(Nov. 29) - Every once in a while, something will appear in the night sky that will attract the attention of even those who normally don't bother looking up. It's likely to be that way on Monday evening, Dec. 1.
A slender crescent moon, just 15-percent illuminated, will appear in very close proximity to the two brightest planets in our sky, Venus and Jupiter.
Also on Monday evening, you may be able to see the full globe of the moon, its darkened portion glowing with a bluish-gray hue interposed between the sunlit crescent and not much darker sky. This vision is sometimes called "the old moon in the young moon's arms." Leonardo da Vinci (1452-1519) was the first to recognize it as what we now call "earthshine."
Sunlight is responsible for the slender crescent, yet the remainder of the moon appears to shine with a dim blush-gray tone. That part is not receiving sunlight, but shines by virtue of reflected earthlight: the nearly full Earth illuminating the otherwise dark lunar landscape. So earthshine is really sunlight which is reflected off Earth to the moon and then reflected back to Earth.
As beautiful as the view of Venus, Jupiter and the moon will be from North America, an even more spectacular sight awaits those living in parts of Western Europe where the moon will pass in front of Venus.
Astronomers refer to this phenomenon as an "occultation," taken from the Latin word occultare, which means "to conceal." This eye-catching sight will be visible in complete darkness across much of Eastern Europe. Farther west, Venus will disappear behind the dark part of the moon either during evening twilight or just before the Sun sets. When Venus emerges, it will look like a brightening jewel on the slender lunar crescent. For virtually all of Europe, the Sun will have set by then, the exception being southern Portugal (including Lisbon).
Such favorable circumstances are quite rare for any given location. For example, the last time London was treated to such a favorably placed Venus occultation such was back on October 7, 1961. And after 2008, there will not be another similarly favorable Venus occultation for the United Kingdom until January 10, 2032. So be sure to make the most of this upcoming opportunity. More detailed information, including maps of the occultation zone, as well as times for dozens of European cities, are here.