In the Shadows
Observing >
There is a long dark vacuous shaft in space, thousands of miles across and hundreds of thousands of miles long. No sunlight has ever directly entered its domain. And our Moon is headed straight for it.
Of course that shaft of darkness is the shadow of our own planet extending out from our orb deep into space on the opposite side as the sun. On the evening of the 27th, next Wednesday, our Moon will venture into the Great Shadow and, if conditions are right, nearly disappear from the skies.
Unlike a solar eclipse which only a privileged few can enjoy, a lunar eclipse can be seen by half the planet. And it doesn't last the fleeting seconds that a solar eclipse takes to complete. No, a lunar eclipse can take the Moon out of commission for more than an hour.
Here are some interesting things to watch for in the next days leading up to the eclipse.
Start watching the Moon every evening starting tonight over in the western skies after sunset. It is at crescent phase now, but you'll notice that every evening it has creeped more and more to the east growing, or "waxing," fuller and fuller in the light of the sun. You are observing it traveling in its lethargic orbit around Earth. It is headed for the side of our planet where the shadow lies in wait.
When the meeting of shadow and satellite finally occurs the eclipse will already be in progress as the now full moon finally rises into view above the horizon at about 6 PM. The Moon will be in what is called the penumbra. The what?
Imagine being on the Moon looking back toward Earth. The sun is beginning to creep behind the Earth. Around you the landscape is getting darker. The sun is disappearing but not gone; you are in partial shadow, the penumbra.
At about 6:20 and for the next hour, the Moon will crawl from penumbra right into the Shadow proper, the umbra. And by 7:30 the Moon will be completely shadowed. No one living on the Moon would see the now hidden sun.
You'll by now pick out an optical illusion due to the simultaneous movements of both planet and moon. The Moon is rising in the east and the shadow appears to be gobbling the Moon from below. Actually the Moon is moving eastward into the shadow. But the earth is spinning during the whole show effectively bringing the eclipse event higher for us to get a better view of it.
Something else to notice as the Moon slips into shadow, if the atmospheric conditions allow it, is the curved shadow of the Earth on the Moon. The ancients noted that every time there was a lunar eclipse the earth cast a curved shadow. Why is this important?
Because the only geometric shape that always forms a curved shadow is a sphere. They deduced, thousands of years ago, that we must be living on a round ball.
As the Moon is in shadow from about 7:30 to 8:30 (and again, if the atmosphere allows it) notice that the Moon isn't black. It may be a reddish hue. Surely this is because of the schmutz in our air making it appear red! Well ... sorta.
Imagine yourself back on the Moon for a moment with the Sun completely behind the Earth. What you'd see in effect is the sun beyond the horizon all over the planet. What happens when we on Earth have the sun behind the horizon? We call it sunset, of course. Moon people see one big round sunset completely encircling the earth like a bright red ring around a black ball!
What's weirder is that our atmosphere actually acts like a giant lens and focuses all this reddened light right onto the Moon. It sounds like science fiction but it is beautiful fact.
After 8:30 the Moon will be moving into the other side of the umbra back into the partial shadow again and before 11 o'clock the whole show will be over.
You may ask why the Moon doesn't move into the earth's shadow every month. Well if the Moon's orbit were on the same line of sight as the plane on which the sun and earth reside, it would indeed happen every month. And Earth would also experience a solar eclipse monthly as the Moon passes in front of the Sun.
But the Moon's orbital plane is slightly tipped. So sometimes the Moon passes by the shadow slightly below it, and sometimes it skims just above it, as will happen next month. The next lunar eclipse is 6 months from now when the Moon only manages to slip through the lighter penumbra.
Next week is a perfect time to take family or students or youth group and go out to witness, from the comfort of your own backyard, a beautiful extraterrestrial phenomenon involving our nearest neighbor.
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2004.10.16 12:03 PM | Comments (0)
