Monday, May 17, 2010
...venus in transit...!
As it moves around its orbit, Venus displays phases in a telescopic view like those of the Moon: In the phases of Venus the planet presents a small "full" image when it is on the opposite side of the Sun. It shows a larger "quarter phase" when it is at its maximum elongations from the Sun. Venus is at its brightest in the night sky and presents a much larger "thin crescent" in telescopic views as it comes around to the near side between the Earth and the Sun. Venus is at its largest and presents its "new phase" when it is between the Earth and the Sun. Since it has an atmosphere it can be seen in a telescope by the halo of light refracted around the planet.
Venus's orbit is slightly inclined relative to the Earth's orbit; thus, when the planet passes between the Earth and the Sun, it usually does not cross the face of the Sun. However, transits of Venus do occur when the planet's inferior conjunction coincides with its presence in the plane of the Earth's orbit. Transits of Venus occur in cycles of 243 years with the current pattern of transits being pairs of transits separated by eight years, at intervals of about 105.5 years or 121.5 years. The most recent transit was in June 2004; the next will be in June 2012. The preceding pair of transits occurred in December 1874 and December 1882; the following pair will occur in December 2117 and December 2125. Historically, transits of Venus were important, because they allowed astronomers to directly determine the size of the astronomical unit, and hence the size of the Solar System. Captain Cook's exploration of the east coast of Australia came after he had sailed to Tahiti in 1768 to observe a transit of Venus.