Thursday, June 16, 2011

Lunar Eclipse

A lunar eclipse doesn't make the moon disappear.

On Earth, we humans enjoy a celestial juxtaposition of distances and sizes such that, when eclipses occur, we are treated to an especially remarkable show.  From the Earth's surface, the Moon and the Sun appear to be the same size.  They're not, of course, but since the Moon is a mere quarter-million miles away its disc of 2100+ miles is sufficient to block the Sun's 865,000 mile disc.  It's almost a perfect overlay.

What does that mean for us?  Well.  From the Moon's surface during a full eclipse of the sun, you wouldn't see any Sun at all.  But unlike the Moon, Earth has an atmosphere which leads to some startling effects.

During a full lunar Eclipse, the Moon seems to fade away as the Earth's shadow slides across it.  But then, just as you thought it was going to be completely blacked out, the Moon makes a startling reappearance.  But now it isn't the pale silvery orb we know so well, but a haunting ruddy orange.  How come?

In that moment, the Moon is experiencing a sunset like you wouldn't believe.  Just as we see the sun fade over the western horizon into darker and redder shades as its more oblique rays are deflected and defracted until only the longer, redder wavelengths reach us on the ground, the Moon gets the weaker but longer wavelengths of the Sun shone on it, and that's what we see reflected back to us during the lunar eclipse.  I imagine an eclipse from the Moon's surface must look like a fiery ring in the sky where the sun ought to be.

The solar eclipses we experience on the ground are one of those fortunate happenstances that make the religious nod knowingly.  During a full eclipse, our planet's moon - so ridiculously large compared to its primary that many astronomers consider the Earth-Moon system a double planet - perfectly blocks out the Sun.  What's left?  Coronal mass discharges, the coronasphere, Baily's beads.  All kinds of visual phenomena you can't see without the eclipse.

Baily's beads in particular require not just an eclipse, but the Moon.  Baily's beads are a little bit of extra light making its way to us on the ground as it gets past lunar mountains.  But back in the day, the eclipse was not just a stunning phenomenon, but a scientific opportunity.  "I could learn so much about the Sun," thought early astronomers, "if I could just block some of it out."  Hey presto, a little blocking coming right up.

Of course, the difficulty is that an eclipse doesn't last.  Just like the Moon last night being orange for less than an hour, the Sun is totally blocked for just a few minutes.  Then the Moon's orbit and the Earth's orbit carry everything back out of alignment and the light comes back.

Enjoy the show.

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