Thursday, March 05, 2009

A Little More Venus Talk

A: Then let us think of the future. We’re still the same people.

PRISONER: Working for different sides.

A: Sides don’t matter. Only success.

PRISONER: In that case we should have a great deal in common.

A: We do the same jobs.

PRISONER: For different reasons, yes.

A: (laughing) I see you still over-rate absolute truth.

A.B. And C.

In yesterday’s poem about Venus, I included a few astronomy bits. Those astronomy bits were generally true, but they weren’t the truth, the whole truth and nothing but the truth.

Today I’m going to go into some details about a couple of points that I mentioned just in passing yesterday.

Venus in the Evening and Morning

Right now Venus is an evening object. After sunset, Venus appears very bright in the western sky about twenty degrees above the horizon.

Out in space, Venus is drawing closer to Earth. From the perspective of Earth, the image of Venus appears to move closer to the Sun. Every day as Venus moves closer to Earth Venus will appear lower and lower above the western horizon. Toward the end of the month, Venus ‘catches up’ to Earth and passes Earth. Venus then will change from an evening object to a morning object.

Some years during that change when Venus catches up to Earth in their orbits the image of Venus appears to pass in front of the Sun, to transit the solar disk.

Once a decade or so, however, Venus appears to pass north of the Sun.

This is one of those years.

Since Venus will appear to pass north of the Sun, for observers on Earth who live north of about latitude forty degrees, there will be a two or three day window around March 23 when we can observe Venus both in the western sky right after sunset and in the eastern sky right before dawn. Venus will be within six degrees of the horizon in both cases. That means in order to see Venus you need a completely unobstructed view of the horizon with no mountains, no trees and no buildings in the way.

I won’t be able to see Venus either in the west or east under those conditions. But some people will.

That will be cool. It won’t be visually dramatic or directly exciting, but it will be a great example of the celestial mechanics of the solar system working themselves out right before our eyes.

Venus in the Daytime Sky

Yesterday I said that when Venus is near the Sun, Venus becomes invisible to Earthbound telescopes because glare from the Sun overwhelms telescopes. That’s generally true and most astronomers don’t observe astronomical objects when the Sun is out.

However, in fact, if you know exactly where to look—or if you have a modern, computer-driven telescope that you calibrate at night and do not move—you can see Venus during the day through a telescope.

I’ve done it.

However it is very, very dangerous to point a telescope near the Sun.

Venus this month is going to be within nine degrees of the Sun. If you attempt to look at Venus during the day and you inadvertently bump your telescope or trip or have something else unexpected happen, light from the Sun can pass through your telescope or your telescope’s finder scope and enter your eye and that can blind you for life.

It is kind of cool to look up into a clear blue sky and see an astronomical object like Venus during the day. But it is not so cool that I would risk my eyesight for it.

I don’t recommend anybody try to view Venus during the day this month when Venus is so close to the Sun.

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