Elongation.

Morning and evening star.

Phases.

elongation. When moving from V toward C' it apparently approaches the sun. C' is the point of superior conjunction. After passing C' Venus is at the left of the sun, rising and setting after the sun does. V' is the point of greatest eastern elongation. After passing V' the planet swings back toward the sun.

In this explanation we have tacitly assumed that the earth is at rest; in reality

C

S

FIG. 107.-CONJUNCTION AND ELON

GATION.

it is moving in the same direction as Venus, but

more slowly. This simply lengthens the time which elapses between inferior conjunction and greatest western elongation, or between any two of the positions which we have just defined.

Greatest western elongation really comes when Venus has arrived at V",

the earth meanwhile having moved on to E'. When an inferior planet is west of the sun it is a morning star; when east of the sun it is an evening star and is to be looked for in the west.

Since Mercury and Venus shine by reflecting the sunlight, and have no intrinsic radiance, they exhibit phases similar to those of the moon. At inferior conjunction the dark side of the planet is toward us; as the planet moves out toward western elongation its phase is a crescent like that of the young moon.

At greatest elongation the phase is a semicircle, like the moon at one of its quarters. When the planet is at superior conjunction we look full in its illuminated face,

which is a complete circle. Afterward it descends through the gibbous phase, to a semicircle, and thence to a narrow crescent again, as it approaches inferior conjunction.

Of Mercury little is known, for it is coy and keeps Mercury. close to the sun. The most favorable times for seeing it in the evening are those when it reaches its greatest eastern elongation in March or April. For it is then nearly above the sun at sunset ; at such a time it may be seen every night for two successive weeks, one of which immediately precedes the time of elongation. It is then very plain, even in strong twilight, and is not likely to be confounded with any fixed star.

Its mean distance from the sun is 36,000,000 miles. Its orbit is more eccentric than that of any other of the large planets, so that its actual distance from the sun ranges from 28,500,000 to 43,500,000 miles.

Sunlight upon Mercury is more than twice as intense when it is nearest the sun as when it is farthest away. The average intensity is seven times that which we experience. The diameter of the planet is 3,000 miles, and eighty-eight days are consumed in making a revolution about the sun.

It is very difficult to make out any markings on Mercury's disc. The Italian astronomer, Schiaparelli,* whose observations of the canals of Mars have proven that he is exceptionally keen of sight, has observed certain dim and ill-defined spots whose motion renders it probable that Mercury rotates on its axis in eighty-eight days, and thus presents the same face continually to the

sun.

Its distance.

Intensity of

sunlight.

Rotation.

There is great uncertainty about the presence of air or Air, water, water; certain spectroscopic observations indicate that

* Astronomer at Milan, Italy.

and mountains.

Venus.

Revolution

and rotation.

Shadings.

Ice and snow.

there may be a thin atmosphere, in which water vapor is present. If these be accepted as correct, the dim shadings described by Schiaparelli may be the outlines of seas or continents.

One imaginative astronomer discovered mountains on the planet about a century ago. Though his telescope was a pigmy compared with those of to-day, modern observers have not verified the existence of the mountains.

VENUS.

Venus is a more interesting object than Mercury because it comes nearer to us and is larger and brighter, giving more light than any other planet. Its distance from the sun is 67,000,000 miles, and its orbit is nearly a circle. It is almost as large as the earth, having a diameter of 7,700 miles.

Two hundred and twenty-five days are consumed in making a revolution about the sun. The time of rotation is generally given as about twenty-four hours, this period having been derived from old observations, which have received some confirmation in recent times. Schiaparelli's investigations cast discredit upon this value, and tend to show that Venus, like Mercury, keeps the same face toward the sun.

Many astronomers have seen shadings upon the planet's surface, but they are so ill defined that their cause is unknown. When the planet is a crescent, the horns are brighter than the rest of the surface. Possibly ice and snow at the planet's poles cause this appearance. On the whole, it may be said that telescopic scrutiny of Venus has decided nothing as to the configuration of its surface.

It seems to be covered with a dense atmosphere, which is an effectual bar to our curiosity. The existence

of the atmosphere is shown at the times of its transits.

When Venus is just about to enter upon the sun's disc, Atmosphere.

or has just passed off, it is surrounded by a tiny rim of

light. The sunlight has pierced through the planet's atmosphere and come on to our eyes. It is probable that the atmosphere is denser than our own, but not more than twice as dense. The spectrum contains lines which indicate the presence of water vapor. It is a reasonable inference that Venus is a planet whose sky is almost totally cloudy, and whose atmosphere is continually laden with moisture. On

a day when the entire earth is enveloped in a cloudshell, to an inhabitant of the moon it would present, on a huge scale, the appearance of Venus.

Mars.

Perplexities.

CHAPTER XIV.

MARS AND THE ASTEROIDS.

"And earnest thoughts within me rise,
When I behold afar,

Suspended in the evening skies,

The shield of that red star."

-Longfellow.

MARS is perhaps the most interesting planet, because of the tantalizing chase on which he has led observers. He is at times almost as near as Venus when the latter is in inferior conjunction; yet he is even then so far away that the more delicate features of his surface, like

the canals, are seen with great difficulty, and are the source of much perplexity. Even the marked features which have for generations passed unchallenged under the names of continents and seas are now subjected to rigid. scrutiny, and in

some quarters are denied their time-honored appellations. While there is a fair consensus of opinion as to the majority of appearances seen upon the planet, there is considerable diver