Part 11

The next long period when Venus will shine as an evening star will comprise the spring and early summer of 1916. She will be at her greatest distance from the sun during the last week of April, and will not pass from view until about the first of July. Then again she will be an evening star, and so seen in the west during the autumn of 1917 and the winter of 1917–18, reaching greatest eastern elongation during the first few days of December, 1917. Her next return to the evening sky will be for the first eight months of 1919, and the next will be for the winter of 1920–21 and the spring of 1921.

The synodic period of Venus is nearly five hundred and eighty-four days, or a little more than one year and seven months. That is, the planet returns to the same position with relation to the sun and the earth at intervals of about that length. The intervals do vary, however, as much as a week or more, owing to the various motions and situations of the planet and the earth. But every eight years Venus and the earth come around to almost exactly the same relative position with each other and the sun and the stars, and thus the appearances of Venus at the various seasons practically repeat themselves every eight years. The full splendor that she is to offer us in the summer of 1914 will be repeated in 1922, just as that of 1914 will but repeat that which she showed in 1906. And in each of the intervening years she will have again the same appearances that she had eight years before.

With the following table as a guide, the appearances of Venus can be followed through a number of years with sufficient accuracy for any but a close student of her movements. The exact dates of elongations and conjunctions will vary a few days, but for at least two or three multiples of eight years not enough to make any material difference in her various aspects.

1913--1921--1929--1937

Greatest eastern elongation, February 12th. Inferior conjunction, April 24th. Greatest western elongation, July 3d.

1914--1922--1930--1938

Superior conjunction, February 11th. Greatest eastern elongation, September 17th. Inferior conjunction, November 27th.

1915--1923--1931

Greatest western elongation, February 8th. Superior conjunction, September 14th.

1916--1924--1932

Greatest eastern elongation, April 26th. Inferior conjunction, July 5th. Greatest western elongation, September 14th.

1917--1925--1933

Superior conjunction, April 28th. Greatest eastern elongation, December 2d.

1918--1926--1934

Inferior conjunction, February 11th. Greatest eastern elongation, April 22d. Superior conjunction, November 25th.

1919--1927--1935

Greatest eastern elongation, July 6th. Inferior conjunction, September 14th. Greatest western elongation, November 25th.

1920--1928

Superior conjunction, July 5th.

The meetings of Venus with the other planets do not, however, occur with this delightful regularity. They all are moving about in their own ways, and engaged in their own affairs, and only the earth gets back to repeat the meeting with her in just eight years. These eight-year cycles are due to the fact that Venus makes thirteen revolutions around the sun while the earth makes eight. Her journey around the sun requires a little less than two hundred and twenty-five days (224.70), and the earth completes its revolution in a little more than three hundred and sixty-five days (365.25). So at the end of about two thousand nine hundred and twenty-two days--which equals eight years--they come into almost exactly the same relative positions in their orbits with which they started out, and begin the cycle anew.

DISTANCE AND BRILLIANCY

The mean distance of Venus from the sun is 67,269,000 miles. Her orbit more nearly approaches the form of a circle than that of any other planet. It is, like the orbits of the other planets, an ellipse, but of such small eccentricity that the difference between her greatest and least distance from the sun is scarcely more than a million miles. Light, traveling as it does, at the rate of a little more than one hundred and eighty-six thousand miles a second, goes from the sun to Venus in about six minutes. It takes something more than eight minutes for light-rays to come from the sun to us. When Venus is nearest the earth, her silvery beams come swiftly across to us in a little more than two minutes. When she is farthest from us, the rays of light require a few seconds more than fourteen minutes to travel over the distance. She is, when at her greatest distance, more than one hundred and thirty-five million miles farther from us than when at her nearest. This difference is due not to any great eccentricity in her orbit, or in that of the earth, such as causes Mercury’s great variations of distance, but to the situation of the two bodies in their orbits: they are nearest together when they are on the same side of the sun, and farthest apart when on opposite sides.

Usually at inferior conjunction Venus is a little more than twenty-five million miles from the earth. At her nearest possible approach to us, however, which takes place at inferior conjunction, when the earth is nearest the sun and Venus is farthest from it, a situation which occurs only once or twice in a century, the distance between us and the planet is only a little more than twenty-three million miles. This is nearer than any other heavenly body ever approaches us, except the moon and, so far as we now know, one small asteroid. Also, it is nearer than Venus comes to any other heavenly body except perhaps Mercury. Her nearest approach to that planet is also about twenty-three million miles.

Unfortunately, our comparative proximity to this beautiful planet does not much aid us in learning anything about her personal peculiarities. Shining only by reflected light, and being, like Mercury, situated nearer to the sun than the earth is, when she comes around to the same side of the sun on which we are, her unillumined side is turned toward us, and at the point of very closest approach she is absolutely invisible to the naked eye. Through a telescope, however, she can be seen up to the very point of inferior conjunction. What we see then is a mere curved line of light, so thin is the crescent she presents; but it is always apparent except when the planet makes a transit. During a transit she is actually in our line of sight with the bright disc of the sun, and is neither above nor below it, as at the ordinary times of inferior conjunction. The slender crescent that we ordinarily see offers a very narrow field for observation.

If there is any one on Venus who is studying the earth, he has a much easier task than we have in our effort to learn something about her. The earth is not only somewhat larger than the planet, but when the two bodies are nearest together the disc of the earth is fully illuminated, and so must show a splendid face; and then, our atmosphere probably interferes less with close observation than that of Venus. This little terrestrial system would undoubtedly shine as a magnificent pair of stars if observed from Venus. At that distance our moon would appear considerably larger than Venus appears to us when at superior conjunction, the earth would seem much larger than Venus ever does to us, and the distance between them would seem to be a little more than the apparent diameter of the full moon as we see it. The light of the earth must cause much more of a shadow than we ever get from the light of Venus.

It has been suggested that light from the earth is responsible for a dusky illumination of the dark side of Venus, which is occasionally seen, and which enables us to distinguish her entire outline even when only the merest line of a crescent is really illuminated. It is known to be earth-shine that causes what is apparently the same phenomenon often seen by us on the moon; but it seems that there is no reason to think that our earth, at its distance, would be sufficiently brilliant to illuminate Venus even so slightly. The cause of the illumination is not known; but it is thought that it may have some electrical origin, probably similar to that of our aurora.

Venus has the same phases that Mercury has. She shows her full face when at superior conjunction, and is then farthest away and smallest to our view. As she moves toward us she first becomes gibbous, and then, at eastern elongation, like a half-moon. As she comes nearer to inferior conjunction, and hence nearer to us, she becomes a thinner and thinner crescent, and as she goes from inferior to superior conjunction these phases are repeated in reverse order. We see less than half of her face when she is at her greatest brilliancy, a phase which usually occurs when she is about forty degrees from the sun, as she is a few weeks before and after inferior conjunction. A very small glass will show the phases of Venus. They have occasionally been seen without artificial aid to vision by an exceptionally good eye. They were not known, however, until they were discovered by Galileo after the invention of the telescope in 1610.

Venus would be many times brighter than she ever appears if the entire disc of the planet could be seen when it is nearest to us. The apparent diameter of the disc at that time is nearly seven times larger than when we see it at the planet’s greatest distance from us. When Venus is in superior conjunction and farthest from the earth the disc measures only ten seconds, while at inferior conjunction its measure is nearly sixty-seven seconds. The diameter of the moon is about 1,868 seconds, so one could string across the diameter of the moon one hundred and eighty-six such planets as Venus appears to be when at her smallest, and only twenty-seven of the size that she appears to be when at her largest. Between these two extremes of size she changes gradually, day by day, from large to small and small to large, in ceaseless succession, as she approaches the earth and recedes from it in her orbital journey. Apparent diameter is determined by an actual measurement of the disc of a planet, and in the case of Venus indicates nothing as to brightness. When the apparent diameter is largest she is not visible to the naked eye.