that when the earth is exactly between Jupiter and the sun, the eclipses are seen 83 minutes sooner than the time calculated; and that when the earth is in the opposite point of its orbit, most distant from Jupiter, they happen 84 minutes later than the time calculated. Hence it is inferred that light takes about 16 minutes to travel across the earth's orbit, which is a distance of 190,000,000 miles ;* for if the effects of light were instantaneous, the eclipses would be seen at the same instant in every point of the earth's orbit.

Saturn shines with a pale, feeble light, and may be seen, like Jupiter and Mars, in any quarter of the heavens. He revolves round the sun at the distance of 900,000,000 miles, and finishes his revolution in 29 years, 161 days. His diameter is 79,042 miles, and his magnitude nearly 1000 times that of the earth. He revolves upon his axis in 10 hours 16 minutes. The inclination of his orbit to the plane of the ecliptic is 2o 29', and the place of his ascending node about 21o in Cancer. This planet, when viewed through a telescope, always engages attention by the singular appearance of two concentric rings surrounding the body of the planet, without touching it. The breadth of the inner ring is estimated at 20,000 miles; that of the outer at 7,200, and that of the vacant space between them at 2,839 miles. These rings revolve round the axis of Saturn, and in the plane of his equator, in 10 hours 32 minutes. Various conjectures have been

*This is a velocity of nearly 200,000 miles per second.

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formed, but nothing is known with certainty, respecting the nature and properties of these rings. The disk of Saturn, like that of Jupiter, appears crossed by faint streaks or belts, and its form is even more oblate than that of Jupiter, its polar diameter being to its equatorial in the proportion of 11 to 12. This planet is, moreover, attended by seven moons or satellites, which, with one exception, revolve in the same plane with the ring, in orbits inclined to that of Saturn at an angle of about 30 degrees.

The most remote of all the known planets belonging to the solar system was discovered at Bath by Dr. Herschel, on the 13th of March, 1781. It was named Georgium Sidus by its discoverer, in honour of the king; but by foreigners it is more commonly known by the name of the Herschel Planet, or Uranus. It is invisible to the naked eye, but when viewed through a telescope of small magnifying power, it appears like a star of the sixth or seventh magnitude. Its distance from the sun is estimated at 1,800,000,000 miles, and its periodic revolution is performed in about 84 of our years. Its diameter is 35,112 miles, and its magnitude 87 times that of the earth. The time of its rotation upon its axis has not been ascertained. The inclination of its orbit to the ecliptic is 46 minutes 20 seconds. This planet is attended by six satellites, which revolve round it in orbits nearly perpendicular both to its own orbit and to that of the earth.

PROBLEMS RELATING TO THE PLANETS.

PROBLEM I.

To find the Place of any Planet in the Heavens, at any given time.

Find the planet's longitude by means of an Ephemeris; then its latitude; and thus its place on the celestial sphere will be determined.*

PROBLEM II.

To find the Time of any Planet's Rising, Setting, and Southing.

Find the place of the planet as above, mark that place on the celestial globe, and then proceed as in the case of the moon. See page 91.

PROBLEM III.

To find what Planets are visible at any given hour.

Place the globe so as to represent the situation of the heavens at the given hour, by a former problem, (see page 59,) and by consulting an Ephemeris, it will be found what planets are then in the visible hemisphere.

* In White's Ephemeris, the tables showing the longitudes of the planets for every day of each month, will be found on the righthand page; and the tables of latitude at the top of the same page, but only for every seventh day, as the latitude varies much more slowly than the longitude.

PROBLEM IV.

To represent, by an Orrery or Diagram, the true Situation of the Planets with respect to the Sun, at any given time.

In White's Ephemeris, besides the tables of longitude, mentioned above, there will be found, at the foot of the left-hand page, a set of tables showing the Heliocentric longitudes of the earth and other planets for every seventh day. By the Heliocentric longitude of a planet is meant the place in which it would be seen in the heavens if viewed from the sun. The longitude spoken of in the first problem is the planet's place as seen from the earth, and is sometimes called, by way of distinction, the Geocentric longitude. Find, then, in the table, the heliocentric longitudes of the several planets on the day nearest the given day, and place their representatives in the orrery accordingly, by means of the ecliptic signs which will be found in its circumference. Or, in lieu of an orrery, construct a diagram representing the several orbits of the planets, at their proportional distances from the sun, and surround the whole by a larger circle, which divide into twelve equal parts, marking them with the signs of the ecliptic, and subdividing them into degrees, as in the wooden horizon of the globe. Then, having found by the table the heliocentric longitude of the earth, and marked it upon the outer circle, lay a ruler from that point to the sun's place, and mark where it crosses the earth's orbit. Proceed in the same manner with the

other planets, transferring the longitude of each to its own orbit; and the points thus marked upon the orbits of the planets will show their true relative situation in the solar system at the given time.

CHAPTER XXIII.

OF THE DIMENSIONS OF THE SOLAR SYSTEM, AND THE MODE OF ASCERTAINING THEM.

IN the foregoing account of the solar system, the distances and dimensions of the planets have been stated, without any attempt being made to show the truth of the statements, or the means by which such information has been obtained. On this subject, however, the learner will naturally feel some curiosity; for certainly there is no attainment of human ingenuity more wonderful than that a being so diminutive, and confined to so narrow a corner of the great universe, should have succeeded in measuring magnitudes and distances so far beyond his grasp or reach. The subject, indeed, cannot be fully treated of, without entering into mathematical investigations which would be unsuitable to those for whom this treatise is designed. Nevertheless, to those unversed in mathematics, some information on it may be conveyed, which, while it serves to gratify a laudable curiosity, may excite a