This month, the big "cold, long nights” moon makes the highest tides of the year.
Ocean tides are caused primarily by the moon’s gravity, which pulls on the oceans directly under it a bit harder than it does on the rest of the planet because that’s the closest part. The slight gravitational tug sloshes the oceans up toward the moon.
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The rigid body of the Earth can’t flex like the oceans, so it is pulled by the moon as a solid body.
But what comes as a surprise to many people is that the part of the ocean opposite the moon gets left behind, being pulled slightly less by the moon’s gravity, creating another high tide.
The gravitational tug Earth and its oceans feel from the moon depends on the separation of the two. The moon’s orbit around Earth isn’t circular; it is slightly oval. And because the sun’s gravity pulls on both Earth and the moon, the shape of that oval orbit varies during the year. And Earth’s orbit around the sun isn’t perfectly circular, either. Earth, and therefore the moon, is closest to the sun Jan. 4 every year. Although the sun has a far more powerful gravitational strength than the moon, it’s also much farther away. The sun affects Earth’s tides about half as much as the moon does.
When the sun and moon pull together, which they do at every new and full moon, tides are higher than average, what mariners call the "spring tide.” When the gravitational tugs of the two bodies are at right angles to each other, at first and third quarter moon, they counteract each other, causing relatively weak "neap tides.”
The moon’s closest approach to Earth is called perigee, and Earth’s closest point to the sun marks perihelion. When a spring tide coincides with either perigee or perihelion, we get particularly high tides. If all three occur together, we get the highest tides. Perigee for 2008 occurs Dec. 12 with the moon a mere 221,559 miles away, making the moon appear larger than it has all year. On that date, we are barely three weeks from perihelion, only 91,346,156 miles from the sun. Dec. 12 also is the date of the full moon, commonly called the "cold moon” or the "long night moon.”
The combination of the long night moon, perigee and perihelion makes for the highest tides of this year. It won’t affect us so much, being in the throes of winter, but beachgoers in Africa, Australia or South America might want to put their umbrellas just a bit farther from the shoreline than normal.
Wayne Harris-Wyrick is director of the Kirkpatrick Planetarium at Science Museum Oklahoma.
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Something else about the Earth is that its cooling and has been for a couple years. The Sun is also cooling and has not generated sunspots or solar flares for at least 4 years. Its so bad that amateur radio operators are having trouble reaching Europe and Asia with basic equipment. It takes sunspots and flares to destabilize the ionosphere and form a type of hard shell the reflects radio waves back to Earth. As long as this global cooling takes effect, amateur radio operators are getting more involved in repeaters, Echolink, and IRLP. That's where the radio towers of the world are connected via the internet. You can tune up a tower in Paris or Argentina and talk on your little FM hand held radio to someone in the states, Europe or Asia. All operators are hoping for sunspots to reappear so we can get back to world of HF.
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→Jupiter and Venus continue to dominate the evening twilight as the two brilliant "stars” in the west at sunset. Saturn rises around midnight and shines brightly until sunrise. The full moon occurs Dec. 12, and the new moon is Dec. 27.
→"Star of Wonder” makes its annual holiday appearance in December. Just what was the object shepherds saw on that night more than 2,000 years ago? Call 602-3761 or go online to www.sciencemuseumok.org for details.
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