What is an “underground moon”?

In the days before modern weather forecasting, Nantucketers anticipated bad weather around the time of an underground moon. The Inquirer and Mirror’s twelve-month calendar not only indicated the phases of the moon throughout each month but the dates on which an underground moon would occur. This led to yearly inquiries from seasonal visitors about what the underground moon signified. Periodically the newspaper would print an explanation, prefacing it with a grumpy remark that this information had already been printed more than once in the past.

Unfortunately, the explanation was ineffective in explaining how the dates and times of an underground moon were computed and what the astronomical event had to do with local weather. The standard explanation was that an underground moon occurs when the moon “makes its change” between midnight and one AM when the moon is below the horizon. This begs the question of what it means for the moon to “make its change” and how one knows precisely when it happens.

Approximately every 29.5 days the moon passes from new to full and back to new. The standard “phases” imposed on this continuum are: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent, and back to new. Because the calendar month (30—31 days, except for February) is a different length than the lunar month (29.5 days), the changes of the moon’s phases do not occur on the same day of every month.

There is a formula that determines exactly when one phase ends and the next begins, and these times can be found in the Old Farmer’s Almanac (full moon only) and—currently—on several websites that provide the hours and minutes for the beginning of every one of the moon’s phases day-by-day, month-by-month throughout the year. In the past the Inquirer and Mirror would compute these times and indicate on its calendar the dates when the transition to certain phases occurred between midnight and one AM with the moon below the horizon.

Nantucketers were concerned only with four phases—new, first quarter, full, and third quarter—and they anticipated bad weather more around the new moon and the full moon than around the quarter moons.

When explaining the underground moon, the newspaper would occasionally include an interview with someone on the staff of the Maria Mitchell Observatory who would state unequivocally that there was no connection whatsoever between the phases of the moon and local weather. Nonetheless, Nantucketers held to the belief that an underground moon portended bad weather and would plan accordingly. It is said that in the 1800s departure of whaleships on voyages would be delayed until after the passing of an underground moon.

The Nantucket Historical Association preserves and interprets the history of Nantucket through its programs, collections, and properties, in order to promote the island’s significance and foster an appreciation of it among all audiences.

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