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.
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.
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.
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.
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 has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.