Part 13 (1/2)
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...o...b..t. 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...o...b..tal 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 pa.s.ses 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 s.p.a.ce, 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 pa.s.sed 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 s.h.i.+ning 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.
[Ill.u.s.tration: THE SURFACE OF THE MOON IN THE REGION OF COPERNICUS.
Photograph made with the Hooker 100-inch reflecting telescope.
(_Photo, Mt. Wilson Solar Observatory._)]
[Ill.u.s.tration: A VIEW OF THE SOUTH CENTRAL PORTION OF THE MOON AT LAST QUARTER. (_Photo, Mt. Wilson Solar Observatory._)]
CHAPTER XXVIII
THE MOON AND HER SURFACE
As the sun has always reigned as king of day, so is the moon queen of night. Observation of her phases, now waxing, now waning, with her stately motion always eastward among the stars, began with the earliest ages. Often when near the full she must have been seen herself eclipsed, and much more rarely the occurrence of total eclipses of the sun are certain to have suggested the moon's intervention between earth and sun, shutting off the sunlight completely, because these eclipses never took place except when the moon was in the same part of the sky with the sun.
If we watch the nightly march of the moon, we shall find that she travels over her own breadth in about an hour's time. By using a telescope on the stars just eastward or to the left of her, she will now and then be seen to pa.s.s between us and a star--on very rare occasions a planet--extinguis.h.i.+ng its light with great suddenness, the most nearly instantaneous of all phenomena in nature. Draw a line connecting the cusps, or horns of the lunar crescent, and then a line eastward at right angles to this, and it will show the direction of the moon's own motion in its...o...b..t round the earth quite accurately.
As the phase advances, note the inside edge of the advancing crescent: this will be quite rough and jagged, compared to the outside edge which is the moon's real contour and relatively very smooth. The position of the inside curve will change from night to night, and it marks the line of sunrise on the moon during the fortnight elapsing between new moon and full; while from full through last quarter and back to new moon, this advancing line marks the region of sunset on the moon. The general shape of this line is never a circle but always elliptical, and astronomers call it the terminator. All along the terminator, sunlight strikes the lunar surface at a small angle, whether near sunrise or sunset; so that owing to the mountains and other high ma.s.ses of the moon's surface, the terminator is always a more or less jagged and irregular line.
Onward from new moon toward full the horns of the crescent are always turned upward or eastward. When the general line of the terminator becomes a straight line from cusp to cusp, the moon is said to have reached first quarter or quadrature. Onward toward full the terminator will be seen to bend the other way, and in about a week's time it will have merged itself with the moon's limb. The moon is then said to be full. Afterward the phase phenomena recur in the reverse order, with third quarter midway between full and new moon again; the phase of the moon being called gibbous all the way from first quarter to third quarter, except when exactly full.
As we know that the moon is, like the earth, a nonluminous body, and s.h.i.+nes only by virtue of the sunlight falling upon it, clearly an entire half of the moon's globe must be perpetually illumined by sunlight. The varying phases then are due simply to that part of the illuminated hemisphere which is turned toward us. New moon is entirely invisible because the sunward hemisphere is turned wholly away from us, while at full moon we see the lunar disk complete because we are on the same side of the moon that the sun is and practically in line with both sun and moon.
If we could visit the moon, we should see the earth in exactly complementary phase. At new moon here we should be enjoying full earth there, and full moon here would be coincident with new or dark earth there. The narrow crescent of new moon here would be the period of gibbous earth there; and it is the reflection of sunlight from this gibbous earth which illuminates the part of the moon but faintly seen at this time, popularly known as the ”old moon in the new moon's arms.” Its greater visibility at some times than at others is due to greater prevalence of clouded area in the reflecting regions of the earth turned toward the moon, and the higher reflective power of clouds than that possessed by mere land and water.
As the moon goes all the way round the sky every month, the same as the sun does in a year, and travels in nearly the same path, clearly it must also go north and south every month as the sun does. So in midsummer when the sun runs high upon the meridian, we expect to find full moons running low, and likewise in midwinter the full moon always runs high, as almost everyone has sometimes or other noticed.
This eastward or true orbital motion of the moon is responsible for another relation which soon comes to light when we begin to observe the moon; and that is the later hour of rising or setting each night. Our clock time is regulated by the sun, which also is moving eastward about 1 daily, or twice its own breadth. So the moon's eastward gain on the sun amounts to about 12 degrees daily, and one degree being equal to 4 minutes, the r.e.t.a.r.ded time of moonrise or moonset each day amounts to very nearly 50 minutes on the average; though sometimes the delay will be less than a half hour and at other times it will exceed an hour and a quarter. The season of least r.e.t.a.r.dation of rising of the full moon is in the autumn, and so the moon that falls in late September or October is known as the Harvest moon, and the next succeeding full moon is called the Hunter's moon.