Before sunrise these next several mornings – August 14, 15 and 16, 2020 – watch for the waning crescent moon to pair up with the dazzling planet Venus in the predawn/dawn sky. It’d be hard to miss these brilliant beauties, as the moon and Venus rank as the 2nd-brightest and 3rd-brightest celestial bodies, respectively, after the sun. Moreover, you might still see a sprinkling of Perseid meteors, although the Perseid shower is past its peak.
The above sky chart shows the moon and Venus as they appear at mid-northern latitudes in North America. At dawn on a given date, someone in the world’s Eastern Hemisphere will see the moon somewhat offset toward the previous date. For example, someone in Europe and Africa, at dawn on August 15, will see the moon roughly one-fourth the way between August 15 and August 14. For someone in Asia, at dawn August 15, the moon will be roughly one-half the way between August 15 and August 14, and someone in New Zealand, at dawn August 15, will see the moon about three-fourths the way between these two dates.
Keep in mind, however, that the moon appears larger in the chart than it does in the real sky. But no matter where you live, the moon will be in the vicinity of Venus on all three dates: August 14, 15 and 16. And no matter where you live, the moon travels in front of the constellations of the zodiac at the rate of about 1/2-degree (the moon’s own angular diameter) eastward per hour, or about 13 degrees eastward per day.
Sharped-eyed people can actually see Venus in a daytime sky with the unaided eye. Want to try your luck? First look for the moon, then seek out Venus. Or aim binoculars at the moon to view nearby Venus. The moon will pass 4 degrees north of Venus on August 15, at 13 hours UTC. Translate to your time zone. A typical binocular field spans about 5 degrees of sky. With the moon there to assist you, this is a great time to look for Venus in a daytime sky, with Venus just a hair’s-breadth shy of its greatest elongation from the sun (45.8 degrees west).
Although the moon and Venus appear close together on the sky’s dome, they are not truly close together in space. At the time that the moon swings by Venus, the moon will be very close to its average distance from Earth (238,855 miles or 384,400 km). Venus, at a distance of about 0.73 astronomical units (one astronomical unit = sun/Earth distance), is over 280 times the moon’s distance from Earth.
Because Venus is an inferior planet – a planet that orbits the sun inside of Earth’s orbit – Venus goes through phases, like our moon. However, you need to use a telescope to see Venus’ phases.
You might think that the moon and Venus should display a similar phase in the morning sky, given that these two luminaries are nearly on the same line of sight. But no, that is not be the case. As the moon sweeps by Venus, the waxing crescent moon will be about 15 percent illuminated by sunshine, whereas Venus will be just a touch past (51 percent illuminated) its half-illuminated first quarter phase.
Whenever you see the moon and Venus together in the morning sky, the moon is always waning while Venus is always waxing. The converse is also true: whenever you see the moon with Venus in the evening sky, the moon is always waxing while Venus is always waning. That’s because the moon orbits Earth whereas Venus orbits the sun.
When the moon is at first or last quarter phase, the sun-Earth-moon make a 90-degree right angle in space, with Earth at the vertex of this right angle. (See illustration above.) On the other hand, when Venus is at first or last quarter phase, the sun-Venus-Earth also form a 90-degree right angle in space, except that it’s Venus that’s at the vertex of the right angle. (See below.)
Enjoy an eyeful of the moon and Venus as they beautify the morning sky in mid-August 2020!