Every once in a while, something will appear in the sky that will attract the attention of even those who normally don't bother looking up. It's likely to be that way on Saturday evening, May 19, when a slender crescent moon, just 15-percent illuminated, will appear in very close proximity to the brightest planet in the sky.
Those who have no advance notice will almost certainly wonder, as they cast a casual glance toward the moon on that night, what that "huge silvery star" happens to be? Sometimes, such an occasion brings with it a sudden spike of phone calls to local planetariums, weather offices and even police precincts. Not a few of these calls excitedly inquire about "the UFO" that's hovering closely in the vicinity of our natural satellite.
The planet that will keep close company with the moon on that Saturday evening will be dazzling Venus, which has adorned the western twilight sky since late December. No other star or planet can come close to matching Venus in brilliance. During World War II, aircraft spotters sometimes mistook Venus for an enemy airplane. There were even cases where Venus drew antiaircraft fire. Venus currently is setting about three and a half hours after sunset, which means it's in the sky until past 11 p.m. For some locations it remains visible until after midnight.
But a very close conjunction of the crescent moon and a bright star or planet can be an awe-inspiring naked-eye spectacle. The English poet, critic, and philosopher, Samuel Taylor Coleridge (1772-1834) used just such a celestial sight as an ominous portent in his epic, "The Rime of the Ancient Mariner." In addition, there are juxtaposed crescent moon and star symbols that have appeared on the flags of many nations, including Turkey, Pakistan, Malaysia, Algeria, Mauritania, and Tunisia.
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Also on May 19, 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."
As seen from the moon, the Earth would loom in the sky nearly 3.7 times larger than the moon does for us. In addition, the land masses, the oceans and clouds make the Earth a much better reflector of sunlight as compared to the moon.
In fact, the Earth's reflectivity varies as clouds, which appear far more brilliant than the land and seas, cover greater or lesser parts of the visible hemisphere. The result is that the Earth shines between 45 and 100 times more brightly than the moon. The Earth also goes through phases, just as the moon does for us, although they are opposite from what we see from Earth. The term for this is called "complementary phases."
On May 16, for example, there's a New moon for us, but as seen from the surface of the moon that day, there will appear in the sky a brilliant Full Earth. A few nights later, as the sliver of a crescent moon begins to appear in the western twilight sky, its entire globe may be glimpsed. sunlight is responsible for the 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 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 reflected back to Earth.
Much of North America will be favored with the best view of this moon-Venus pairing. Across the northern and central U.S. and southeast Canada, the minimum distance separating the two will be no more than one-degree (the apparent width of the moon is roughly one-half degree). Keep in mind that this display will be an illusion of perspective: the moon will be only about 233,000 miles (375,000 kilometers) from Earth, while Venus is nearly 347 times farther away, at 80.9 million miles (130 million kilometers).
And because the moon is so much closer to us than Venus, it will be possible to see it change its position relative to Venus over a span of just a few hours. From New York City, for instance, at sunset, the moon will appear to be directly to the right of Venus. But three hours later, the moon will have moved to a point above and to the right while the gap between the two will have been halved.
For most locations, the moment of closest approach between the moon and Venus will come after sunset. The exceptions will be over the Pacific Northwest States and western Canada, where the closest approach occurs before sunset; for these regions, the moon and Venus will be slowly separating during the early evening hours.
In the table below, I have calculated the viewing circumstances for 18 selected U.S. and Canadian cities. The time of local sunset is given, as well as the time when Venus and the moon will appear closest together (keep in mind the moon moves in an easterly direction against the star background at its own apparent width per hour). Notice that for eastern locations nearly three hours will elapse between sunset and closest approach, while for some far-western locations these events will nearly coincide. A "D" indicates that closest approach comes before sunset. The minimum separation distance (Sep.) and the moon's altitude (Alt.) above the horizon are provided as well.
While those using binoculars or a small telescope will enjoy the almost three-dimensional aspect of the moon, Venus will be rather disappointing, appearing as brilliant blob of light, for right now, Venus appears as a tiny, featureless gibbous disk. That will change in the coming weeks, however, as Venus approaches Earth and the angle it makes between us and the sun allows it to evolve into a "half-moon" phase in early June . . . and a lovely crescent phase of its own during July.