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Venus at her brightest

Probing the Venusian clouds, the European Space Agency’s spacecraft Venus Express detected electromagnetic signals that evidence lightning, as depicted in this painting
ESA (image by Christophe Carreau) |

The moon is a cold rock. Cold rocks don’t give off a whole lot of light.

The only reason we can see the moon it that it’s lit by the sun.

As the moon orbits the Earth, the sun lights up different parts of it. Lit full-face, the moon appears as a perfectly-round “full moon”; lit from the side, we see a D-shaped “quarter moon”; and when the moon is lit from behind, we see only a sliver: a “crescent moon.”



The more of the moon we see lit, the more light the moon sheds on us. A thin crescent reflects only a small amount of light into our eyes; a half-lit, D-shaped quarter moon reflects more light; the fully-lit full moon reflects almost enough light to read by.

When Galileo Galilei turned the newly-invented telescope toward the planet Venus in the early 1600s, he saw that Venus, like the moon, goes through phases.




Like the moon, Venus reflects a lot of sunlight toward Earth when lit full-face; less light when lit from the side; and very little light when crescent, lit from behind.

When Venus came around the sun last spring, to become the “evening star” – in the west after sunset – she was lit full-face, reflecting lots of sunlight toward Earth.

Earth orbits 93 million miles from the sun; Venus, nearly 70 million.

When Earth and Venus were on opposite sides of the sun last spring, the two planets were separated by more than 150 million miles.

Despite Venus reflecting sunlight full-on, she was far away, and the distance diminished her brightness.

The closer a planet orbits the sun, the faster it moves. Earth circles the sun at roughly 65,000 miles an hour; Venus, at better than 75,000. And since Venus has an inside track, she circumscribes a smaller circle – she covers fewer miles in her race around the sun. So even though Venus was on the other side of the sun from us last spring, she’s been running faster on a shorter track, and she’s been catching up to us, drawing closer and closer. Which should make Venus grow brighter.

But even as Venus draws closer, she changes the angle between herself, Earth, and sun — which is why Venus shows phases, like the moon.

Right now, through a telescope, Venus is becoming a thinner and thinner crescent … which should make Venus appear dimmer.

All through spring, summer and fall, the brightening effect of Venus getting closer has overwhelmed the dimming effect of her thinning crescent.

Ever since she first appeared as the evening star last spring, Venus has been growing brighter. But not for long.

On Friday, the brightening effect of getting closer will just match the dimming effect of getting skinnier. This Friday marks Venus at her brightest.

After Friday, Venus dims. Early next year, Venus will come between Earth and sun. Lit entirely from behind, the planet will be invisible.

Continuing around the sun, by next February Venus will shine brightly again — in the east, before dawn, as the “morning star.”

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He teaches classes to students of all ages, and may be reached at stahler@kvmr.org.


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