ON Dec. 16, a thin crescent moon will grace the southwestern early evening sky. If you see it, you might give a thought to the Boston Tea Party. Rebellious colonists threw a shipment of tea into the harbor rather than pay an obnoxious tariff on that same December date 220 years ago.
You will see in the sky what the colonial raiders saw. The moon will show the same phase and be in the same location. But thanks to the computer power now available, we can know a crucial fact about this moon that surprised the colonists. It was positioned to produce a far lower tide than they expected.
Physicists Donald W. Olson and Russell L. Doescher of Southwest Texas State University in San Marcos have uncovered the lunar story. They used calculating methods developed by Belgian meteorologist Jean Meeus that work even with the pocket-size ``palmtop'' computer on which I wrote this column. The astronomical sleuths explain in the December issue of Sky & Telescope that, not only was the young moon nearly in line with Earth and the sun, but it also was at perigee (closest approach to Earth). This substantially enhanced its tidal power.
It works this way. When the moon is new or full it lines up with Earth and sun. Astronomers call this line up syzygy. Solar and lunar tide-raising forces then add together. They create the largest ordinary low-high tidal ranges - the so-called spring tides. When the moon also is at perigee - that is, when its orbit brings it closest to Earth - its tide-raising force is even stronger. A coincidence of perigee and syzygy can bring the highest high tides and lowest low tides known.
That's essentially what happened when the colonists boarded the tea-carrying cargo ships Beaver, Dartmouth, and Eleanor at Boston's Griffin's Wharf. Drs. Olson and Doescher's calculations show that lunar perigee occurred about 3 a.m. Dec. 13, 1773, in terms of modern Eastern Standard Time. New moon occurred 14 hours later. The influence of this near coincidence of lunar perigee and syzygy was still strong Dec. 16. It increased the tidal range from an average of about 11 feet for ordinary spring tides to more than 14 feet.
The Tea Party lasted from 6 to 9 p.m. The extraordinarily low tide came right in the middle of it. The raiders slit open 340 tea chests, dumped 90,000 pounds of tea leaves, and threw the chests after them. With little water at the docks, this created an unexpected mess on the mud flats.
Commenting on this research in a telephone conversation, Olson explained that the tidal story was a byproduct of other research. He was originally interested in what the moon had looked like on historic occasions. Old pictures and accounts of historic events may get the lunar phase wrong.
Olson and Doescher had earlier checked out the moon on the night of Paul Revere's famous ride. Accounts and illustrations had implied a nearly full moon. They wondered if this were right. A nearly full moon might have revealed Revere when he snuck across Boston Harbor under the nose of a British warship. Their calculations showed that the moon was indeed 87 percent full. But it rose too far to the south to give Revere away.
In checking out the Tea Party moon, Olson says they found that some accounts and pictures had it wrong. These speak of bright moonlight or show a nearly full moon. But there was only a thin crescent.
The analogy to 1773 isn't exact. This time, perigee doesn't coincide with a new moon. The tides will be ordinary. Moreover, anyone standing where actors now reenact the Tea Party won't see the moon at all. Landfilling and subsequent construction have obscured that view.
Why did the raiders risk low tide and not wait for higher water? The reason may have been political. The colonists had negotiated with the British governor until the last minute. Customs officers would seize the tea at midnight Dec. 16 if the tax were not paid. Waiting for high tide would have run over that deadline.
The raiders had to move fast to make their protest demonstration. Even if they had known how bad low water would be, they couldn't risk delay. It would have been another case where political wisdom outweighed scientific knowledge.