Saturday Night Special: Biggest Full Moon of 2009

This view of a full moon was photographed by an Expedition 14 crewmember onboard the International Space Station. Earth's horizon and airglow is visible at left. CREDIT: NASA

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 moons 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.

• Gallery: Moon Images
• Skywatcher's Guide to the Moon
• Top 10 Strangest Things in Space