Sister Planet Struts Her Stuff

During April and most of May, you can't help but notice the brilliant beacon in the western sky at sunset. Venus, named after the goddess of love, and often called Earth's sister planet, has long been the "evening star" to wish upon. Venus is approaching a time of maximum brightness that makes it nearly impossible to miss, and an event which hasn't taken place for over over one hundred years!

When Venus is at maximum brightness, only the Sun and Moon can be brighter on the astronomical brightness scale. Observers may notice Venus in the sky, but many people don't also realize that Venus goes through phases just like we see in our Moon. Unfortunately, you normally need a small telescope to see the phases.

Venus is the second planet from the Sun. Because it's so close to the Sun, it always appears near the Sun as seen from Earth. Venus will spend a little over nine months as a "morning star" and then about the same length of time as an "evening star."

Our ancestors actually thought they were seeing two different celestial objects. They named the morning star Phosphorus, the god of light, and the evening star was named Hesperus, a name which means close of the day. It was the Greek philosopher and mathematician Pythagoras who first realized that Phosphorus and Hesperus were the same object, but still thought that Venus revolved around the Earth.

It was Galileo Galilei, in 1610 who began observing Venus through his telescope and noticed one night that rather than being a full disk, Venus was a crescent. He continued watching the planet and watched Venus display the same phases as the Moon. Galileo correctly concluded that this could only happen if Venus was orbiting the Sun, and he used this as evidence that all planets, even Earth, revolve around the Sun.

When Venus is on the opposite side of the Sun from us, it would appear to us as a round disk, and would also appear small because it's farther away from us. Venus is traveling at a faster rate than us around the Sun, so it does catch up to us. As it gets closer, it appears both larger and brighter. But as Venus approaches us, the angle of sunlight striking it also changes as seen by Earthly observers. The closer Venus gets, the more it turns its shadowed side to us.

Venus' disk continues to shrink as it prepares to pass between the Earth and Sun. When it appears as a crescent, it is about six times closer to us than when it was on the other side of the Sun. So even though the lit portion is shrinking, to us, Venus appears brighter.

Venus reaches its maximum brightness on May second. At this time, if you know where to look, you can spot Venus in a deep blue afternoon sky. And after sunset, if your skies are dark, Venus is so brilliant that it actually casts a shadow! The planet is about 42 million miles from Earth.

On May fifth, Venus' disk is about twenty-five percent lit and the planet appears nearly forty percent larger than it did a month ago. As the crescent continues to shrink, Venus appears closer to the Sun in the sky each night. By May ninteenth, Venus is only 32 million miles from Earth, closer than Mars was to Earth last August. Now, the illuminated portion of Venus is only twelve percent.

By the last week in May, Venus appears only about fifteen degrees from the Sun in the sky.

As Venus gets closer to the Sun, it is headed for a June eighth transit, when it will pass directly between the Sun and Earth. Such an event is very rare and has not occurred since 1882. If you have a telescope with a solar filter on it, you can watch the tiny dot of Venus pass across the Sun's disk. To everyone else, Venus will disappear from the evening skies and we will have to wait to see it until the planet reappears as a morning star.

Constellations

The Solar System