If you’ve ventured outside after sunset recently and happened to glance to the west, you may have noticed an astonishingly bright “star” glaring down on you, seemingly hovering in the sky. Is it a helicopter, a supernova, a—gasp—UFO?
Nope. That’s Venus, the second rock from the sun, Earth’s evil twin and frequent UFO impersonator.
If you haven’t seen the planet before, right now is the best time to take a gander. It’s not hard to spot: go out when the sky is getting dark and look west and then up. Venus is incredibly bright, shockingly so, which is why it’s commonly mistaken for a UFO. I get e-mails pretty often from slightly panicked people about it. They can’t believe it’s real.
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It’s not only real, it’s a whole planet, and it orbits the sun closer than Earth does. Venus is 110 million kilometers from the sun, compared with our 150 million km. It moves faster around our home star, too, so its year is shorter than ours, lasting only about 225 Earth days.
How we see Venus in our sky depends on where it is in its orbit. Imagine standing a short distance away from a race car going around a track. For reference, let’s say there’s a flag marking the center of the track. The car starts between you and the flag and passes in front of it, moving left to right. When it reaches the right side of the track, you see it round the bend and then start moving right to left. It moves along the far side of the track, behind the flag, until it reaches the left side. It rounds that part, moving toward you, and then you see it moving left to right again. It passes in front of the flag, and the cycle starts again.
The situation is the same with Venus, except its orbit is the track, and the sun is the flag in the center. Sometimes we see it passing very near the sun in the sky, when it’s closest to us in space. We call this point inferior conjunction. Venus moves “to the right”—technically westward—until it reaches the point where it’s farthest from the sun, called greatest western elongation. Then it reverses direction as it rounds that part of its orbit, moving eastward, or “to the left.” It passes behind the sun on the far side of its orbit—reaching superior conjunction—and then continues on until it’s as far east as it gets: the point of greatest eastern elongation. It rounds the bend again, starting to head west until it passes Earth on the near side of the sun once more. Then the dance begins anew. (Incidentally, this back-and-forth motion helped inspire the term “planets” itself, which derives from “planētēs”—Greek for “wanderers.”)
The easiest time to see Venus is when it’s at maximum elongation from the sun. At greatest western elongation, it appears to be a morning star, rising well before the sun does, and at greatest eastern elongation, it’s an evening star, setting late. That’s where we are now: Venus reaches greatest eastern elongation on June 4, when it will be 45 degrees away from the sun. As our star dips below the horizon, darkening the sky, Venus becomes an unmissably bright planetary beacon.
It’s not just celestial mechanics that makes Venus “pop,” though—planetary science is a factor, too. Venus is about the same size as Earth, but unlike our world, it is covered with a tremendously dense carbon dioxide atmosphere. This thick blanket of gas absorbs infrared light, trapping it as globe-warming heat. But Venus’s clouds also efficiently reflect visible light—a situation that sets the planet gleaming in our skies, with the downside of pressure-cooking it at a temperature of circa 475 degrees Celsius. That’s hot enough to melt lead, leading to Venus’s eminently deserved status as “Earth’s evil twin.”
But there’s more. Carefully looking at the interplay of light and geometry on Venus can reveal something simple yet profound about our solar system. Like our moon, Venus undergoes phases (although you need a telescope or binoculars to see them). When it’s on the far side of the sun, we see it fully lit like a full moon. As it moves to eastern elongation—as it is now—we see it half lit, and then it becomes an ever thinning crescent as it moves between us and the sun. When it’s closest to the sun in the sky, we see the back side of it, the unilluminated half, so it’s in the “new” phase. At western elongation, it’s half lit again. Then it approaches the far side of the sun, where the cycle repeats.
Those phases are more than just a lovely sight at the eyepiece. Galileo noted the phases of Venus as he observed it early in the 17th century, and he used it as an argument against the then popular geocentric models of the solar system. In such models, Venus and the sun both orbited Earth, with Venus closer to us. But if that were the case, Venus would never look full because it could never be on the far side of the sun, as seen from Earth. A full Venus, Galileo showed, was clinching evidence for the competing sun-focused “heliocentric” solar system model developed by Copernicus.
Galileo paid for this later in life when the Catholic Church accused him of heresy, but happily, there’s no such penalty for you now. Watch Venus while you can, and enjoy the view!
Special Events to Watch For
June 2: Mars will pass directly through the Beehive star cluster, an easy “binoculars object” that contains dozens or even hundreds of visible stars.
June 11–13: Venus will pass about a degree away from the Beehive cluster.
June 21: Venus, the thin crescent moon and Mars will make a tight triangle in the west after sunset. Mars and Venus wil lbe close together for several weeks around this time.
July 9: Mars and the bright star Regulus will be less than a degree apart to the upper left of Venus.
July 26: Venus and Mercury will be about 5 degrees apart, low to the horizon, after sunset. Regulus will be close to the pair as well.