Coming up on June 5-6 of this year 2012 is a very special astronomical event, the transit of Venus across the face of the sun.
Here is a previous post discussing the orbits of Venus and our earth and the "synodic cycle" of the planet Venus. That post explains that Venus reached its point of "maximum eastern elongation" (point 2 on the diagram included in that post) on March 27 of this year. As the earth and Venus have proceeded along their respective orbits, Venus is now nearing point 3 on that diagram, when it begins to pass between the earth and the sun.
Just as the moon passes between earth and our sun each month (at new moon) but does not create a solar eclipse because its orbital plane is tilted by just over five degrees from earth's orbital plane (it only creates a solar eclipse when it passes through new moon at the same time that it is intersecting the earth's orbital plane around the sun at one of the "lunar nodes"), so also Venus often passes between the earth and the sun without transiting the solar disc from our perspective, because the orbital plane of Venus is inclined by about 3.4° from the plane of earth's orbit around the sun.
The pattern of the orbits of our earth and Venus bring about a visible transit of Venus across the sun's disc in a 243-year cycle, consisting of two transits separated by eight years. A transit occurred in June of 2004 and another will occur this June 5-6, but the next one after that will not take place until the year 2117.
Here is a useful and fascinating website that discusses the upcoming transit of Venus and how to safely and successfully observe this rare and important celestial event, TransitofVenus.org. It contains a global map showing the visibility zones for the transit, and (even more helpful) a link to the Transit of Venus Project which will calculate your approximate location through your IP address and give you times for the transit of Venus for your point on the globe (along with a little diagram of the sun and the track of the planet Venus across its solar disc).
The website also contains some excellent historical resources discussing the importance of the Venus transit to modern science (from the seventeenth century onwards). Future blog posts on this site will discuss the importance to ancient civilizations and the possible links between the important phenomenon of the Venus transit and ancient knowledge.