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April 15, 2024

Backyard Astronomy: Venus

By Cheryl Haimann

You've probably seen Venus, even if you don't realize it. If you've looked at the southwestern sky in the early evening recently, you've seen Venus. If you've ever seen a really, really bright "star" in the early evening or just before sunrise, that was probably Venus. If you've ever made a wish on the first star you saw, there's a good chance you wished instead on Venus.

The planet is our nearest planetary neighbor, passing less than 26 million miles from Earth at its closest. It is almost exactly the same size as Earth, only about 400 miles smaller in diameter. Venus outshines everything except the Sun and Moon. Its brilliant appearance prompted the ancients to name it after the goddess of love.

Surely it must be a beautiful, Earth-like place, right? Science fiction writers and movie makers thought so, often presenting Venus as a tropical wonderland populated by beautiful and pliant women. Telescopes revealed almost no details about the planet, further enhancing the mystery and romance.

This fantasy lasted until 1962, when Mariner II, the first space probe to another planet, visited Venus. What Mariner and subsequent explorations discovered was about as far from paradise as you could get. You've heard of "hell on Earth"? Forget that. If hell is anywhere in our solar system, it is on Venus.

Earth and Venus probably started out being quite similar, hot, volcanic rocks. But while Earth eventually cooled down, allowing oceans to form and absorb its carbon dioxide, Venus never did. Its atmosphere is largely carbon dioxide, and incredibly dense, making it a planet-sized model of the greenhouse effect. Sunlight comes in, warms the ground, and is radiated back up as heat. But instead letting the radiant heat escape into space, the crushing atmosphere traps some of it, reflecting it back to the ground. Because of this, the temperature on Venus is about 850 degrees F, day or night. It is even hotter than Mercury, which is 30 million miles closer to the Sun.

If the searing heat isn't enough, there's the matter of the clouds. Venus once had, and may still have, active volcanoes. Those volcanoes belched sulfur into the atmosphere, which, as we know, does not like to let anything escape. As a result, the clouds are filled with strong sulfuric acid. If there is a plus side to this toxic veil, it is that the clouds reflect light very well. That is why Venus appears so bright to us.

Because Venus is inside Earth's orbit, we are able to observe phases, just as we can with our moon. Unlike our moon, though, the apparent size of the planet varies dramatically. When Venus is on the far side of the Sun, the entire disc is illuminated, but it appears small. As its orbit brings it closer, it goes through gibbous and crescent phases, but also appears larger.

Venus was half illuminated on January 16, and now is going into its crescent phase. It will remain high in the southwest, visible for several hours after sunset, until early February. As it moves closer to Earth, the crescent will become more slender, the apparent size of the disk will become larger, and it will move toward the west and be lower in the sky, disappearing into the Sun's glare in late March.

As the crescent becomes larger and more slender, you may be able to see it with steady handheld or tripod-mounted binoculars. Look before it is completely dark. Like the Moon, it is bright enough to cause an uncomfortable glare when it is dark. The effect is lessened when it is still daylight or dusky.


Cool Thing of the Week: 365 Days of Astronomy

365 Days of Astronomy is a daily podcast being produced in support of the International Year of Astronomy. The ten-minute episodes are contributed by volunteers, and if the first couple of weeks are any indication, they will cover a wide variety of topics.

The podcast is available on iTunes, or you can listen to the episides at the website. The website also has written transcripts of the episodes.


Backyard Astronomy updates are available on Twitter!

Article © Cheryl Haimann. All rights reserved.
Published on 2009-01-19
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