Venus Reaches -4.8 Peak Brilliance on Sept. 18, Brightest Until 2028

Q: Why does a shrinking, crescent-shaped Venus shine brighter tonight than almost anything else in the entire night sky?

Venus looks brightest tonight because it is at the sweet spot between being closer to Earth and still showing enough of its sunlit side to reflect a great deal of light. That may sound counterintuitive, because Venus is not full tonight. Through a telescope it appears as a large crescent, roughly a quarter illuminated. But as Venus moves toward inferior conjunction in late October, it is drawing much nearer to Earth, so its apparent disk is growing rapidly. Even though the illuminated fraction is shrinking, the planet’s visible crescent covers enough apparent area in our sky to maximize the total reflected light reaching us. Astronomers call this “greatest brilliancy” or “greatest illuminated extent.” Venus also has an exceptional built-in advantage: its thick global cloud deck is extremely reflective. Those sulfuric-acid clouds give Venus the highest albedo of any planet, reflecting roughly 75% to 90% of the sunlight that hits it. Combined with Venus’s proximity to both the Sun and Earth, that makes it the brightest planet and usually the third-brightest natural object in the sky after the Sun and Moon. So the reason a shrinking Venus can outshine nearly everything overhead is geometry plus reflectivity: it is close enough to look much larger, and its clouds bounce back an extraordinary amount of sunlight. Tonight, those factors briefly align at their most favorable balance.

Q: If Venus is barely illuminated right now, how is it creating a spectacular light show that won't return until 2028?

Venus is dazzling now precisely because it is not full. Its peak brightness happens when two factors line up: the planet is much closer to Earth, so its apparent disk looks much larger, while enough of its sunlit side is still visible to reflect a great deal of light toward us. Around Sept. 18, Venus is only about one-quarter illuminated, but that crescent is large enough in the sky to produce its “greatest brilliancy.” That effect is strengthened by Venus’ exceptionally reflective cloud cover. The planet is wrapped in thick clouds of sulfuric acid droplets that reflect roughly 75% to 90% of the sunlight that hits them, giving Venus the highest reflectivity of any planet. That is why it can outshine every star and appear as a steady white beacon after sunset. Astronomers often call this moment Venus’ “greatest illuminated extent.” A nearly full Venus is farther away, so it looks smaller and does not appear as bright from Earth. Near inferior conjunction in late October, Venus will come even closer, but by then its lit side is turned mostly away from us, so brightness drops again. The sweet spot is now, when distance and phase are balanced. At peak evening brightness, Venus is around magnitude -4.8, more than 20 times brighter than Sirius. It will not appear this bright again in the evening sky until 2028, though it will have another peak in the morning sky later this year.

3 articles · Updated · 10best.usatoday.com · Sep 18

Venus hits its greatest brilliancy on Sept. 18, appearing brighter than almost anything in the evening sky and marking its brightest showing until 2028.
NASA says the planet will shine at magnitude -4.8 and sit unusually low above the western horizon, where it should look like a steady white light.
30 to 60 minutes after sunset offers the best viewing window, and the event should be easy to spot with the naked eye.
Binoculars or a telescope can sharpen the view, but unlike an earlier fall solar eclipse, no special protective glasses are needed.