Weather Watch

How the 2017 supermoon and Earth’s rotation combine to create a King Tide

View of a full moon from near Harmony.
View of a full moon from near Harmony. Special to The Tribune

The only supermoon of 2017 will occur Sunday night. It will rise in the evening at 5:25 p.m. and set Monday at 6:43 a.m. The gale-force Santa Lucia (offshore) winds should produce mostly clear skies for spectacular viewing.

When this full, cold moon is directly overhead, it will reach perigee (the closest point in its orbit to the earth) at 12:45 a.m. Monday and will appear approximately 14 percent larger and 30 percent brighter than usual. Correspondingly, the highest tide (King Tide) of the year will happen at 9:30 a.m. Monday when the tide will reach 6.8 feet above the Mean Higher High Water (MHHW) level.

So why doesn’t the high tide occur around midnight Sunday when the moon is directly overhead or over our meridian, but happens about nine hours later? Here’s why.

The Central Coast usually experiences two high and low tides per day, but due to the tidal cycle being over 24 hours, some days have only one high or low tide. These “tidal forces” are not the total gravitational forces exerted by the sun and moon on Earth, but the difference between these gravitational forces over the surface of the planet. The bottom topography or underwater bathymetry of our coastline also plays an essential part in the changing tides.

This tugging produces a tidal “bulge,” or area of higher sea level on the ocean’s surface. As Earth rotates eastward on its axis, the Central Coast moves into this bulge, which produces a flood tide and eventually a high tide. As Earth continues to spin, we move into an area of below-normal sea level, or nodes, which produces an ebb tide, eventually reaching low tide. The slack tide is when the tide is not coming in or going out.

It may seem counter intuitive, but the gravitational forces of the moon and sun tug at the oceans primarily on the horizontal tangent and not the vertical plane to the Earth’s surface. This is partly why the high tide lags between eight and nine hours on the Central Coast — also referred to as the lunitidal interval. This value can be used to program specific tide watches that you may have received during the holidays.

During a full and new moon, the sun and moon are aligned, producing strong tidal forces on the Earth; this is also referred to as spring tide, even though it doesn’t have anything to do with the spring season. These are the highest and lowest tides of the month. During the moon’s quarter phases, the gravitational forces of the moon and the sun are perpendicular to each other. Nearly canceling each other’s tidal forces out, the result is smaller high and low tides. This is referred to as neap tide.

Dr. Ray Weymann , retired Director and Chair of the Astronomy Department at the University of Arizona, told me that Earth’s orbit around the sun is an ellipse, a shape that can be thought of as a “stretched out” circle or oval. The moon’s orbit has moderate (and variable) eccentricity, and this also has an impact on tides. The eccentricity of the orbit is changing almost on a daily basis due to complex variations in gravitational effects, which can alone change the tides.

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John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.

The year ends with a trio of full moons at their closest points to Earth.

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