Animation of the phases of Venus during half a synodic period, created with twenty-six telescopic views (including one of the 2004 transit of Venus, in which it passed in front of the Sun at "new" phase). At the start of the animation, Venus is on the other side of the Sun, relative to our position, and looks relatively small, but nearly fully lit, since we are viewing it from nearly the same direction as the Sun. As Venus moves around its orbit, gaining on us, we see it larger and larger, until it passes in between us and the Sun; at the same time, the direction we view Venus from becomes increasingly different from that of the Sun, until (at transit) we see Venus from the opposite direction the Sun does, causing us to see only the night-time side. In other words, as Venus moves from the opposite side of the Sun to "our" side, the fraction we see lit steadily shrinks, while the apparent size of the planet steadily grows. After transit, the planet moves away from us, and the process is exactly reversed. (The original animation, taken by "Wah!" in Hong Kong, includes an additional twenty-five views, showing the other half of the synodic period; see apod060110)
A selection of frames from Wah!'s series of images, showing phases on either side of inferior conjunction, and the transit at inferior conjunction, in 2004. Again, note that the closer to us Venus is (and the closer to transit), the larger it looks, but less of the lit side faces us, and more of the dark side.
Transits only occur when Venus is between us and the Sun, at inferior conjunction -- conjunction meaning that it has the same celestial longitude as the Sun, and inferior meaning that it is "lower" (closer to us) than the Sun is. However, at most inferior conjunctions, Venus does not actually transit the Sun. Its orbit is tilted slightly relative to ours, and if it isn't almost exactly at one of the places where its orbital plane crosses ours (the nodes of its orbit), it will appear to pass above or below the Sun, instead of in front of it. As a result, transits only occur twice, separated by almost eight years, in alternating periods of 113 1/2 and 129 1/2 years. The last pair of transits of Venus occurred in December of 1874 and 1882; the current pair, 129 1/2 years later, in early June of 2004 and 2012; the next pair 113 1/2 years later, in early December of 2117 and 2125; the following pair 129 1/2 years later, in early June of 2247 and 2255; and the following pair 113 1/2 years later, in early December of 2360 and 2368. (See Planetary Transits)
The phases of Venus on various dates during 2004, again showing that as Venus grows in size, the lit portion shrinks. (John Rummel, apod040521)