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Frothingham,
Siege of Boston,
59, 84n; the Stiles Diary and Jeffries Diary, in Ludlum,
The Country Journal New England Weather Book, 127
.
    APPENDIX I
     
    The Moon, April 18-19, 1775
     
    Many participants remembered that the moon was nearly full on the night of April 18-19, 1775. “The moon shone bright,” Paul Revere wrote in his deposition. His fellow captive Sanderson wrote independently, “It was a bright moon-light after the rising of the moon, and a pleasant evening” (Phinney,
Lexington,
31).
    These memories have been confirmed by two American astronomers, Donald W. Olson and Russell L. Doescher, who studied this subject in detail, and found that the nearest full moon occurred on April 15, 1775, and the last quarter on April 22. On the night of the midnight ride, they calculate that “there was indeed a bright waning-gibbous moon, 87 percent sunlit, in Boston on the night of April 18, 1775.” They estimate that the moon rose over Boston at approximately 9:53 p.m., local apparent solar time (approximately 25 minutes later than our modern Eastern Standard Time).
    On that night Olson and Doescher reckon that the moon had a strong southern declination of -18 degrees. If Revere’s friends were rowing him across the Charles River at approximately 45 minutes after moonrise, the moon would have been very low in the southern sky—6 degrees above the horizon, on a true bearing of 121 degrees.
    Revere’s course from the North End of Boston to Charlestown’s ferry landing was approximately 330 degrees. The moon was almost directly behind him, and as it was only 6 degrees above the horizon, his boat would have been shrouded in the dark moonshadow of the town’s skyline, and very difficult to see from the deck of HMS
Somerset,
or even from British guardboats that were patrolling the river that night.
    The transit of the moon occurred at 2:42 a.m. according to the computations of two other astronomers, Jacques Vialle and Darrel Hoff. The marker in the Minute Man National Historical Park that refers to a third quarter moon is not correct.
    See Donald W. Olson and Russell L. Doescher, “Astronomical Computing: Paul Revere’s Midnight Ride,”
Sky and Telescope,
April 1992, pp. 437-40; also Jacques Vialle and Darrel Hoff, “The Astronomy of Paul Revere’s Ride,”
Astronomy
20 (1992): 13-18.
    APPENDIX J
     
    Tidal Movements, the British March, and the Midnight Ride, April 18-19, 1775
     
    Shakespeare observes that “there is a tide in the affairs of men; which, taken at the flood, leads on to fortune”
(Julius Caesar, IV, iii, 217)
. So it was for Paul Revere, but General Gage would have been luckier at the ebb.
    Tidal movements in the Charles River and Boston’s Back Bay had an important impact on these events. Paul Revere remembered that when he crossed the Charles River at approximately 10:30 to 11:00 p.m., “It was then young flood.” The strong Boston tide was flowing into the harbor and running westward up the estuary of the Charles River.
    At that same hour, the British troops were moving across the Charles River to Lechmere Point in Cambridge. Lt. William Sutherland wrote that when they marched along the river at 2 a.m., “the tide being in we were up to our middles.” Mackenzie remembered that the men were “obliged to wade, halfway up to their thighs, through two inlets, the tide being by that time up.”
    Modern computations confirm the accuracy of these accounts. Professors Olson and Doescher, using a method of harmonic analysis, estimate that on the afternoon of April 18, 1775, high tide in Boston harbor occurred at 1:14 p.m. local apparent solar time. Low water followed that evening at 7:19 p.m. and high tide again at 1:26 a.m. in the morning. These results confirm that the tide was rising when Revere and the British soldiers were crossing the river.
    Early American almanacs were even closer to the descriptions of the participants. Three colonial almanacs variously estimated the time of high tide on