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Tides, and the pull of the moon and sun

Expect higher-than-usual tides in the days following the January 31, 2018, full 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 and sun are 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 also depends on the shape of your specific beach, the angle of the seabed leading up to your beach, plus your larger coastline and the prevailing ocean currents and winds. Following the January 31, 2018, supermoon, you can expect higher-than-usual tides. Click the links below to learn more.

Some background. What are spring tides?

Why does a supermoon cause more extreme tides?

What part does the sun play, in early 2018?

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.

Some background. 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 always happen – every month – around full and new moon.

First full moon of 2018 – and closest supermoon of 2018 – was January 1-2. Here it is – at 99.9% illumination – as captured from Karachi, Pakistan, by Talha Zia.

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 2018, the January 1-2 full moon closely aligned with perigee to bring forth especially high tides. As it happened, on the day after the January 1-2 supermoon, Storm Eleanor hit Europe with winds of up to 100 mph (160 km/h). The wind and extra-high tides caused flooding, hampered travel, injured and killed people, left tens of thousands of homes without power across the UK, Ireland and other parts of Europe. No doubt the extra-high tides contributed to the severity of the storm. Read more: High tides and winter storms

Why are the tides at their strongest around supermoons? 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? Probably not. The highest tides tend to follow the supermoon by a day or two.

Do the most extreme high tides – high 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: 2018’s largest supermoon on January 1-2

Gary Peltz in Seattle, Washington, caught these beautiful sunset reflections and the nearly full moon on December 31, 2017.

What part does the sun play, in early 2018? Not only the moon, but also the sun plays a role in Earth’s tides. You might see that – when Earth is closest to the sun, as it is every early January – the pull on Earth’s tides by the sun is strongest. We reach Earth’s closest point to the sun for 2018 on January 3 at 5:35 UTC; translate to your time zone. Astronomers call this special point in our orbit perihelion, from the Greek roots peri meaning near and helios meaning sun.

The closer-than-usual sun and closer-than-usual full moon will almost surely increase the high of high tides in the first few days of January, 2018.

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 on 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.

Looking for a tide almanac? EarthSky recommends

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 January 31, 2018, full supermoon.

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Deborah Byrd

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