Besides the craters, the principal features of the Moon are: (a) Dark plains, supposed at first to be surfaces of water and hence called seas, bays, lakes, etc. (b) Ranges of mountains and isolated peaks. (c) "Rills" or cracks, and (d) "Rays" or white streaks, which radiate from certain craters.

The Moon ought to be studied with the aid of a map, for general use; the one in Webb's Celestial Objects for Common Telescopes is probably the best.

The origin of craters, etc., probably has been volcanic; absence of air and water on the Moon; if any changes have occurred on the Moon it has been very difficult to be sure of their reality.

REFERENCES:

Newcomb, Popular Astronomy; Langley, The New Astronomy; Young, General Astronomy, Chapter VII; Webb, Celestial Objects for Common Telescopes; Neison, The Moon; Nasmyth and Carpenter, The Moon; Proctor, The Moon.

QUESTIONS ON LECTURE III

1. At what time of the year does the full Moon remain longest above the horizon?

2. Does the Moon rise every day?

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3. Explain the phases of the Moon. What is the harvest" and "hunter's" moon, and what is the cause?

4. What determines the direction of the horns of the crescent Moon? Can a star ever be seen within the horns?

5. How much of the Moon has actually been seen from the Earth?
Explain the reasons.

6. Describe the general appearance of the surface of the Moon?
7. What reasons are there for believing the Moon has no atmos-
phere?

8. What influence has the Moon on weather?

LECTURE IV

THE PLANETS

Distances, Dimensions: In order of distance from the Sun, the planets are: The Terrestrial Planets: Mercury, Venus, Earth and Mars. The Major Planets: Jupiter, Saturn, Uranus and Neptune. The "Asteroids," "Planetoids " or " minor planets,” about five hundred in number, occupy the wide gap between the two classes.

Relative Distances of Planets from the Sun: Bode's Law: There is a curious approximate relation which makes it easy to remember the distances from the Sun in terms of the EarthSun unit. This is known as Bode's Law. The law is this: Write a series of 4's. To the second 4 add 3; to the third add 3X2, or 6; to the fourth add 3X4, or 12; and so on. The resulting numbers, divided by 10, are very nearly the true mean distances of the planets from the Sun, in terms of the mean distance of the Earth from the Sun, as given in the above table. In the case of Neptune, as is seen from the table, the law utterly breaks down.

Periods: The sidereal period of a planet is the time of its revolution around the Sun from a star to the same star again, as seen from the sun. The synodic period is the time between two successive conjunctions of the planet with the sun, as seen from the earth. If E, P and S are respectively the sidereal periods of the Earth and the planet and the planet's synodic period, then we have the relation

Apparent Motion of the Planets among the Stars: The apparent motion as seen from the Earth is made up of the real motion of the planet about the Sun, and the apparent motion due to the Earth's own movement. Among the stars, the planets' motions are alternately direct (eastward) and retrograde (westward).

Apparent Motions with respect to the Sun: (a) the inferior planets, i. e., those between the Earth and the Sun, oscillate back and forth on each side of the Sun.

(b) The superior planets move continuously westward with respect to the Sun, coming to the meridian earlier each night.

The Ptolomaic system of the universe, the Copernican system, the system of Tycho Brahe, Kepler's Laws of motions. of the planets:

I. The orbits of the planets are ellipses, having the sun in the focus.

2. The radius-vector describes equal areas in equal times. 3. The squares of the periods are proportional to the cubes of the mean distances.

Sir Isaac Newton and the Law of Universal Gravitation. INDIVIDUAL PLANETS. Mercury: the exceptional planet of the solar system. It is the nearest planet to the Sun, receives the most light and heat, is the swiftest in its movement, and (excepting some of the asteroids) has the most eccentric orbit, with the greatest inclination to the ecliptic. It has a diameter smaller and mass less than any of the planets. It is rarely seen by the people generally, because it never goes far from the Sun. Period, 88 days; synodic period, 116 days; distance from Sun, 36,000,000 miles, with a variation of 15,000,000 miles each way; its phases, difficulty of observation.

Venus: the Earth's twin sister, the brightest and most conspicuous of all the planets. Distance, 67,200,000 miles; period, 225 days; synodic period, 584 days; phases, first announced by Galileo; surface markings; evidences of atmosphere, no satellites.

Mars: its mean distance from the Sun is 141,500,000 miles, but the eccentricity of the orbit is so considerable that the distance varies about 13,000,000 miles. The light and heat it

receives from the Sun is somewhat less than half that received

by the Earth. At opposition its average distance from the Earth is 48,600,000 miles (141,500,000 — 92,900,000). When the opposition occurs when near the planet's perihelium this distance is reduced to 35,000,000 miles; if near aphelion, it is over 61,000,000 miles. At conjunction the average distance from the Earth is 224,400,000 miles (141,500,000 + 92,900,000). The apparent diameter and brilliance of the plant, of course, vary enormously with these great changes of distance. If R is the planet's distance from the Sun, D its distance from the

The apparent diameter varies from 3".6 at conjunction to 25".0 at favorable opposition; its real diameter is 4,200 miles. Its surface 0.28; its volume, 1/; its mass, 1; its density, 73 per cent; its superficial gravity, 38 per cent of the Earth's. Rotation 24 h. 37 m. 22s.67, the inclination of axis to its orbit, 24° 50'; both nearly the same as for the Earth.

Telescopic appearance and surface markings; polar caps; recent discoveries, the "canals "; atmosphere and temperature. Is Mars inhabited?

Two satellites; one rises in the east and sets in the west, the other rises in the west and sets in the east.

REFERENCES:

Newcomb, Popular Astronomy; Young, General Astronomy and
Manual of Astronomy; Webb, Celestial Objects for Common
Telescopes; Langley, The New Astronomy; Clerke, History of
Astronomy during the Nineteenth Century (edition of 1902);
Lowell, Mars.

QUESTIONS ON LECTURE IV:

1. What are Kepler's laws of planetary motions, and what inferences flow from them?

2. What is the law of gravitation?

3. Give the names, distances in miles from the Sun, distances in terms of earth distance = 1 from the Sun, periods, diameters, etc., of the bodies of the solar system.

4. Describe the apparent motion of Venus among the fixed stars. 5. Why is it we can never see Venus at midnight?

6. Give a general description of the planet Mars. Write an account of the scientific investigation regarding the so-called “canals.” 7. What do you think of the inhabitability of the planet Mars?

LECTURE V

THE GIANT PLANETS

Jupiter: Jupiter, while not as brilliant as Venus, is next to her in this respect, being on the average about five times brighter than the brightest of the fixed stars. Moreover, Jupiter being a "superior" planet, is visible, unlike Venus, in the midnight sky.

Distance, 483,000,000 miles from the Sun, with a variation of 21,000,000 miles each way. Period, 11.86 years; synodic period, 399 days. Mean diameter is 86,500 miles, almost eleven times that of the Earth; its surface is 119 times, its volume 1,300 times that of the Earth; mass, 318 times the Earth's; density, about one-fourth that of the Earth's, or about the same as the Sun's.

Jupiter is the giant of the Sun's family; and whether we regard its bulk or its mass, it is larger than all the rest of the planets put together.

The planet rotates on its axis in about 9 h. 55 m. The time can be given only approximately, not because there are not distinct enough markings to determine the time more closely, but for the reason that different times are obtained from different spots, according to their nature and their distance from the planet's equator. Like the Sun, Jupiter rotates more swiftly at the equator. However, white and dark spots which are at nearly the same latitudes on Jupiter rotate at entirely different rates of speed.

Telescopic appearance, most noticeable feature the belts. The markings upon the planet are almost, if not wholly, atmospheric, as is proved by the manner in which they change their shapes and relative positions. It is hardly probable that we ever see anything upon the solid surface of the planet underneath, nor is it even certain that the planet has anything solid about it. Jupiter is, therefore, a semi-sun."

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Satellites, five in number. Four of them the first heavenly bodies ever "discovered" with a telescope. Found by Galileo, January 7, 1610. The "fifth" satellite discovered by Barnard, September, 1892, a very "difficult" object, even for a large

telescope