CHAPTER XXVII

THE INNER PLANETS

VULCAN

About the middle of the last century, Le Verrier, a great French astronomer, having added the planet Neptune beyond the outside confines of the solar system, sought evidence of a lesser planet traveling round the sun within the orbit of Mercury. For many years close watch was kept on the sun in the hope of discovering such a body in the act of passing across the disk, or in transit, as it is technically termed. Lescarbault, a French physician, announced that he had actually seen such a planet, Vulcan it was called, passing over the sun in 1859. Total eclipses of the sun would afford the best opportunity for seeing such a body, and on several such occasions astronomers thought they had found it. But the signal advantages of photography have been applied so often to this search, and always unsuccessfully, that the existence of Vulcan, or the intramercurian planet, is now regarded as mythical.

MERCURY

This planet is an elusive body that very few, even astronomers, have ever seen. It is not very bright, has a rapid motion and never retreats far from the sun, so that it was a puzzle to the ancients who saw it, sometimes in the twilight after sunset and again in the twilight of dawn. When following the sun down in the west, in March or April, Mercury is likely to be best seen; twinkling rather violently and nearly as bright as a star of the first magnitude.

Very little is to be seen on the minute disk of this planet, except that it goes through all the phases of the moon--crescent, gibbous, full, gibbous, crescent. Whether Mercury turns round on its axis or not, cannot be said to be known, because the markings that are suspected on its surface are too indefinite to permit exact observation. More than likely the planet presents always the same side or face to the sun, so that it turns round on its axis once, while traveling once around the sun in its orbit. Mercury's day and year would therefore be equal in length. Nor have we much evidence on the question of an atmosphere surrounding Mercury; probably it is very thin, if indeed there is any at all. When Mercury comes directly between us and the sun, crossing in transit, the edge of the planet as projected against the sun is very sharply defined, and this would indicate an absence of atmosphere on Mercury.

Transits of Mercury can occur in May and November only: there was one on November 7, 1914, and there will be one on May 7, 1924. The latter will be nearly eight hours in length, which is almost the limit. Mercury's distance from the sun averages 36 million miles, the diameter of the planet is 3,000 miles, and his orbital speed is 30 miles per second, the swiftest of all the planets. No moon of Mercury is known to exist, although many times diligently searched for, especially during transits of the planet.

VENUS

Brightest of all the planets, and the most beautiful of all is Venus. Its path is next outside the orbit of Mercury, but within that of the earth, so that it partakes of all the phases of the moon. Like Mercury it sometimes passes exactly between us and the sun, a rare phenomenon which is known as a transit of Venus.

Being without telescopes, the ancients knew nothing about these occurrences, but they were puzzled for centuries over the appearance of the planet in the west after sunset, when they called it Hesperus, and in early dawn in the east when they gave it the name Phosphorus.

Venus is known to be girdled with an atmosphere denser than ours, and it seems to be always filled with dense clouds. It is the reflection of sunlight from this perpetually cloudy exterior which gives Venus her singular radiance. So brilliant is she that even full daylight is not strong enough to overpower her rays; and she may often be seen glistening in the clear blue daytime sky, if one knows pretty nearly in what direction to look for her.

Venus is 67 million miles from the sun, and as our own distance is 93 million miles, this planet can come within 26 million miles of the earth. It is therefore at times our nearest known neighbor in space, excepting only the Moon and Eros, one of the erratic little planets that travel round the sun between Mars and Jupiter. Also possibly a comet might come much nearer.

Astronomers always take advantage of this nearness of Venus to us, if a transit across the sun takes place; because it affords an excellent method of finding out what the distance of the sun is from the Earth. A pair of these transits happens about once a century, there were transits in 1874 and 1882, and the next pair occur in 2004 and 2012. In actual size, Venus is almost as large a planet as our own, being 7,700 miles in diameter, as compared with 7,920 for the earth. Her velocity in her orbit is twenty-two miles per second, and she travels all the way round the sun in seven and one half months or 225 days.

Venus from her striking brilliancy always leads the novice to expect to see great things on applying the telescope. But aside from a brilliant disk, now a slender crescent, now half full like the moon at quarter, and again gibbous as the moon is between quarter and full, the telescope reveals but little. There is pretty good evidence that the markings thought to have been seen on the planet's surface are illusory, and so it is wholly uncertain in what direction the planet's axis lies; also there is great uncertainty about the length of the day on Venus, or the period of turning round on its axis. Probably it is the same in length as the planet's year.

Once when Venus passed very close to the sun, just barely escaping a transit, Lyman of Yale University caught sight of it by hiding the sun behind a tall building or church spire. The dark side of Venus was turned toward us and he could not of course see that. But the planet was clearly there, completely encircled by a narrow delicate luminous ring, which was due to sunlight shining through the atmosphere that surrounds the planet. Similar ring effects were seen by observers of the transits of Venus in 1874 and 1882; and from all their observations it is concluded that Venus has an atmosphere probably at least twice as dense and extensive as that which encircles the earth. Spurious satellites of Venus are many, but no real moon is known to attend this planet.