Next Tuesday/Wednesday is the last transit of Venus in the lifetime of anyone alive now.  During a transit, Venus appears as a black circle crossing the sun.  These events come in 8-year separated pairs–there was one in 2004–with over a century (2117) until the next pair.

Some fun facts:

• The seventeenth and eighteenth century transits were historically important because they allowed astronomers to measure the scale of the solar system.  It turns out that the ratios of distances in the solar system are easier to fix than the overall scale.  In 1678, Edmund Halley proposed to the scientific community that the coming 1761 transit would provide an opportunity to measure the distance to Venus by parallax, and from that get the distance to the sun (the Astronomical Unit).  In preparation for the event, European astronomers traveled throughout the world, which was a major undertaking at the time.  It was worth it though:  they figured out the AU to within about a million km.
• Why are Venus transits so rare?  It’s similar to the reason we don’t have a solar eclipse every month:  the orbital planes don’t quite coincide.  (They nearly do, but given how small planets are compared to the sizes of their orbits, tiny tilts have big effects on transits.)  Usually, Venus appears to pass “above” or “below” the sun.
• As we all know, Venus is the hottest planet, with a surface temperature of 860 F.  The reason is the greenhouse effect from its incredibly thick carbon dioxide atmosphere (surface pressure nearly a hundred times that of Earth).  Why the devil should Venus have so much more CO2 than Earth.  The main difference between the two planets, it turns out, is liquid water.  On Earth, the oceans trap huge amounts of CO2.  In fact, if all the trapped CO2 on Earth were released into the atmosphere, our atmosphere would have a composition pretty similar to that of Venus and Mars (i.e. mostly CO2).
• Water vapor is also a potent greenhouse gas.  More water vapor makes the lower atmosphere hotter, which makes more water evaporate into the atmosphere, which makes it hotter, which…uh oh!  It seems like there’s a potential for an instability where the oceans of a terrestrial planet end up boiling off and the planet turns into something like Venus.  This is called the “runaway greenhouse effect”.  Fortunately, negative feedback mechanisms keep this doomsday scenario from happening on Earth.  However, some have speculated that something like this may have happened in Venus’ early history.
• Liquid water very likely also played a crucial role in the evolution of the Martian atmosphere.  The Martian oceans removed large amounts of CO2 from Mars’ atmosphere billions of years ago, which would have decreased the pressure and temperature of the atmosphere–ironically destroying the conditions for the continued existence of liquid water.
• The Soviet Union actually landed probes on the surface of Venus; Russians should take pride in the Venera missions, an impressive achievement of the Soviet space program.  Given the extreme conditions on the surface, these probes couldn’t survive long (20 min – 2 hr).  NASA has also carried out several successful orbiter missions to study the planet.