Mass Transit

The transit of Venus happens again today. The next time up...105 years from now in 2117.

From Wired...

Nearly 130 years ago, the premier event in astrophysics involved watching a tiny dot slowly sail across the surface of the sun. That dot was our sister planet, Venus, and observing its transit as it passed directly between the Earth and sun was a momentous scientific undertaking. 

A transit of Venus is one of the rarest astronomical phenomena, occurring in a cycle that repeats every 243 years, with pairs of transits eight years apart separated by about 121 years and 105 years. 

The above image comes from 1882, when astronomers were very keen to make accurate measurements of the occurrence. That’s because, until this time, they did not know the exact size of the solar system. Using geometry, astronomers knew they could use the transit of Venus to determine how far the Earth was from the sun, producing the Astronomical Unit.

Update: My brother-in-law's beautiful photo of the transit (nice job AGS!) taken with an iPhone through a telescope.  I wonder what photos of the next transit in 105 years will be taken with...