Earth and Sky【20】

The planet Venus now shines as a eerily brilliant light low in the western twilight sky. We'll talk about why Venus appears at greatest brilliancy this week —— on today's Earth and Sky.

　　DB: This is Earth and Sky —— Monday, September 23. If you've been watching the evening sky, you've probably seen a bright object up after sunset.

　　JB: It's the planet Venus —— now lower in the sky after sunset than it's been for several months. Venus might be hidden by trees or buildings on your horizon. But, if you do see the planet this week, it'll surprise you —— because Venus is now at greatest brilliancy —— shining most brightly for this evening apparition.

　　DB: And for Venus that's very, very bright. You might think that Venus has to show us its fully lighted hemisphere —— its day side —— to look so bright. But if you had a telescope, you could see that Venus is now in a crescent phase as seen from Earth. Venus will pass between us and sun next month. As it's come closer to doing this, Venus has shown us a decreasing fraction of its lighted half. Tonight that brilliant object in the west after sunset is really only about 24% illuminated, as seen from Earth. This crescent Venus manages to shine so brightly because it's now nearly at its closest to Earth —— and so the disk of Venus looms especially large.

　　JB: Sunlight reflecting from the cloudtops of Venus takes over three-and-a-half minutes to reach us now. These clouds, by the way, help make Venus bright —— because they're so good at reflecting sunlight. For today, that's our show. Special thanks to Research Corporation —— a foundation for the advancement of science. We're Block and Byrd for Earth and Sky.