That imaginary great circle in the heavens, which the sun appears to describe in the course of the year, is called the ecliptic. The apparent diurnal, or daily motion of the sun is very different from the path which appears to traverse in the course of a year. The former 98 CELESTIAL LATITUDE AND LONGITUDE. is observed by the most inattentive spectator; but the knowledge of the latter must be the result of patient observation. The other primary planets, when seen from the sun, do not describe exactly the same circle among the stars, that the earth does; but are sometimes on one side of the ecliptic and sometimes on the other. But none of them, except Juno, Pallas, and Ceres, are ever farther distant from the ecliptic than eight degrees. So that within a zone or belt of sixteen degrees, that is, eight degrees on each side of the ecliptic, the planets, except those just named, are always to be found. This zone, or broad belt, is called the Zodiac. The ecliptic then is an imaginary circle in the heavens passing through the middle of the zodiac, and situated in the plane of the earth's orbit. A plane is an even level surface. If you suppose a smooth thin solid plane cutting the sun through the centre, extending out as far as the fixed stars, and terminating in a circle which passes through the middle of the zodiac; in this plane the earth would move in its revolution round the sun; it is therefore called the plane of the earth's orbit. The points, where the orbit of any hea venly body cuts the plane of the ecliptic, are called the nodes of that body. The point, where the body passes from the north side of the plane of the ecliptic to the south, is called its descending node; where it passes from the south to the north, its ascending node. The ecliptic, as well as every other circle, great or small, is divided into three hundred and sixty degrees; but it has also another division into twelve signs, of thirty degrees each, called the twelve signs of the zodiac. These signs derive their names from clusters of stars, or constellations, which, as the ancients imagined, resembled certain animals. They are most commonly represented by characters, and the names given them should be made familiar; for the sun, as he appears to move round in the ecliptic, seems to enter these clusters of stars, and is therefore said to be in this or that sign. If the axis of the earth be supposed to extend both ways to the starry heavens, its places or points among the stars are the celestial poles, one north and the other south, directly over or beyond the poles of the earth of the same name. If the plane of the earth's equator were extended every way to the starry heavens, the circle it would make among the CELESTIAL LATITUDE AND LONGITUDE. 99 stars is called the celestial equator. Now the celestial equator does not coincide with the ecliptic, but makes an angle with it of twenty-three degrees and twenty-eight minutes, that is, the axis of the earth is not perpendicular to the plane of the ecliptic, but is inclined twenty-three degrees and twenty-eight minutes. Thus we have two great circles, the ecliptic and equator, passing through the heavens eastwardly and westwardly, from either of which the latitude of the heavenly bodies might be estimated. But astronomers have se lected the ecliptic for this purpose, and have supposed lines or circles to cross it at right angles, as the meridians do the equator; which lines or circles are called secondaries to the ecliptic. The points where all the secondaries meet, are called the poles of the ecliptic; which points are twenty-three degrees twenty-eight minutes from the celestial poles. Hence the latitude of a heavenly body is its distance from the ecliptic, measured on a secondary to the ecliptic; and like latitude on the earth, it can never exceed ninety degrees. The longitude of a heavenly body is the distance of a secondary to the ecliptic, reckoned from some given uniform secondary, called the prime secondary. But the longitude of heavenly bodies, unlike longitude on the earth, is reckoned only eastward; it may extend, therefore, to three hundred and sixty degrees. It is usually stated in signs, degrees, minutes, and so forth; and the prime secondary, from which it is reckoned, cuts the ecliptic in the beginning of the sign Aries, a point where the celestial equator crosses the ecliptic. If a secondary, for instance, passing through a heavenly body, cuts the ecliptic eighteen degrees in the sign Capricorn, then, since the first point of Capricorn is nine signs eastward from the first point of Aries, the longitude of that body is nine signs, eighteen degrees. But it is often important to know the distance of a heavenly body from the celestial equator, as well as from the ecliptic. This distance is its declination, and is reckoned on a meridian, as latitude is on the earth. Its distance from the beginning of Aries, reckoned on the equator, is its right ascension; which, like celestial longitude, is reckoned through the whole circle, or three hundred and sixty degrees. Two planets are said to be in conjunction with each other, when they have the same longitude, or are in the same degree of the ecliptic on the same side of the heavens, though their latitude be different. They are said to be in opposition, when their longitudes differ half a circle, or they are in opposite sides of the heavens. QUESTIONS.-1. What is the ecliptic ?-explain. 2. What is the zodiac?explain. 3. What is meant by the plane of the earth's orbit? 4. What are nodes? 5. What are the divisions of the ecliptic? 6. What are the celestial poles? 7. What is the celestial equator? 8. How is the axis of the earth situated with regard to the plane of the ecliptic? 9. What are the poles of the ecliptic? 10. What is the latitude of a heavenly body? 11. The longitude? 12. How is the longitude of a heavenly body reckoned and stated? 13. What example is given? 14. What is the declination of á heavenly body. 15. Right ascension? 16. When are two planets said to be in conjunc tion? 17. In opposition? [NOTE. The points at which the ecliptic cuts the celestial equator are called the equinoctial points. Those two points of the ecliptic farthest from the equator are called solstices. Ap'ogee, that point of the orbit of the moon which is farthest from the earth. Perigee, that point which is nearest to the earth.] 18. Look at fig. 40. and point out the ecliptic, zodiac, and signs of the zodiae. LESSON 45. Day and Night. Ver'nal, belonging to the spring. Intersect', to cut, to divide each other mutually. By the diurnal motion of the earth, the same phenomena appear as if all the celestial bodies turned round it; so that in its rotation from west to east, when the sun or a star just appears on the eastern side of the horizon, it is said to be rising, and as the earth continues its revolution, it seems gradually to ascend till it has reached its meridian; here the object has its greatest elevation, and begins to decline till it set, or become invisible on the western side. In the same manner the sun appears to rise and run his course to the western horizon, where he disappears and night ensues, till he again illuminate the same part of the earth in another diurnal revolution. One half of the earth's surface is constantly illuminated, and by the regular motion of the earth on its axis, every place is successively brought into light and immersed in darkness. If the axis of the earth were always perpendicular to the plane of the ecliptic, the days would every where be of the same length, and just as long as the |