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April and May 2012 present a grand time to observe the planet Venus' changing phase in the evening sky with the telescope. Because Venus lies inside the Earth's orbit, this world shows the full range of phases - like our Moon. Planets residing outside of Earth's orbit always appear full or close to full in the telescope.
Friday, April 13 & the Calendar
In early April, sunshine illuminates 48% of Venus' disk, while the darkness of night covers the other 52%. By the end of May, sunshine only illuminates about 1% of Venus' disk but it should be visible through the telescope. As its phase wanes (thins), the angular diameter of Venus' disk more than doubles in these two months. In fact, you might even be able to see the whisker-thin slender crescent through binoculars in late May or early June!
|Venus' Disk Through the Telescope|
The sharpest view of Venus' phase is at early dusk or before sunset. At nigtttime, the glare from this dazzling world - the brightest heavenly body after the Sun and Moon - obscures the clear division between day and night on Venus' disk. If you can't find Venus in the daytime, then aim your telescope at Venus as soon as you spot it after sunset.
Venus Wanes in Evening Sky
Whenever Venus appears in the evening sky after sunset, this inferior planet is waning from full to new phase. Look at the illustration below. We are looking down upon the north side of Venus orbital plane, in which case Venus circles the Sun in a counter-clockwise direction. Venus transitions from the morning to the evening sky at superior conjunction (full phase), or when Venus is on the other side of the Sun.
After Venus reaches superior conjunction, it heads towards Earth in its smaller and swifter orbit. As Venus gets closer to Earth, its phase shrinks yet its angular diameter increases. Venus last reached superior conjunction on August 16, 2011.
Surprisingly, Venus is brightest in our sky when about one-quarter of its daytime side and three-quarters of its nighttime side faces Earth. That's because, at this juncture, Venus' illuminated portion covers the most square area of sky. That will happen in late April and early May.
Venus brightest in evening sky late April and early May
Venus Waxes in Morning Sky
Venus will reach inferior conjunction (new phase) on June 5, 2012 (for the world's Western Hemisphere), to transition from the evening to the morning sky. After inferior conjunction, Venus goes away from Earth in its smaller, faster orbit around the Sun. Venus waxes in phase yet its angular diameter decreases in the morning sky.
Transit of Venus
Much more often than not, Venus swings north or south of the solar disk at inferior conjunction. But not this time! On June 5, 2012, Venus passes directly in front of the Sun, to stage the last transit of Venus in the 21st century.
copyright 2012 by Bruce McClure
|Last Transit of Venus in 21st Century on June 5, 2012|
March 2012 Feature * May 2012 Feature