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Quarter moon, Earth perihelion, neap tide

The last quarter moon combined with Earth at perihelion should usher in a gentle neap tide, with little variation between high and low tide over the next few days.

The moon reaches its last quarter phase and swings out to lunar apogee – its most distant point from Earth in its orbit – on January 2, 2016. In addition, the Earth sweeps to perihelion – its closest point to the sun in its orbit – on this same date: January 2, 2016. The combination of the last quarter moon at lunar apogee, and Earth at perihelion, should usher in a relatively even-keeled neap tide over the next few days.

Close and far moons in 2016

At neap tide, the variation between high tide and low tide is at a bare minimum. High tide doesn’t climb all that high and low tide doesn’t fall all that low. At quarter moon, the sun and moon make a 90o angle in Earth’s sky, so the tidal influence of the sun partially cancels out the tidal influence of the moon. Therefore, the range between high and low tides is quite subdued.

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 called neap tides. Image via physicalgeography.net

On the other hand, at new moon or full moon the tidal influence of the sun and moon combine to create wide-ranging spring tides. The high tide soars way up high whereas the low tide sinks way down low.

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 called spring tides. Image via physicalgeography.net

The moon’s tidal force is about two times greater than that of the sun. Yet, with the quarter moon closely aligning with apogee on January 2, 2016, the moon’s tide-generating force is less than average. Also, the sun comes closest to Earth for the year on January 2, 2016, so the sun’s tide-generating force is at its greatest at this time of year. Unless a storm surge throws a monkey-wrench into things, the ocean coastlines should display little change between high and low tide during the next few days.

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Two years from now – in early January 2018 – the full moon will closely align with lunar perigee – the moon’s closest point to Earth in its orbit. And, as always, Earth will be at perihelion in early January. At that time, we should expect a wide-ranging perigean spring tide, as the extra-close full moon teams up with the extra-close sun to produce a maximum variation in high and low tide in early January 2018.

In the meantime, enjoy the relatively level waters that accompany the early January 2016 neap tide!

Bruce McClure

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