The planet Venus continues to shine like a silvery-white lantern in the southwestern sky for about 2.5 hours after sundown. On Saturday evening (Dec. 28), a lovely crescent moon will join Venus in the twilight sky, making for an eye-catching post-Christmas celestial ornament.
Although by the standards of most Venus-moon pairings they will not be unusually close together, they still will likely draw the attention of even those who normally do not look up. Venus will appear to hover about 2.5 degrees almost directly above and ever-so-slightly to the left of the moon.
If it is cloudy in your area on Saturday evening, do not despair! There will be four more opportunities to see the crescent moon and Venus in our evening sky during the first half of 2020.
The future dates to note are: Jan. 28, Feb.27, March 28 and April 26.
Look for Earthshine
Also on Saturday night, as well as for several evenings thereafter, you may be able to see the full globe of the moon, with 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 earthshine. That faint bluish-gray light is light from the Earth reflected back to the moon. The Earth's light of course is reflected sunlight, so earthshine is really sunlight which is reflected off Earth to the moon and reflected back to Earth.
Another way to explain this is to imagine yourself standing on the surface of the Moon on Saturday night. Looking up into the sky, you would see a nearly full Earth. In fact, the phases of the Earth are the exact opposite of those of the moon as seen from the Earth.
A new moon on Earth, for example, translates into a full Earth as seen from the moon. This effect is known as "complimentary phases." So, if you were are standing on that part of the moon that night not in sunlight, your only light source would come from that nearly full (92% illuminated) Earth.
Now, if you've ever been outside around the time of a full moon, you've probably noticed how it can light-up the surrounding landscape. Now, imagine seeing the full Earth from the moon: a disk nearly four times larger in apparent size and shining far brighter because of sunlight shining off of its bluish oceans and whitish clouds. As a consequence, the surrounding lunar landscape would appear illuminated by a bluish-gray glow: Earthshine!
Related: The Phases of the Moon Explained
Which is the brighter?
Incidentally, as you're admiring this "dynamic duo," try determining which of the two appears to be the brighter object: moon or Venus?
In most cases, people will usually choose Venus, but actually it's the moon. There is no question that Venus is dazzlingly bright, shining at magnitude -4.0. That's 11 times is brighter than Sirius, the brightest star in the nighttime sky.
Yet that slender sliver of the moon glows 19 times brighter than Venus. The reason Venus looks brighter, is that its brilliance is condensed into a point of light compared to the moon whose brightness is spread out over a much-larger area.
Now in broad daylight!
And here's a final challenge: during the mid-afternoon, say around 2 o'clock, try locating the crescent moon. It will be due south, about one-third of the way up from the horizon to the point directly overhead.
Once you've found it, try making a daylight sighting of Venus. At that hour it will be less than 4 degrees to the upper left of the moon. If you use binoculars, train them on the moon ... look to the upper left of the moon and you'll immediately see Venus shining like a bright white speck; you’ll be surprised how easy it will be to see.
Once sighted, try seeing Venus with your naked eye; now that you know where it is you should be able to pick it out against the clear, blue winter sky.
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