Closest Moon of the year

(Photo by Josh Kelahan)

This Tuesday, February 19, three hours before sunrise, the Moon reaches 2019’s closest approach to Earth. The media will call it a Supermoon, astronomers will call it lunar perigee, and Hudson River tides will be stronger than usual.

It brings up the topic of lunar influence. As the nearest celestial object, the Moon does affect us. But how it works is, for virtually everyone, deeply mysterious.

Ancient civilizations saw that the tidal range was larger at Full and New Moon and called these “spring tides,” as if the water leapt up at them like a spring. A week later, during the Half Moon (which is also oddly termed the Quarter Moon), tides were wimpy, and these were called “neap tides” – from the Old English nep, as in “nipped in the bud.”


If the Moon ventures unusually close during a spring tide, as it will on Tuesday, the waters rise an extra few inches: enough to be the year’s strongest. That’s what we can expect on Wednesday, since the maximum tides happen a day after a perigee Full Moon, or Supermoon. This is still not enough to cause flooding; for that we need one additional factor.

Two things can supply that extra “straw” to push coastlines over the edge to experience devastation. The first would be a low-pressure (storm) system, since a one-inch drop in barometric pressure creates a one-foot rise in sea level.

An even-bigger factor would be onshore winds. During a storm, if you face into the wind and raise your right arm, you will point to the storm’s location. Winds whirl counterclockwise around low pressure, so that a storm over Atlantic City will blow its strongest winds into Long Island Sound and New York Harbor.

It’s all fascinating. Where people go astray is to think that when the Moon is overhead, it gravitationally pulls the ocean toward itself. It doesn’t – not even by one thousandth of an inch. That’s because, at sea level, Earth’s gravity is pulling downward with nine million times more strength than the Moon’s gravity is pulling upward. It’s no contest: Earth’s gravity wins, and water does not rise toward the sky.

What happens is something else: When the Moon is on the horizon, rising or setting, it’s then pulling sideways on seawater. So, while the Moon cannot pull water straight up because it is opposed by Earth’s gravity, it can indeed pull water sideways, since this torque is opposed by nothing at all.

It’s a tiny nudge. But it adds up over thousands of miles, until all this sideways torque produces a three-foot mound of seawater roughly located under the Moon. When this yard-high bulge approaches the seashore, thanks to Earth’s rotation, it rises to become a five-foot bulge, which is the typical displacement seen at high tide.

None of this happens to the water in your body. It doesn’t budge. So lunar gravity doesn’t affect you. But its light probably can.

The Full Moon is 450,000 times less bright than the Sun. But this was enough to provide sufficient safe nocturnal illumination in olden days to let courtship proceed. Over time, the female body apparently became attuned to the 29-day lunar synodic cycle: the interval from Full Moon to Full Moon. And while the human 29-day estrous cycle is shared by no other animal except the opossum, this moonlight explanation still seems like a credible way to account for the similarity between the lunar phase cycle and the human menstrual cycle.

Other lunar influences? Surveys examining many other areas of putative lunar power have yielded largely negative results. A huge New York City study of public hospitals in the late ’50s showed a one-percent increase in births during the bright half of each month. However, it best correlated not with the Full Moon, but the Last Quarter phase.

Studies looking for psychological links were even more negative. There is no correlation between lunar phase and calls to crisis centers, or with admissions to mental hospitals. And a large study in Dade County, Florida found a link between violent crime and air temperature, and also a correlation with the calendar (a strong increase during weekends), but none with lunar phase.

On the other hand, other studies reveal the tidal effect to have an even-greater influence than previously known, especially among animals such as clams in the vast intertidal marshes, and the behavior of their predators, like gulls. So, while the Moon’s gravity may be negligible – it exerts a smaller tug on a newborn baby than the gravity of the attending physician – its overall influence on our world and its inhabitants remains vast and still not fully known.

Want to know more? To read Bob’s previous columns, visit our Almanac Weekly website at Check out Bob’s new podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.

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