Casually speaking, a month is 30 or 31 days. But astronomers describe several months, each of different duration.

“Month” derives from pre-Germanic “menoth” (a complete cycle of lunar phases), what astronomers call a synodic month, averaging 29.53 days.

But, because Earth’s orbital speed varies throughout the year, the sun appears to move faster compared to the stars at some times, and slower at others. The moon, in turn, takes between 29.18 and 29.93 days to complete its phase cycle.

The simplest is the sidereal month, the time for the moon to move through 360 degrees (using the fixed stars as reference): 27.32 days.

This month would be fixed if not for the fact that the moon is slowly spiraling away from Earth, a consequence of the ocean tides’

gravitational tug on our natural satellite. So, the sidereal month lengthens by about 0.04 seconds per century.

The tropical month is the time for the moon to return to the Vernal Equinox, the location among the stars of the sun on the first day of spring.

The moon’s gravitational pull on Earth’s equatorial bulge makes Earth’s axis slowly precess (wobble like a top), so the Vernal Equinox creeps westward, making the tropical month about 7 seconds shorter than a sidereal month.

The sun’s gravity makes the moon’s orbit precess, so the time for the moon to return to its point of crossing Earth’s orbital plane —the “draconic month”—is 27.21 days.

Finally, the moon’s elliptical orbit slowly shifts its orientation, so the period between successive perigees (closest point to earth) averages 27.55 days, the so-called “anomalistic month.”

This all may seem like minutiae, but every one of these months figures into the cycles of eclipses, allowing us to predict far in advance their duration, location, and frequency.

Next column: Gravitational waves and kilonovae.Sky calendar through November 8th:Planets:One hour after sunset:Saturn: SW, very lowOne hour before sunrise:Venus: ESE, extremely lowMars: ESE, lowMoon: First quarter 4:22 p.m. 10/27. Full moon 11:23 p.m. 11/3.Other data: Daylight Saving Time ends 2:00 a.m. 11/5 (set clocks one hour earlier).

Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or


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