The Copernican theory, which once and forever "brushed the cobwebs out of the sky," by clearing away the mists of pre-existing error, first completely explained the varying positions of the Shepherd's star, irradiating the first or last watch of night, according to her alternate function as the follower or precursor of the sun. As she travels on a path nearer to him by more than twenty-five and a half million miles than that of the earth, she is seen by us on each side of him in turn after passing behind or in front of him. The points at which her orbit expands most widely to our eyes—an effect of course entirely due to perspective, as her distance from the sun is not then actually increased—are called her eastern and western elongations; that at which she passes by the sun on the hither side her inferior, and on the farther side her superior conjunction. At both conjunctions she is lost to our view, since she accompanies the sun so closely as to be lost in his beams, rising and setting at the same time, and travelling with him in his path through the heavens during the day. When at inferior conjunction, or between us and the sun, she turns her dark hemisphere to us like the new moon, and would consequently be invisible in any case, but when in the opposite position, shows us her illuminated face, and is literally a day star, invisible only because effaced by the solar splendor. It is as she gradually separates from him, after leaving this latter position, circling over that half of her orbit which lies to the east of him, that she begins to come into view as an evening star, following him at a greater and greater distance, and consequently setting later, until she attains her greatest eastern elongation, divided from the sun about 45° of his visible circuit through the heavens, and consequently remaining above the horizon for some four hours after him. From this point she again appears to draw nearer to him until she passes on his hither side in inferior conjunction, from which she emerges on the opposite side to the westward, and begins to shine as a morning star, preceding him on his track, at a gradually increasing distance, until attaining her greatest westward elongation, and finally completing her cycle by returning to superior conjunction once more in a period of about five hundred and eighty-four days.
Venus is thus Hesperus or Vesper, the evening star, when following the sun as she passes from beyond him in superior conjunction to inferior conjunction where she is nearest to the earth. As she again leaves him behind in her course from this point to the opposite one of superior conjunction, she appears in her second aspect as Phosphorus or Lucifer, "the sun of morning," and herald of the day, shining as
"The fair star
That gems the glittering coronet of morn."
II.
THE PHASES OF VENUS.
But the changes in the aspect of Venus due to her varying positions in her orbit are not confined to those which cause her to oscillate with a pendulum movement eastward and westward from the sun. The discovery that she undergoes phases exactly like those of the moon, followed that of the existence of Jupiter's satellites as the second great result achieved by the use of the telescope in the hands of Galileo. The fact that the planets were intrinsically dark bodies revolving round the sun, and reflecting its light, as he and Copernicus had maintained, thus received a further ocular demonstration. The Florentine astronomer describes in a letter to a friend how the planet, after emerging from superior conjunction as a morning star, gradually loses her rotundity on the side remote from the luminary, changing first to a half sphere and then to a waning crescent; until, after passing through the stage of absolute extinction when intervening between us and the sun, she re-appears as a morning star, and undergoes the same series of transformations in inverse order. The revelation was indeed so novel and unexpected, that when the slight deformation of the planet's shape was first detected by him, he did not venture to announce it in plain terms but veiled it, according to the prevailing fashion of the time, under a Latin anagram. His celebrated sentence—
"Haec immatura a me jam frustra leguntur."
("Those incomplete observations are as yet read by me in vain.")
forms, by transposing the letters, the more definite statement,