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Tides, and the pull of the moon and sun

Expect higher-than-usual tides for a few days following the May 25, 2017 new supermoon.

This beautiful image is from EarthSky Facebook friend John Lloyd Griffith

In most places, but not everywhere, there are two high tides and two low tides a day. The difference in height between high and low tides varies, as the moon waxes and wanes from new to full and back to new again. The moon is primarily responsible for the rising and falling of ocean tides, but, for any particular spot on Earth’s surface, the height of the tides and their fluctuation in time depends not only on the moon, but also on the sun – and also on the shape of the specific beach, the larger coastline, the angle of the seabed leading up to land, and the prevailing ocean currents and winds. Click the links below to learn more about the tides.

What are spring tides?

Why does a supermoon cause more extreme tides?

What are neap tides?

Why are there two high tides and two low tides each day?

Around each new moon and full moon – when the sun, Earth, and moon are located more or less on a line in space – the range between high and low tides is greatest. These are the spring tides. Image via physicalgeography.net

What are spring tides? Around each new moon and full moon, the sun, Earth, and moon arrange themselves more or less along a line in space. Then the pull on the tides increases, because the gravity of the sun reinforces the moon’s gravity. In fact, the height of the average solar tide is about 50% the average lunar tide.

Thus, at new moon or full moon, the tide’s range is at its maximum. This is the spring tide: the highest (and lowest) tide. Spring tides are not named for the season. This is spring in the sense of jump, burst forth, rise. So spring tides bring the most extreme high and low tides every month, and they happen around full and new moon.

Why does a supermoon cause more extreme tides? When the new moon or full moon closely aligns with perigee – closest point to Earth in the moon’s orbit – then we have a supermoon and extra-large spring tides. Some call these perigean spring tides. But since, in recent years, these close new or full moons have come to be called supermoons, it’s also likely some are already calling them supermoon tides, and we’ve also heard the term king tides.

In 2017, the May 25 new moon will closely align with perigee and the December 3 full moon will closely coincide with perigee, to bring forth perigean spring tides.

Why are the tides at their strongest around this time? It’s simply because the moon is at its closest to Earth, and thus the Earth’s oceans are feeling the pull of the moon’s gravity most powerfully.

Should you expect these extra-high tides on the day of the supermoon itself? No. The highest tides tend to follow the supermoon by a day or two.

Do the most extreme of the high tides – tides bringing floods – always occur at supermoons? Not necessarily. It’s when a spring tide coincides with a time of heavy winds and rain – flooding due to a weather extreme – that the most extreme flooding occurs.

Read more: 2017’s closest supermoon is May 25

Read more: What is a supermoon?

Around each first quarter moon and last quarter moon – when the sun and moon are at a right angle to Earth – the range between high and low tides is least. These are the neap tides. Image via physicalgeography.net

What are neap tides? There’s about a seven-day interval between spring tides and neap tides, when the tide’s range is at its minimum. Neap tides occur halfway between each new and full moon – at the first quarter and last quarter moon phase – when the sun and moon are at right angles as seen from Earth. Then the sun’s gravity is working against the gravity of the moon, as the moon pulls on the sea. Neap tides happen approximately twice a month, once around first quarter moon and once around last quarter moon.

Earth has two tidal bulges, one of the side of Earth nearest the moon (where the moon’s gravity pulls hardest), and the other on the side of Earth farthest from the moon (where the moon’s gravity pulls least).

Why are there two high tides and two low tides each day? If the moon is primarily responsible for the tides, why are there two high tides and two low tides each day in most places, for example, the U.S. eastern seaboard? It seems as if there should just be one. If you picture the part of Earth closest to the moon, it’s easy to see that the ocean is drawn toward the moon. That’s because gravity depends in part on how close two objects are.

But then why – on the opposite side of Earth – is there another tidal bulge, in the direction opposite the moon? It seems counterintuitive, until you realize that this second bulge happens at the part of Earth where the moon’s gravity is pulling the least.

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Earth spins once every 24 hours. So a given location on Earth will pass “through” both bulges of water each day. Of course, the bulges don’t stay fixed in time. They move at the slow rate of about 13.1 degrees per day – the same rate as the monthly motion of the moon relative to the stars. Other factors, including the shape of coastlines, etc., also influence the time of the tides, which is why people who live near coastlines like to have a good tide almanac.

Bottom line: The sun, the moon, the shape of a beach and larger coastline, the angle of a seabed leading up to land, and the prevailing ocean currents and winds all affect the height of the tides. Expect higher-than-usual tides for a few days following the May 25, 2017 new supermoon.

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Deborah Byrd

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