Revolutionary Moon

ON the night of April 18-19, the moon will re-enact an intriguing bit of history.

That's the anniversary of "the midnight ride" of Paul Revere and two fellow "Patriots" and of the fight in Concord, Mass., between "the embattled farmers" and British occupation troops who started the American Revolutionary War. The ride and fight are duly celebrated in legend and poetry. But the moon is a relatively unsung player in that seminal event. Yet it had an important supporting role for the main actors.

Being a little past full - 87 percent sunlit - it illumined the way from Boston to Concord. Astronomers Donald W. Olsen and Russell L. Doescher of Southwest Texas State University in San Marcos now find that the position of the moon's rising also was crucial in helping Revere evade the British guard when he left Boston. As they explain in the astronomy magazine Sky & Telescope, the moon will repeat that 1775 performance this year.

Dr. Olsen said in a telephone interview that illustrations in a children's book had aroused his curiosity about the April 18 moon. They depicted Revere's ride with a nearly full moon in the background. He also noted that Henry Wadsworth Longfellow's poem celebrating the ride mentioned the moon five times. He wondered whether the moon actually was prominent that night or had illustrator and poet exercised artistic license?

Using computing methods developed for pocket calculators by Belgian meteorologist Jean Meeus, the Texas astronomers calculated the moon's phase and course for April 18-19, 1775. They found that the moon not only was prominent that night but, fortunately for Revere, it rose relatively far to the southeast.

To recap the events: the British occupation force at Boston learned that colonists were caching munitions at Concord. The colonists expected the British to march on Concord. Revere, William Dawes, and Samuel Prescott stood by to ride west and warn the colonists whenever the British set out.

Orders came for British troops to assemble at 10 p.m. on April 18, just after moon rise at about 9:37 p.m. Revere warned Dawes and Prescott and then crossed Boston Harbor to reach his own horse. His rowboat passed to the east of the British naval ship HMS Somerset. For most risings of a nearly full moon, sentinels on the ship could have seen the boat. But, this time, the moon rose far enough to the south so that, 45 minutes after rising, it stood 6 degrees above the horizon at an azimuth of 31 degrees s outh of east. Sentinels on the Somerset saw it coming up over Boston, not over the water. Revere slipped past in the dark.

This year, the moon will rise a little past full - 96 percent sunlit - at about 9:38 p.m. Eastern Daylight Time on April 18. It will stand 6 degrees above the horizon at an azimuth of 38 degrees south of east 45 minutes later.

A modern Revere probably wouldn't make it across the harbor, however. He would be silhouetted - and thus visible to observers - by the lights of metropolitan Boston, even though the moon, once again, is favorably placed.