Plus, The Strongest Tides of the Year
On Tuesday, February 19, 2019, the full Moon reaches 2019’s closest approach to Earth. The media will call it a supermoon, astronomers will call it lunar perigee. Regardless of the name, it will be the nearest, largest, and brightest full Moon of the year—and will produce the strongest tides.
This nearest full Moon of the year will reach its peak on Tuesday, February 19, at 10:53 A.M. EST. “Perigee,” the official astronomical term for the nearest point in the Moon’s orbit to Earth, is three hours before sunrise. This is the closest alignment for the year; whether you can tell if the Moon’s appears biggest or brighter is up for speculation but it’s technically true.
Given the timing, the Moon should appear full both Monday night (February 18) and Tuesday night (February 19). See the Almanac’s February 2019 Moon Guide for more viewing times, facts, and folklore about this event.
How Moons Affect Tides
Something that’s not talked about a lot is the lunar influence on the tides. As the nearest celestial object, the Moon does indeed affect our oceans (and us).
- Ancient civilizations, especially sea faring ones like the Greeks, 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, added to that spring tide business, the Moon happens to come unusually close the same day, as it will on Tuesday, the waters rise an extra few inches—enough to be the year’s strongest tides. That’s what we can look for next Wednesday, since the maximum tides need a day after the astronomical influences to fully catch up. See customized Tide Calculator. But this is still usually not enough to cause flooding; for that we need one additional “push.”
Two phenomena can supply that final “straw” to push coastlines over the edge to experience damage or even devastation. The first is simply a low pressure (storm) system. That’s because ocean water acts like the liquid at the base of a primitive barometer. A one inch drop in barometric air pressure creates a one foot rise in sea level.
An even bigger factor would be onshore winds. Winds whirl counterclockwise around low pressure so that, to use one common example, a deep storm over Southern New Jersey will blow its strongest winds into Long Island Sound and New York Harbor, boosting the tides. If you want to know where a storm is located, just face into the wind and raise your right arm straight out. You’ll be pointing to the storm’s location!
Tides Do Not Actually “Rise”
Where people get it wrong is when they imagine that when the Moon is overhead it gravitationally pulls the ocean up toward itself. It actually cannot do this. The reason is fascinating. At sea level, Earth’s gravity pulls the ocean downward with nine million times more force than the Moon’s gravity pulls those waters upward. So it’s not a fair contest. Earth’s gravity wins and water doesn’t 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 because this torque is opposed by nothing at all.
It’s a tiny nudge on each tiny bit of ocean water. But it adds up over thousands of miles until all this sideways torque produces a 3-foot mound of seawater located under the moon. When Earth’s rotation brings this tidal bulge to where the sea bottom is shallower, near the shoreline, the mound rises to become a 5-foot bulge—the typical increase 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. Indeed, the Moon exerts a smaller tug on a newborn baby than the gravity of the attending physician. But moonlight probably does exert an influence.Although the full Moon is 450,000 times less bright than the Sun, this may have been enough to affect our biological rhythms.
Does it seem as if there have been a lot of “supermoons” recently? Learn how I define a supermoon.