January 10, 2009 - The Perigean Full Moon

The full moon Saturday night will be the biggest one of the year as Earth's natural satellite reaches its closest point to our planet.

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.

The moon's average distance from us is about 238,855 miles (384,400 km). Friday night it will be just 221,560 miles (356,567 km) away. It will be 14 percent bigger in our sky and 30 percent brighter than some other full moons during the year, according to NASA.

Tides will be higher Saturday night, 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 the moon's closest point to Earth is called perigee. The farthest point on the lunar orbit is called apogee.

Some other strange lunar facts:
The moon is moving away as you read this, by about 1.6 inches (4 centimeters) a year. Eventually it'll be torn apart as an expanding sun pushes the moon back toward Earth for a wrenching close encounter.

There is no proof the full moon makes people crazy.

Beaches are more polluted during full moon, owing to the higher tides.

The moon will rise Saturday evening 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.
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.

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This Saturday night, Jan. 10th, another perigee Moon is coming. It's the biggest full Moon of 2009, almost identical to the one that impressed onlookers in Dec. 2008.

Johannes Kepler explained the phenomenon 400 years ago. The Moon's orbit around Earth is not a circle; it is an ellipse, with one side 50,000 km closer to Earth than the other. Astronomers call the point of closest approach "perigee," and that is where the Moon will be this weekend.
Perigee full Moons come along once or twice a year. 2008 ended with one and now 2009 is beginning with another. It's the best kind of déjà vu for people who love the magic of a moonlit landscape.

January is a snowy month in the northern hemisphere, and the combination of snow + perigee moonlight is simply amazing. When the Moon soars overhead at midnight, the white terrain springs to life with a reflected glow that banishes night, yet is not the same as day. You can read a newspaper, ride a bike, write a letter, and at the same time count the stars overhead. It is an otherworldly experience that really must be sampled first hand.

Another magic moment happens when the perigee 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. This weekend, 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 catch yourself reaching out to touch it.

You won't be the only one. Even at perigee, the Moon is 360,000 km away, yet the distant beauty beckons to poets, stargazers and NASA with equal force: "Come back," it seems to say, "I'm really not so far away."

Edit: Dana's Muse