The sun is accompanied by a phenomenon called the zodiacal light. It is a beam of light of a triangular form, visible a little after sunset and before sunrise, with the base towards the sun. It is most clear about the beginning of March in the evening, and in September in the morning, but in the torrid zone it is constantly seen. It is generally supposed to proceed from the sun's atmosphere.

QUESTIONS.-1. What is the figure of the sun? 2. Describe the motions of the sun. 3. How is it made evident that the sun has a rotation round his axis? 4. What is said of the spots that have been seen in the sun? 5. What does Dr. Herschel consider the sun to be? -The spots? 6. Describe the zodiacal light. 7. In what proportion do the planets receive light and heat from the sun? (see Appendix.) 8. What rule is given? 9. What is said of the attraction of bodies? 10. What is the rule for finding the distances of the planets from the sun? 11. What was ascertained by Kepler? 12. What is the rule for finding how many times one planet is greater than another? [NOTE. When any body, revolving round the sun, is nearest to him, it is said to be in its perihe'lion; and when it is most distant, in its aphe'lion (pron. ǎf-e'le-un.) The common centre about which the sun revolves in its periodical motion is always found to be exceedingly near the sun, and most commonly within it: it may, therefore, without any material error, be regarded as the centre of the planetary system.]

LESSON 43.

Mercury and Venus.

Elonga'tion, a planet's elongation, or its angular distance from the sun, is an angle formed at the earth by two lines, one drawn from the earth to the sun, and one from the earth to the planet. Disk, the face of the sun and moon, as it appears to us on the earth.

MERCURY is seldom visible to the inhabitants of the earth, for its greatest apparent distance from the sun, or its greatest elongation, is not more than twenty-eight degrees, and its reflected light is absorbed in the more powerful rays of the sun. He always appears on the same side of the heavens with the sun; of course, he can be seen in the east, only in the morning a little before sunrise, and in the west in the evening a little after sunset. When viewed with a telescope of high magnifying power, he exhibits nearly the same phases as the moon, and they are to be accounted for in the same manner. Mercury revolves round the sun at nearly

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;
Friend to mankind, she glitters from afar,
Now the bright evening, now the morning star.
BAKER.

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 Ele 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 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 atmosphere 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

ap

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 cir'cumstance of Venus having an atmosphere far more dense than that of the moon.

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 seen? 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.

THE planet which we inhabit is called the earth. It re volves about the sun at the mean distance of ninety-five, or, as some state, of ninety-three millions of miles. It com pletes 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, circle among the stars. But to us on the earth, the sun apa pears 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 it 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 ob

servation.

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 heavenly 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 selected 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 hun- ' dred 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.