Supermoon

[HenryC]

No doubt you have heard the term "supermoon" on the news lately.  Miami and Miami beach have been getting a lot of these lately when above-average tides, coupled with high water from rising sea levels due to global warming and offshore winds, cause flooding. Needless to say, this is not just a nuisance, the damage and inconvenience in a seaside community is severe, and the mitigation costs can be enormous.  If king tides coincide with a tropical storm the flood impact could be disastrous.

The supermoon is associated with "astronomical tides" or  "king tides".  The correct nautical term is a "spring tide". These are extreme tides that we get when the sun, earth and moon are lined up.  Incidentally, a tide lower than average is called a "neap tide".  A ship is said to be "neaped" when a spring tide allows it to enter a shallow harbor and it must then wait at least a half month for the next spring tide to allow it to float free.  There's an old story about how this happened to Columbus in Jamaica, and only his knowledge of the tides and eclipses (from his Almanac) allowed him to intimidate hostile natives and save his crew until he could float his ship.

The moon circles earth in an elliptical, not circular, orbit and when it is extremely close AND lined up with the sun, the tide can be quite extreme.  This effect is quite noticeable with a sextant, which can actually be used to measure the size of the lunar disk.   The moon is roughly a half-degree (30 minutes of arc) in diameter, but it can appear larger or smaller depending on how far away from earth it is. 

Using the moon in celestial navigation requires that the apparent size of the orb be taken into account.   The moon's central position is given in the Nautical Almanac, but with the sextant you can only measure where the edge of the moon appears to touch the horizon, so the lunar semidiameter (or radius) must be either added or subtracted to the sextant reading to derive its central value.  Consequently, both the lunar and solar semidiameters are published in the Almanac for every day of the year.

For example, tomorrow, 14 Nov, the lunar semidiameter is listed as 16.8'.  Two weeks ago, when the moon was on the opposite side of its orbit, the Almanac gave a value of 14.7' of arc.  This is quite a difference (it translates to over two nautical miles!).  This is not noticeable because the moon at apogee and perigee are not side by side and cannot be directly compared, but it is easily measurable in a sextant and must be corrected for.

Normally this is no big deal, but if you live in an area where the tidal variation is critical to navigation, you need to take this into account.  The Fed publishes Tide and Current Tables which can be used to make sure you have enough water under your keel to get back to your anchorage, as well as the speed, set and times of the currents.  You can't rely on the TV or newspaper because in a large harbor the water takes time to slosh back and forth.  The tide tables give you the times and depths of the tides and corrections you can apply to different parts of the harbor.  In areas of extreme tidal variation and killer tidal currents (like San Francisco Bay) this can be critical, even life-saving  info.  SF Bay often has 6 foot tidal ranges and 6 knot currents, and it takes time for the various basins to fill up and empty through narrow channels and obstructions.  You can't sail against forces like that, especially if you're dealing with a 20 knot blow and an eight foot chop.  This is by no means unusual for the N Bay in summer. Even in a gentler environment, spending a few hours aground is a nuisance, but if you're seriously neaped, you could miss a couple of days of work, or worse, a couple of weeks waiting for the next spring tide to float your boat.

You have been warned...

[Bob23]

Thank you Henry. I always enjoy (and read in full) your most informative posts. I was hoping to combine Koinoina's last sail of the season with watching the sunset and supermoon rise tonight but the forecast up here (NJ) is for cloud cover.
Bob23   Clouds happen.

[Tim Gardner]

I was just out taking the garbage cans to the end of the driveway.  Yesterday was cloudy , today is clear.. Huge, just past full moon,, nearly as bright as day. Cool.
TG

[Bob23]

Perfect moon this morning...good walking in the dark accompaniment.

[kickingbug1]

    the coyotes loved it