Saturday night special:  Biggest full moon of '09

Moon will be nearly full, rising earlier Friday night and later Sunday night...

By Robert Roy Britt

updated 1:36 p.m. CT, Fri., Jan. 9, 2009

If
skies are clear Saturday, go out at sunset and look for the giant moon
rising in the east. It will be the biggest and brightest one of 2009,
sure to wow even seasoned observers.

Earth,
the moon and the sun are all bound together by gravity, which keeps us
going around the sun and keeps the moon going around us as it goes through phases. The moon makes a trip around Earth every 29.5 days.

But
the orbit is not a perfect circle. One portion is about 31,000 miles
(50,000 km) closer to our planet than the farthest part, so the moon's
apparent size in the sky changes. Saturday night (Jan. 10) the moon
will be at perigee, the closest point to us on this orbit.

It will appear about 14 percent bigger in our sky and 30 percent brighter than some other full moons during 2009, according to NASA. (A similar setup occurred in December, making that month's full moon the largest of 2008.)

High tides
Tides will be higher, too. Earth's oceans are pulled by the gravity of the moon and the sun. So when the moon is closer, tides are pulled
higher. Scientists call these perigean tides, because they occur when
the moon is at or near perigee. (The farthest point on the lunar orbit
is called apogee.)

This month's full moon is known as the Wolf Moon from Native American folklore. The full moon's of each month are named. January's is also known as the Old Moon and the Snow Moon.

A
full moon rises right around sunset, no matter where you are. That's
because of the celestial mechanics that produce a full moon: The moon
and the sun are on opposite sides of the Earth, so that sunlight hits
the full face of the moon and bounces back to our eyes.

At
moonrise, the moon will appear even larger than it will later in the
night when it's higher in the sky. This is an illusion that scientists
can't fully explain. Some think it has to do with our perception of
things on the horizon vs. stuff overhead.

Try
this trick, though: Using a pencil eraser or similar object held at
arm's length, gauge the size of the moon when it's near the horizon and
again later when it's higher up and seems smaller. You'll see that when
compared to a fixed object, the moon will be the same size in both
cases.

More lunacy
If
you have other plans for Saturday night, take heart: You can see all
this on each night surrounding the full moon, too, because the moon
will be nearly full, rising earlier Friday night and later Sunday
night.
Interestingly, because of the mechanics of all
this, the moon is never truly 100 percent full. For that to happen, all
three objects have to be in a perfect line, and when that rare
circumstance occurs, there is a total eclipse of the moon.

A departing fact: The moon is moving away as you read this, by about 1.6 inches (4 centimeters) a year.
Eventually this drift will force the moon to take 47 days to circle our
world.

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