Expert: Tom Whiting - 5/27/2010

Question
How long will we be able to see Venus in the sky over the west and how often does it appear.

Thank you

Answer
Hi Mike,
Us Earthlings will be able to observe Venus through the end of September 2010, at which time
our sister planet will pass between Earth and Sun (not exactly, but close)... an event called
inferior conjunction on October 29th at 0100 hr GMT, then rapidly reappears into our early dawn sky the first week of November 2010.  Maximum brilliance occurs on the dates of September 23rd
in the western evening sky, and again December 4th in the early morning eastern sky.
Venus runs in an 8 year cycle, orbiting the sun once every 224.7 days to our 365.25 days, going through 13 orbits to the Earth's 8 orbits.
365.25 days times 8 = 2922 days
224.7 days times 13 = 2921 days
So every 8 years Venus cycles repeat, not exactly, but almost...see
http://www.lunarplanner.com/HCpages/Venus.html
or if that seems too complicated, just Google "Venus 8 year cycle".

So see, it's not the time of the year or season... Venus is actually visible to us most of any year but not always in the western evening sky.  The only time we can't see the planet is when
it is very close to the sun, either passing between us and the sun, or behind the sun for a few months.

You can easily see the 8 year cycle with transits of Venus across the disk of the sun. We already had one on June 8, 2004... with one more to go on "D-day" of 2012, or June 6, 2012 in the late afternoon hours for USA.  The pairs are separated by over a century. Miss the June 6,
2012 event, and you'll be waiting until 2117 (that's correct... 105.5 years later!) and 2125 AD for the next pair of transits, and those are in December.  The last pair was 1874 and 1882 and they were December also.  (Because of orbital alignments, transits of Venus across the sun's disk can only occur in June and December, just like transits of Mercury can only occur in May
and November. But that's another story.
Clear Skies,
Tom Whiting
Erie, PA USA