Galileo had already found that his new telescope provided a means of testing astronomical theories. As soon as he had turned it on to the Milky Way, a whole crowd of legends and fables as to its nature and structure had vanished into thin air; it proved to be nothing more than a swarm of faint stars scattered like golden dust on the black background of the sky. Another glance through the telescope had disclosed the true nature of the moon. It had on it mountains which cast shadows, and so proved, as Giordano Bruno had maintained, to be a world like our own. What if the telescope should now in some way prove able to decide between the orthodox doctrine that the earth formed the hub of the universe, and the new doctrine that the earth was only one of a number of bodies, all circling round the sun like moths round a candle-flame? And now Galileo catches Jupiter in the field of his telescope and sees four small bodies circling around the great mass of the planet—like moths round a candle-flame. What he sees is an exact replica of the solar system as imagined by Copernicus, and it provides direct visual proof that such systems are at least not alien to the architectural plan of the universe. On January 30th he writes to Belisario Vinta that these small bodies move round the far greater mass of Jupiter “just as Venus and Mercury, and perhaps the other planets, move round the sun.” Any lingering doubts that Galileo may have felt as to the significance of his discovery are removed nine months later when he observes the phases of Venus. Venus might have been self-luminous, in which case she would always appear as a full circle of light. If she were not self-luminous but moved in a Ptolemaic epicycle, then, as Ptolemy had himself pointed out, she could never shew more than half her surface illuminated. On the other hand, the Copernican view of the solar system required that both Venus and Mercury should exhibit “phases” like those of the moon, their shining surfaces ranging in appearance from crescent-shape through half moon to full moon, and then back through half moon to crescent-shape. That such phases were not shewn by Venus had indeed been urged as an objection to the Copernican theory. Galileo’s telescope now shews that, as Copernicus had foretold, Venus passes through the full cycle of phases, so that, in Galileo’s own words, we “are now supplied with a determination most conclusive, and appealing to the evidence of our senses, of two very important problems, which up to this day have been discussed by the greatest intellects with different conclusions. One is that the planets are not self-luminous. The other is that we are absolutely compelled to say that Venus, and Mercury also, revolve around the sun, as do also all the rest of the planets, a truth believed indeed by the Pythagorean school, by Copernicus, and by Kepler, but never proved by the evidence of our senses, as is now proved in the case of Venus and Mercury.”