Eclipse alert… but just barely

The most minimal of penumbral lunar eclipses takes place Saturday night, though with clear skies, anyone can enjoy the nice May full moon. Look for Saturn east of the moon Wednesday night. Sunset triumvirate – Mercury, Jupiter, Venus are positioning themselves for a rare match-up.

May 20, 2013

[Dave Heller] Night sky gazers can start tracking a trio of planets in seemingly close proximity. Let’s set our sight lines with Derrick Pitts, chief astronomer at the Franklin Institute. Derrick, we’re preparing for a show that takes place over several nights.

[Derrick Pitts] One could actually argue that it runs right through next week and on into June, but we can really start to pick up the members of this triumvirate that are going to be working together 45 minutes after sunset over in the west, northwest.

Do the members know they’re working together?

The members have no idea what’s going on at all. It’s actually is a really interesting tableau that can be seen this way only from planet Earth because we’re talking about these three members of the solar system — Mercury, the planet that’s closest to the sun, Venus, the member between Mercury and Earth, and then the largest planet of the solar system, Jupiter, which is hundreds of millions of miles farther out. At the same time though, for those of us here on Earth viewing this, what we’re going to see is the results of the orbital speeds of these three planets causing them to look as if they’re moving around with each other out there in space. The truth of the matter is, of course, that Mercury and Venus are fairly close to us but their heightened orbital speeds around the sun is what’s going to display most of the action that we’ll see. The one other thing we’ll see is we’ll see that Jupiter appears to move through this section of the sky pretty quickly over the next 7-10 days, but that’s actually the result of the Earth’s motion around the sun, not Jupiter’s motion around the sun.

Separately, the second-largest planet Saturn is around this week as well.

Indeed it is, and it’s over on the other side of the sky where we’ll find it not far from the moon on Wednesday evening. It’ll be just to the east of the moon, to the left of the moon if you will. So if you go out anytime after sunset, as soon as you can identify the moon, look to the left of the moon and you should be able to see Saturn.

One of the difficulties about Saturn is that its distance is almost twice that of Jupiter. So even though it’s similar in size, the distance makes its seem so much smaller. But again, the way to recognize planets from stars in the evening sky is that stars twinkle but planets don’t. So if you’re looking in that portion of the sky, just to the left of the moon, to the east of the moon, you’ll be able to see that little tiny spec, not tiny spec, but that smaller object that does not twinkle — that’s the planet Saturn. Small telescope will let you see the rings.

As you mentioned the moon, there’s a lunar eclipse this week. I think the technical term for this one is the “Ho-Hum” eclipse.

[Laughs] I would certainly agree with that. Because it is actually an eclipse only technically. Here’s what we mean by this: there are different stages for an eclipse. The first phase of an eclipse (of a total solar eclipse) would be the penumbral phase. Then comes the umbral phase of the eclipse. Now the penumbra actually happens to be the outermost ring of shadow created by the Earth when the sun shines onto the surface of the Earth, the shadow’s created out behind the Earth. As the moon moves into this outermost ring of shadow, that’s a penumbral eclipse.

Unfortunately, there isn’t much color associated with penumbral eclipses. A total eclipse, where the moon actually moves into the center portion of the Earth’s shadow, called the umbra, that’s when you get the deep, rich color — reds, maybe even light browns — that you can see on the surface of the moon. However in this case, technically a penumbral eclipse, meaning that the moon only very slightly grazes the outer edge of the penumbra. So for all intensive purposes, that evening you’ll see a full moon but with no color whatsoever. So you couldn’t really recognize it as eclipse if you observed it. The only way you’d be able to recognize the eclipse is if you work the numbers to see that the position of the moon is just grazing the outer edge of the penumbral shadow of the Earth.