Review of Retrograde Motion
      As the Earth and other planets move around the Sun, they sometimes lap each other. At such times, the inner planet sees the outer planet opposite the Sun, at opposition, and the outer planet sees the inner planet more or less in line with the Sun, at conjunction. There are actually two times when an outer planet sees an inner planet more or less in line with the Sun, so to distinguish the two, the one where the inner planet passes in between the Sun and the outer planet, and they are at their closest to each other, is called inferior conjunction, while the one where the inner planet passes on the other side of the Sun, so that the planets are at their furthest from each other, is called superior conjunction.
      At the time that the two planets are at their closest, they are both going in the same direction, but the inner planet is going faster, so the outer planet appears, from the inner planet, to be going backwards, or in retrograde motion. (The outer planet also sees the inner planet in retrograde motion as well, but that requires a little extra discussion, which will be covered at another time.)
      The most easily observed retrograde motions are when the inner planet sees the outer planet, as it is opposite the Sun, and up all night. The retrograde motions that occur when the outer planet looks at the inner planet are harder to see, because the inner planet is close to the Sun, and only appears in the sky shortly after sunset, or shortly before dawn. Still, particularly for Venus, because of its brilliance, it is relatively easy to observe its retrograde motion, as shown in the image below.

Transits of Venus and Mercury
      The image above displays a rare conjunction, one in which Venus passed directly between the Earth and Sun, producing a rare transit of the Sun. In astronomy, transit has two meanings -- an everyday one, referring to the moment when a celestial body crosses the Meridian, and is at the highest point of its diurnal path; and a more specific one, referring to the rare occasions when Mercury or Venus, passing through inferior conjunction, also passes directly in front of the Sun, and is seen silhouetted against it.
      Each time that an inferior planet passes through conjunction, which is every 116 days for Mercury, and every 584 days for Venus, there is a possibility that it will pass in front of the Sun but because the orbits of the planets are tilted relative to ours, transits only occur happens when the planet is also near a node of its orbit (one of the places where its orbital plane crosses the orbital plane of the Earth), which happens every 44 days for Mercury, and every 112 days for Venus; otherwise, the planet will pass above or below the Sun, instead of in front of it. As a result, transits are relatively rare, occurring only around a dozen times per century for Mercury, and for Venus, less than twice per century. In fact, transits of Venus usually occur in pairs 8 years apart, separated by alternating periods of 105 1/2 years and 121 1/2 years, with the latest transits occurring in June of 1761 and 1769, December of 1874 and 1882, and June of 2004 and 2012; and the following transits occurring in December of 2117 and 2125. Note that there were no transits of Venus in the 1900's, and not one person who was alive at the time of the 1882 transit still lived when the next one occurred in 2004, making it a truly rare event.
      Since the image above, showing the 2004 retrograde motion of Venus, includes the inferior conjunction for the 2004 transit, if you look very carefully, you can see Venus in front of the Sun; but in case you can't, the image immediately below shows that portion of the image considerably enlarged; and below that is a spectacular image of the transit, one of many posted on various websites during the weeks following the 2004 transit. Note that in the images below, Venus and Mercury look larger in comparison to the Sun than they really are, because when passing between us and the Sun, they are closer to us than the Sun is.

      Similarly, the image below shows a more common, but still not everyday transit of Mercury which occurred in May of 2003. Unlike the images of Venus' transit, which showed the position of Venus at only one moment, the image below is a superposition of 23 images taken about 15 minutes apart, over a period of more than 5 hours (in an animation, Mercury would be moving down and to the right as time passed, because it was in the middle of its retrograde motion, and therefore appeared to be moving westward relative to the Sun and stars). Recent transits of Mercury occurred in November of 1973, 1986, 1993 and 1999, and in May of 2003. The next few occur in November of 2006, May of 2016, November of 2019, 2032, and 2039, and May of 2049.