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Earth Sense: Eclipse less complex than tides, other natural events
Rudi Kiefer

Monday, the moon will bring us some daytime twilight during a full solar eclipse. The many moon-related Latin terms may need some sorting out. 

The moon revolves around the Earth roughly once every 30 days. The period we see from new moon to new moon is called synodic month. That’s the time from an “invisible” moon (because the sun is shining on the side turned away from us) to the same a month later. 

In between, we see a growing round shape called “waxing gibbous.” Its bright part appears on the right. A full moon follows as a complete illuminated circle. After that, it becomes a waning gibbous, where the bright portion is at left.

Just as the globe rotates, so does the moon. But since it turns, shouldn’t we see something other than the “face” in it, object of many children’s stories? Interestingly, the Earth’s gravity has such a hard grip on it that the moon can rotate, but only in step with its orbit around us. So it seems to be standing still, seen from our ever-rotating position in Hall County.

The Earth puts a strong gravity lock on the moon, but our oceans are in turn affected by the moon’s own gravity. In areas facing the moon during its 24-hour rotation, the water level rises some, causing what’s known as “high tide.”

On the other side of the Earth, centrifugal force makes it slosh in the opposite direction, producing a high tide there. So we end up with two high and two low tide cycles. In other words, the change from high to low tide occurs every six hours.

The effect gets stronger if the moon is closest to the Earth during its out-of-round orbit. This situation, called perigee, causes higher tides than normal. At times the sun, Earth and moon are aligned, called syzygy. When a syzygy and perigee occur simultaneously, and while the Earth is closest to the sun (“perihelion”), tides get extremely high, with effects like beach erosion and destruction of oceanfront homes.

Compared to these complex movements, an eclipse is a simple thing. The moon passes in front of the sun, which in Monday’s case obscures it totally for a few minutes. It’s worth seeing North Georgia getting darker at 2:36 p.m., but don’t look directly at the sun without special protective lenses.


Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.

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