Thursday, November 21, 2013

We Never See All of Venus

We were returning from an outing the other night (on the 19th) and Venus in the sky had a great brilliance. In the wonder of watching it—on the road and then later from home—my early telescopic days back in Kansas returned to me. A bit of knowledge returned as well, but knowledge, unless well-maintained, has a way of eroding. What I remembered was that Venus has phases. And so I said, “It must be a full Venus up there.” Alas.

It turns out that Venus is at its brightest when it is quite close to the earth and shows an intermediate crescent shape. At that point the planet is about 42 million miles from the earth, and now is such a time. The image shown, from the U.S. Naval Observatory’s web site (link), is an apparent image, not a photograph. It is for November 19th. Visible portion of Venus will continue to grow, and grow brighter, until December 10th of this year.

When Venus is closest to us (25 million miles away), it goes dark; it is directly between us and the sun. It is full only when it is on the other side of the sun from us—and therefore we cannot ever see her full face. At that point Venus is 162 million miles from us—and bright although Venus is, almost full as it approaches the sun from the back, its brightness has decreased by more than one fifth.

The next image, from Wikipedia (link), shows images of the planet in 2004 (the date stamps are month-day-year). After “new” Venus is reached (the last image), the same images appear but in reversed orientation.

Beautiful planet—but you have to be outside to experience it. Watching Venus and reading about the planet is a nice illustration of the difference between knowledge and experience.

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