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Dec.
9, 2008: No, you can not see Neil Armstrong's
footprint. But go ahead and look: The full Moon of Dec. 12th
is the biggest and brightest full Moon of the year.
It's
no illusion. Some full Moons are genuinely larger than others
and this Friday's is a whopper. Why? The Moon's orbit is an
ellipse with one side 50,000 km closer to Earth than the other:
diagram. In the
language of astronomy, the two extremes are called "apogee"
(far away) and "perigee" (nearby). On Dec. 12th,
the Moon becomes full a scant 4 hours after reaching perigee,
making it 14% bigger and 30% brighter than lesser full Moons
we've seen earlier in 2008.
Above:
In 2004, Greek amateur astronomer Anthony Ayiomamitis photographed
an apogee Moon and a perigee Moon, and set the images side
by side to show the difference. [Larger
image]
A
perigee Moon brings with it extra-high "perigean tides,"
but this is nothing to worry about, according to NOAA. In
most places, lunar gravity at perigee pulls tide waters only
a few centimeters (an inch or so) higher than usual. Local
geography can amplify the effect to about 15 centimeters (six
inches)--not exactly a great flood.
Okay,
the Moon is 14% bigger, but can you actually tell the difference?
It's tricky. There are no rulers floating in the sky to measure
lunar diameters. Hanging high overhead with no reference points
to provide a sense of scale, one full Moon looks much like
any other.
The
best time to look is when the Moon is near the horizon. That
is when illusion mixes with reality to produce a truly stunning
view. For reasons not fully understood by astronomers or psychologists,
low-hanging Moons look unnaturally large when they beam through
trees, buildings and other foreground objects. On Friday, why
not let the "Moon illusion" amplify a full Moon that's
extra-big to begin with? The swollen orb rising in the east
at sunset may seem so nearby, you can almost reach out and touch
it.
But
you still won't be able to see Armstrong's footprint. Not
even Hubble can do that. The Moon is 384,400 km away (on average).
At that distance, the smallest things Hubble can distinguish
are about 60 meters wide. The biggest pieces of left-behind
Apollo equipment are only about 9 meters across and smaller
than a single pixel in a Hubble image.
What
you will see is the world around you. This is both
the brightest and (in the northern hemisphere) the highest-riding
full Moon of the year. If you go outside around midnight it
will be close to overhead and act like a cosmic floodlamp
making the landscape absolutely brilliant, especially if there's
snow. Full moons are always high during winter and, indeed,
the solstice is right around the corner on Dec. 21st.
A
fun experiment: Take a friend outside on Friday evening and
ask if they notice anything unusual. Is the Moon big and bright
enough to impress the unwary? Explain perigee later....
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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