Going Through a Phase

At the Veen Observatory visitors’ night this past Saturday, one of the main objects shown early in the evening was the planet Venus. It was in several of the telescopes there, including the main telescope in the large dome. But some people were confused upon their gaze through the eyepiece.

“Wow! Look at the moon!” more than one person said. They were really surprised when they were told “no, that’s the planet Venus.”

Just like the moon, the planets Venus and Mercury exhibit phases as they orbit the sun. This has to do with the relative positions of the sun, the planets, and the earth. In fact, when Venus or Mercury are between the earth and the sun, no reflected light can be seen, and those planets are in the “new” phase, just like when our moon is between us and the sun. And just like the moon, Venus and Mercury’s orbits are not exactly the same as the earth’s. The vary compared to the plane of the solar system. So at times Venus and Mercury are “below” or “above” the sun from our vantage point, but lost in the glare.

Now, when those planets are on the other side of the sun from us, they reflect a full disc, just like when our moon is opposite the sun from earth in its orbit. If you notice with a full moon, it rises opposite the sun in the sky, and about the same time the sun sets (give or take a few minutes).

However, unlike the moon, which is extremely bright when full, Venus (and Mercury) is just the opposite. Here’s an interesting fact: When Venus is at its brightest (and largest through a telescope), it shows a crescent phase.

So when you look up in the evening sky over the next few weeks, and see brilliant Venus shining in the twilight, think about how bright it is, and realize that the planet is just going through a phase.

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