IJMC - Let the Lethargy Continue
Just to finish out one night with three relatively effortless posts...I
leave you with this. A bit of a correction to the Solstice bit I sent out
last week. I must say I am amused...all the checks I did to verify the
Solstice moon thing are covered here. All are true, just simply
exaggerated a bit. Oh well, so is life on the Internet. Humor to follow
tomorrow night, assuming I can lift my arms to press keys... -dave
Dave:
I realize that this is less junky than your usual fare, but help fight
ignorance about this "superbright full moon" on the Solstice.
This article is from the website of "Earth and Sky", an astronomical show
on public radio.
Thanks,
David Sparks
--------
"Superstition brings bad luck."
---------- Forwarded message ----------
Earth & Sky
http://www.earthsky.com
Solstice Moon Madness
December 16, 1999
"If you believe this email chain letter about the amazing brightness of
the full moon on December 22, next thing you'll be sending $20 bills
to some address in Burned Barn, Nebraska . . ." -- Alan MacRobert,
Sky & Telescope magazine
In the past few days, we've had a mass of email messages
from our friends and fans about a spectacularly bright full
moon on this year's solstice on December 22. Everyone
wants to know, is it true?
The answer here isn't straightforward.
Will there be a spectacularly bright full moon on the night of the
solstice? Yes! The moon is full on the solstice this year, and every
full moon is spectacularly bright.
But will this month's full moon appear super bright -- much brighter
than usual -- clearly brighter to the eye than any other full moon of
your lifetime? No. Unless you are a very astute observer indeed with
an extremely clear dark sky, the full moon on December 22 won't look
significantly brighter to the eye than any other full moon.
Here's the rumor. According to an email chain letter, "The December
full moon will occur on the solstice, December 22 . . . Since this full
moon is also occurring in conjunction with a lunar perigee (point in
the moon's orbit that is closest to Earth), the moon will appear about
14% larger than it does at apogee (the point in its elliptical orbit
that is farthest from the Earth). What's more, since the Earth is also
several million miles closer to the sun at this time of the year than
in the summer, sunlight striking the moon is about 7% stronger, making
it brighter. Also, this will be the closest perigee of the Moon of the
year since the moon's orbit is constantly deforming. As a result, there
will be a super bright full moon, much more than the usual, on December
22. It hasn't happened this way for 133 years. Our ancestors 133 years
ago saw this. Our descendants 100 or so years from now will see this
again."
It's that phrase "much more than the usual" that's bothersome. The full
moon on December 22 definitely will not be much brighter than usual.
Here are the facts.
Yes, the December full moon will occur on the same day as the
solstice, December 22.
Yes, this full moon does come in conjunction with a lunar perigee.
Perigee comes once each month, and, yes, it's true that some perigees
bring the moon closer than others. On December 22, the moon will be
356,653 kilometers from Earth, its closest for the year.
Yes, the Earth (and with it, the moon) is a few per cent closer to the
sun now than it was in July. Earth's yearly "perihelion," or closest
point to the sun, comes this year on January 3. So because of stronger
sunlight striking the moon, the full moon on December 22 will appear
slightly brighter than a similar full moon in July, when Earth is
farthest from the sun.
All of this is a great coincidence! But it's not as great as it might
sound, and it has happened more than once in the past 133 years. Roger
Sinnott of Sky & Telescope magazine has calculated that there are
three other dates in the last 133 years when the full moon was even
closer and brighter than it will be on December 22, 1999. Those were
the full moons of December 1893 -- January 1912 -- and January
1930
So the full moon will indeed be brighter than usual on December 22.
But only by a trace! As some reports have said, it'll be around 14%
brighter than usual.
But -- here's the important thing -- a 14% change in brightness is not
a very large change! You probably won't be able to detect the change
clearly with your eye alone.
Here are some facts to consider, regarding the brightness of sky
objects. First, even an experienced variable star observer can barely
detect a 10% brightness change in a variable star, when that star is
right next to a comparison star of similar brightness.
Second, the human eye sees over a huge range in brightness. Daylight
is some 500,000 times brighter than full moonlight, for example. Said
another way, the sun is 50 MILLION per cent brighter than the full
moon. In that light, 14% does not represent a big change in brightness.
According to Alan MacRobert at Sky & Telescope magazine, you
would need a sensitive light-measuring device to detect the change
clearly.
We have heard from observers who disagree -- who say they can
detect a change in the moon's brightness between each monthly perigee
and apogee. Michael Cashman wrote from Hawaii, "I can tell when
full moons anyway are relatively distant (closer to apogee). And we
skywatchers out here always look at the sky because the air is so
marvelously clear, so when it comes up on the horizon and is
magnified by the atmosphere, it's really something to behold. It's
wonderful to walk to work and see a big full moon set on the western
horizon of the Pacific . . . "
Most people don't look at the full moon as carefully as Michael and his
fellow skywatchers in Hawaii. Lots of people will probably go outside
on the night of this year's solstice and confirm for themselves that
this full moon is indeed bright! But it won't be significantly brighter
to the eye than any ordinary full moon.
Alan MacRobert called this "an example of the power of the Internet
to spread rumors and confusion."
It would be great if you would pass this information on!
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