| Eastern Elongation | Western Elongation | ||
| (Evening Star). | (Morning Star). | ||
| 18 | November, 1912. | 27 | December, 1912. |
| 10 | March, 1913. | 24 | April, 1913. |
| (Favorable for viewing.) | |||
| 7 | July, 1913. | 22 | August, 1913. |
| (Favorable for viewing.) | |||
| 1 | November, 1913. | 10 | December, 1913. |
| 22 | February, 1914. | 6 | April, 1914. |
| (Favorable for viewing.) | |||
| 18 | June, 1914. | 5 | August, 1914. |
| (Favorable for viewing.) | |||
| 15 | October, 1914. | 23 | November, 1914. |
DISTANCE AND BRIGHTNESS
Of all the planets Mercury is nearest the sun. His average distance is thirty-six million miles. He is nearly eighty times nearer than Neptune, the outermost planet, and more than two and one-half times nearer than we are. But his orbit departs so far from being a circle that his distance from the sun varies as much as fifteen million miles. When he is nearest the sun, or in perihelion, he is only twenty-eight million miles from it; when he is farthest, or in aphelion, his distance is forty-three million miles. There is even greater variation in his distance from us. The difference between his least possible and his greatest possible distance from us is as much as eighty-nine millions of miles. For the earth has an elliptical orbit as well as Mercury, and when we are at perihelion, which occurs in the winter, we are three millions of miles nearer to the sun than we are in mid-summer. If Mercury chances to be then at his greatest distance from the sun, and also at inferior conjunction, or between us and the sun, he is only forty-seven millions of miles from us. If, when we are farthest from the sun, he also is at his greatest distance from it, and is in superior conjunction, or on the other side of the sun from us, he is one hundred and thirty-six millions of miles from us.
These changes in distance from the earth have much to do with Mercury’s changes in apparent brightness to us. At his brightest, when he appears at greatest elongation and we can see him without a telescope, he is brighter than Arcturus, the brilliant first-magnitude star in Boötes, that swings over us nightly from early spring to late autumn. When seen with the naked eye, he is also red in color, somewhat like Arcturus; but through a telescope he is dull silver, like the moon, or even more ashy in his paleness. As he goes farther and farther from us he becomes dimmer and dimmer and can be followed only with a telescope until, even with this aid to vision, he is lost in the rays of the sun at superior conjunction. His apparent diameter as mathematically measured varies from five seconds, when he is farthest away, to thirteen seconds, when he is nearest.
When he is at his nearest possible distance from us, light travels from Mercury to us in a little more than four minutes. At his greatest possible distance we could not receive the waves of light that he sends out in less than twelve minutes. As a matter of fact, we do not receive them at all, for, as we have seen, he is invisible when at his greatest possible distance from us, being then on the far side of the sun.
Another cause of Mercury’s apparent change in brightness is due to the fact that, in common with Venus, he goes through phases from crescent to full like the moon. This is, as we have seen, a result of his shining only by reflected light and of his orbit’s being between ours and the sun. If he shone by his own light, he would be at his nearest approach to us a very brilliant body indeed. As it is, his dark side is turned toward us when he is nearest, and when his full face is illuminated he is on the far side of the sun. We see half of his face when he is at greatest elongation; but he is brightest when we see less than half, because he is then nearer to us, and the difference in distance more than compensates for the difference in illumination.
These phases cannot be seen with the naked eye, but it requires only a small telescope to show them, and a very charming little moon-like body Mercury is when we see them. His horns point toward the east when he is coming toward us and nearing inferior conjunction, and when he is backing away from us and going toward greatest western elongation they point toward the west. It was through the blunting of one of these horns when the planet was in certain positions that a mountainous surface was suspected, so great is the significance of small details in observations.
As a mere place from which to view the other bright bodies Mercury would be far superior to the earth. He not only has the sun nearly seven times larger in appearance at its mean distance than we see it, but, being himself nearest the sun, all the other planets are outer planets in relation to him, and all have their discs fully illuminated.
The earth and the moon, as seen from Mercury, would show as a splendid pair of stars circling about each other, the earth more brilliant than any first-magnitude star, and the moon of the third magnitude, or about as bright as Phecda, the star at the bottom of the bowl of the Big Dipper, just under the beginning of the handle. The earth would show a disc of about twenty seconds, and the moon one of about eight seconds, with a distance between them of about 871 seconds. Some idea of what this distance is may be had if one knows Mizar, the star at the bend of the handle of the Dipper, and its tiny shining attendant, Alcor. These two stars are 708 seconds apart. The distance between them is about equal to one-third of the diameter of the moon as measured from the earth. It does not appear to be nearly so much as that, and some persons have difficulty in separating the two stars; but the moon is not only inconstant but deceptive, and owing to its brilliancy seems always proportionately larger than it really measures.