occultation) behind the moon should be shortened by a lunar atmosphere refracting the light of the star; and the presence of such an atmosphere should cause the moon's light to fade away gradually, at the boundary between her dark and enlightened hemispheres, just as from a similar cause our daylight fades into twilight.

19. Q. What indications are there of Lunar Inhabitants?

A. The absence of an atmosphere would seem to render impossible the existence on the moon of such forms of life as prevail here, and any buildings or other signs of inhabitants, if such exist, could not be perceived from the earth. Yet air, water, and even inhabitants may exist on the opposite side of the moon, though the hemisphere visible from the earth appears bleak and desolate, and without any indications of animal or vegetable life.

20. Q. What is probably the climate of the moon?

A. The lunar summer and winter arise from the moon's rotation on her axis, and the climate probably alternates between intense heat-possibly exceeding that of boiling water-accumulated during a fortnight's continuous sunshine, and an exceedingly fierce frost, resulting from continuous, unobstructed radiation during a fortnight's darkness.

21. Q. If the heat becomes so intense on the moon, why is moonlight attended with no perceptible heat?

A. Most of the heat from the moon is probably absorbed by the upper portions of our atmosphere, and this may be the cause why clouds have a

tendency to disappear under a full moon. It has been estimated that nearly 550,000 full moons would be required to equal the light of the sun, which, therefore, must exceed in brightness the light that would be supplied from a sky covered with full moons.

LESSON 10.-ECLIPSES OF THE MOON.

1. Q. How are Eclipses of the Moon produced? A. An Eclipse of the moon is produced by the earth's passing between the sun and the moon, and thus cutting off the sun's light from the moon. Lunar eclipses only occur at full moon when the earth is between the sun and the moon, and in the direct line joining the two.

m

While one-half of the earth E (Fig.) is illumined by the sun S, the other half is in darkness, and this darkness extends, as a shadow, far beyond the earth. If the moon, when full, happens to be at the Node of her orbit, the Sun, Earth, and Moon (S E and 'm) will be in the same straight line, and the moon, being more distant from the sun, must pass through the Earth's Shadow, as at m, and be eclipsed. The darkly-shaded space which converges behind the earth E, and from which all the sun's light is excluded, is called the Umbra. P P, outside the True Shadow, is the Penumbra, or partial shadow, from which only a portion of the sun's rays are cut off. In passing through the Penumbra the moon continues visible, but only part of the sun being visible from her surface, her light becomes feebler the nearer she is to the Umbra, into which first her eastern edge, and then gradually the rest of her surface passes, when the Eclipse is Total. Should the moon be at a certain distance from her Node, part only of her disc may enter the Umbra and then the Eclipse will be Partial.

2. Q. How does the interposition of the Earth between the Sun and Moon produce a Lunar Eclipse?

A. While one-half the earth receives light from the sun the other half is in darkness, and from the great size and globular form of the earth, this darkness extends, as a Cone-shaped Shadow, to three or four times the moon's distance from the earth. Being an opaque body, and less than the earth, the moon is eclipsed whenever she passes into this shadow.

3. Q. Why is there not an Eclipse at every full moon?

A. The earth's shadow moves in the plane of the ecliptic, and if the moon moved in the same plane, there would be about 25 eclipses a-yeara Lunar Eclipse at every Full, and a Solar Eclipse at every New Moon. But since the moon's orbit does not coincide with the ecliptic, a lunar eclipse can only occur when the moon, at full, is at or near one of the Nodes of her orbit (Lesson 12).

4. Q. What is meant by the Nodes of the moon's orbit?

A. The moon's orbit being inclined about 5° to the plane of the ecliptic, part of her path lies above and part below that plane, and the Nodes are the two points at which the moon's orbit crosses the plane of the ecliptic. The Ascending Node is that at which the moon passes from South to North, and that at which she passes from North to South is the Descending Node (Lesson 2).

5. Q. How is the occurrence of lunar eclipses affected by the moon's position relatively to the nodes of her orbit?

A. When the moon's distance from the node exceeds about 12° there can be no lunar eclipse, because the moon will then pass above or below

the path of the earth's shadow; but there must be an eclipse if, when full, the moon is within about 9° from the node. The distance from the node beyond which the moon cannot be eclipsed is called the Lunar Ecliptic Limit.

6. Q. When must lunar eclipses be total?

A. A Lunar Eclipse is Total when the moon passes wholly-and Partial when she only passes partly within the earth's shadow. If, at full, the moon's distance from her node be less than about 3° there must be a Total Lunar Eclipse; but if, when full, her distance from the node exceed about 6° she can only be partly immersed in the shadow, and an eclipse occurring then must be Partial (Lesson 8).

7. Q. Why does the moon's light grow fainter before any part of her disc appears eclipsed?

A. An eclipse really begins when the moon enters the earth's True Shadow or Umbra, but her light gradually grows fainter from the time she enters the Partial Shadow, called the Penumbra, which, "like a smoky haze," surrounds the Umbra. The moon only becomes sensibly darkened when deeply immersed in the penumbra which receives light from but a part of the solar

disc.

8. Q. Why is the moon often visible while totally eclipsed?

A. Though the geometrical umbra reaches far beyond the moon's orbit, the total darkness of the shadow extends to a point only about twothirds that distance, because a portion of the sun's light is refracted, or bent by the earth's

atmosphere, into the part of the umbra beyond that point. Hence the moon, crossing the shadow where it is faintly illumined, has been seen during a total eclipse of a fiery copper colour, and even of a deep blood-red.

9. Q. Why does the moon exhibit various colours when eclipsed?

A. The variety of colours arises from the light, thrown on the moon by refraction, traversing the earth's atmosphere at a greater or less distance from the surface, in consequence of which, less or more of the violet and blue rays are absorbed. The lowest strata impart the hue of sunset, but of double the depth of tint, because double the thickness of atmosphere is traversed (Lesson 28).

The sun being immensely larger than our globe, the Earth's Shadow must be a cone terminating in a point beyond the earth. The Length of this Shadow, i.e., the distance of its apex from the earth, may be calculated when the sun's distance from the earth and the relative lengths of the diameters of the two bodies are known. Further, if the distance and magnitude of the moon be known, the Point where the moon, when eclipsed, will pass through the Earth's Shadow, the Real and Apparent Diameter, or breadth of the Shadow at that point, and the Duration of the Eclipse may also be determined.

Thus to find the Length of the Earth's Shadow the circles (Fig.), of which S and E are the centres, may represent the Sun and Earth respectively; R S the Sun's, and B E the Earth's radius; RB and D C lines touching the surfaces of the Sun and Earth, and, when produced, meeting in the point A, which will represent the apex of the Earth's Cone-shaped Shadow B A C. If ET be drawn parallel, to B R, then B E, the Earth's radius, will equal R T.