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Treebeard's Stumper Answer
8 January 1999

Low Winter Tides

There was a huge full moon to bring in the New Year last weekend, rising in the east just as the sun set in the west. There was also an extremely low tide at the beach, minus 1.5 feet according to the newspaper tide table. Everyone knows that the moon causes the tides by gravitational attraction, but the details are tricky. We often (usually? always?) get our most extreme tides of the year in midwinter. The lowest tide last summer was only about minus 1 foot. Why would the season make a difference with the tides? After all, there are full moons in the summer as well.

Extreme winter tides happen when several different factors combine. The greatest tides occur when the sun, moon, and earth are all in a line (in syzygy), at full or new moon. The earth is closest to the sun (at perihelion) on about January 2. If the moon is closest to the earth (at perigee) at about the same time, then the tides will be greater. The winter sun is low, but the full moon on the other side of the sky is high, more directly above us. It's complicated. Tide prediction is still based more on observation than astronomical theory!

Notes:

This winter (1998-9), syzygy, perihelion, and perigee all occured within a few days of each other. There was a full moon on January 2, the earth was closest to the sun on January 3, and the moon was closest to the earth on December 30. It's not surprising that we had extreme tides.

Graybear contibutes this clear explanation:

The moon plays the biggest part in determining the tides, but the sun helps out, also. This causes the larger spring tides when Sol and Luna are aligned with Earth (full and new moons), and the weaker neap tides when they form a right angle (first & last quarter moons). As you mentioned, there was a full moon at the new year (Luna set a beautiful backdrop for the fireworks at our First Night celebration!), but there are other factors at work. Gravitational attraction is based on distance and is stronger when the bodies are closer. Earth's perihelion is about 3 million miles closer than aphelion (see 25 Sep 98 stumper). Luna's perigee is about 221,463 miles vs. apogee of 252,710 miles. I haven't found the exact times of perihelion and perigee, but the New Year's full moon was very close to both, and the seasonally high/low tides were caused by a combination of spring tide, lunar perigee, and perihelion.

I first thought of this stumper when I heard a local radio announcer mention that New Year's weekend was a great time for the beach since the tides were so low this year. This has happened before. I immediately thought that the reason was the altitude of the ecliptic in the sky. In the summer, the sun is high and the full moon and planets and zodiac constellations are low at night, but in the winter this situation is reversed. The winter moon rises more nearly vertical for us in the northern hemisphere, and it's more directly above us at night when full. The effect of this on the tides is suggested by this picture (modified from NOAA's fine Our Restless Tides site).

You can see diminishing tidal peaks in this graph of tides for the first four months of 1999 (from the TBONE Tide Calculator):

Now I have second thoughts. I'm not so sure the altitude of the ecliptic is important. I haven't found it mentioned as a factor in my research. In fact, the greatest tides in 1999 will be in July rather than December, so at best we often have our extreme tides in midwinter, not always. I'm confused, but I think I know how to settle it. If winter tides are extreme mostly because of the combined effects of perigee/perihelion, then tides in the southern hemisphere will be most extreme at the same time, during their summer (our winter). But if the altitude of the ecliptic is an important factor, then southern hemisphere tides will be most extreme during their winter (our summer). The real stumper is finding southern hemisphere tide tables!

I don't really understand tides, though I've done some research. It seems appropriate to answer my stumper with more stumpers.

So many questions, so little time. These web sites will help your research efforts:

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Copyright © 1999 by Marc Kummel / mkummel@rain.org