WRAL WeatherCenter Blog

Astronomical timing helps Hurricane Teddy bring Outer Banks flooding

Posted September 21, 2020 7:16 p.m. EDT

Spring tides occur at new and full moons with the Moon's gravity combines with the Suns (image: NOAA)

For coastal residents and visitors, there really isn't a good time for hurricanes, even those which stay offshore. Waves whipped up over the weekend brought serious flooding, closing N.C. 12 in two places.

High tides at the NOAA measurement station at Duck, NC are running about 1.5 feet above predicted level and moderate flooding is being seen from Florida to the Carolinas.

Effects from Hurricane Teddy come at close to the worst time of the year, astronomically speaking. The new moon was on Thursday, September 18, the perigee occurred Friday,September 18 and the equinox occurs on Tuesday, September 22.

Spring tides

In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the Sun and Moon on the Earth's oceans. Earth is pulled back and forth through the Moon's monthly and Earth's yearly orbits, creating bulges in the oceans.

As the Moon orbits around the Earth, it combines gravitation forces at new moon when they line up Earth-Moon-Sun and full moon when they line up Moon-Earth-Sun. Bulge is pulled out bit more than usual causing tides that "spring forth" little higher and low tides are a little lower.

Spring tides, also known as king tides, occur every 14.75 days, but some are springer than others.

Perigean Spring Tides

The Moon's orbit around Earth isn't a perfect circle. Every 27.5 days it reaches perigee, or the point in its orbit where it is closest to Earth. The closer two objects are, the greater their gravitational attraction making tides a little higher close to perigee.

Perigee coincides with full or new Moons about 3-4 times each year, creating higher than normal tides.

Equinoctial Tides

The Sun and Moon work together during new and full moons (spring tides) when the Earth, Sun and Moon are lined up. These effects are at their greatest around the equinox when the Sun is directly over the equator creating equinoctial (pronounced ee·kwuh·naak·shl) tides.

About every 18.6 years, all three of these coincide creating perigean spring equinoctial tides such as the Outer Banks are enduring.

The Sun and Moon's gravitational effects on the tides are greatest at the equinox

October's new moon occurs on the same day as lunar perigee, the second closest of the year. However, the Sun will have moved to a point about 10 degrees below the equator, lessing its gravitational effects.

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