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The image at left shows photos of Venus obtained with the Celestron 8 inch telescope (left) and with the one-inch aperture Galilean telescope (right). The Galilean picture is an average of eleven 1/200th sec exposures taken on April 25, 2004 at 7:11 pm PDT, about an hour before sunset. It is shown at twice its original pixel scale. The 8-inch picture is a single 1/125th sec exposure, reduced in size to match the Galilean photo.
This is a similar sequence taken seventeen days later on May 12, 2004 at 6:05 pm PDT and reproduced at the same scale. The Galilean picture is an average of ten 1/160th sec exposures, while the 8-inch picture is a single 1/250th sec exposure taken 10 minutes later. At the time of these photos, Venus was nearing its closest approach to the Earth. Not only its change in shape, but also the increase in apparent diameter, from 34 arc-seconds on April 25th to 44 arc-seconds on May 12th, is clearly evident.
Galileo was particularly impressed by the observation of Venus changing from these crescent shapes to a full round disk as it receded from the Earth. This confirmed that Venus' orbit extended to the opposite side of the Sun from the Earth, a result predicted by Copernicus, and in contradiction of Ptolemy's theory that Venus moved in a circular orbit about a moving point between the Earth and the Sun. The crescent phases would be expected under either theory, but the small full round disk would be seen only if Venus orbited the Sun. In a variant of Ptolemy's system, dating back at least as far as Aristotle, Venus was supposed to move in a circle entirely on the far side of the Sun. Under that scheme Venus would always appear as a more of less fully illuminated circular disk, and never show a crescent phase.
The changing angular diameter of Venus as revealed by the telescope (the variation during the complete cycle is much greater than in the examples shown above) was also significant, for the Copernican system predicted dramatic changes in the distance to Venus during the year, yet the naked eye had been able to perceive only small changes in brightness. Finally, the fact that the telescope revealed Venus to be less than fully illuminated refuted the then current (and quite plausible) idea that the planets might shine by their own self-generated light, rather than by the reflection of sunlight.
According to astronomer/historian Owen Gingerich, Galileo's observations of Venus by telescope probably began in September or October of 1610, with the changing phases becoming apparent to him by mid-November. His discovery was not officially announced, however, until New Year's Day, 1611, when he decoded the famous anagram "Haec immatura a me iam frustra leguntur o y," which he had included in a previous letter to the Tuscan ambassador in Prague, Giuliano de Medici, written some three weeks earlier. Galileo used the anagram to establish the priority of his discovery while hedging against the possibility that it might be incorrect or incomplete. When unscrambled (and translated from Latin) the anagram announces "Venus has phases like the Moon." Galileo had earlier that year used the same method (an anagram in a letter) to establish the priority of his discovery of the unusual shape of the planet Saturn.
We now know that Venus goes through a complete cycle of phases in a period of about 19 months. The following diagram, constructed with Patrick Chevalley's free Cartes du Ciel sky mapping software, shows how Venus would have appeared on the first day of each even numbered month from August 1609 to April 1611. More detailed simulations, and a very clear discussion of the geometry required to produce the various phases, can be found in several of the articles cited below, especially the one by Paolo Palmieri (2001).
When Galileo completed his first successful nine-power telescope and demonstrated it (as a military instrument) to various dignitaries in Venice between August 21 and 24, 1609 (see Rosen, 1951), he had just missed a chance to see Venus (then a morning star) in its strikingly crescent phase, similar to that in the modern photos shown above. He may well have looked at Venus with the better telescopes which he apparently constructed in the final months of 1609 (and which he used very successfully to explore the Moon and milky way); but by November he would have seen only a small, slightly less than fully illuminated, circle (as pointed out by Peters, the extremely bright glare of the planet makes these shapes very difficult to distinguish unless one looks in twilight when the background sky is not too dark). The round shape would have suggested that Venus was either situated farther away than the Sun, or shining by its own light. Only by early December 1610 did Galileo have a chance to see the shape changing from gibbous (more than half illuminated) to crescent (less than half illuminated) -- a clear proof both that Venus was shining by reflected light and that it had passed from beyond the Sun to closer to the Sun, something that would have been unthinkable to Aristotle or Ptolemy. With clear proof in hand, Galileo promptly announced his discovery.
The fact that this discovery came after the ones announced in Sidereus Nuncius is more a matter of timing than of difficulty. During the very brief portion of each 19-year cycle in when Venus is closest to Earth and exhibits its striking slender crescent phase, its angular size approaches one minute of arc -- almost resolvable by the unaided eye. The crescent shape can be detected with the most modest of instruments. It is somewhat surprising, therefore, that Galileo's predecessors, such as Thomas Harriot, who had a telescope good enough to detect the roughness of the lunar surface by early July of 1609, missed the crescent phase of Venus that year. But then, Harriot announced very little, and perhaps his records have simply been lost.
Galileo's observations of Venus, including a drawing of Saturn, Jupiter, Mars and the phases of Venus that appears in The Assayer (1623), are described by the Museum for the History of Science in Florence. The Venus drawing, which appears in the National Edition of Galileo's Works, can also be seen in context by accessing any of the several copies of The Assayer available on-line. It is also shown on our Photo-Drawing Comparison page. Galileo's discovery of the phases of Venus had earlier been mentioned (but not illustrated) in the opening paragraphs of his 1612 book Discourse on Bodies Floating in Water and in his Sunspot Letters (1613).
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Last modified: October 7, 2006