Venus, when in a favourable position, is to us the most conspicuous member of the planetary system. Its apparent diameter differs considerably at different times: this is owing to the greater extent of variation of distance than occurs in the case of Mercury. Its phases are similar to those of Mercury, to which the reader is referred. The discovery of the phases of Venus is due to Galileo, who announced the fact to his friend Kepler in the following logograph or anagram: –

These things not ripe, as yet concealed from others, are read by me.

Which, when transposed, becomes —

Little is known of the appearance of this planet's surface, owing to its dazzling brilliancy when viewed in a telescope; notwithstanding this, however, spots are occasionally visible. These spots are not supposed to be connected with the planet's surface, but rather to belong to its atmosphere. Schröter has suspected the existence of numerous high mountains on its surface. The rotation of Venus on its axis was discovered by D. Cassini in the year 1667.