The United States had a leading rote in observing the 1874 and 1882 transits.
We are now on the eve of the second transit of a pair, after which there will be no other 'til the twenty-first century of our era has dawned upon the Earth, and the June flowers are blooming in 2004.
Naval Observatory astronomer James Harkness, just before observing the 1882 transit of Venus
Unlike the moon, which looms so large on the sky that it covers the entire sun during a solar eclipse, Venus during a transit masks only one-thousandth the area of the sun's surface and blocks a mere 0.
As seen from Earth, a transit of Venus occurs just twice every century or so, in pairs spaced 8 years apart.
Sun-watching space observatories such as SOHO (Solar and Heliospheric Observatory) and TRACE (Transition Region and Coronal Explorer), which orbit above Earth's turbulent atmosphere, should have ringside seats for the transit.
The duration of the transit and the amount by which starlight is dimmed by the planet's passage provide the only way astronomers can now determine the mass, size, and orbital inclination of these unseen bodies.
By analyzing the specific wavelengths of starlight absorbed during a transit, astronomers have also made the first discovery of an extrasolar planet's atmosphere.
The astronomers observing the June transit will have an ace in the hole: They already know the composition of Venus' atmosphere, thanks to spacecraft that have directly measured it.