Next message: Robert Eno: "Re: Moon lining up for big tides on June 22?"
Jim, you wrote:
"I hear from an astronomer friend of mine that the moon is lining up around
summer solstice to produce unusually big tides June 22 this year. Can
anyone shed light on that?"
It's full moon, near perigee, and the declination of the Moon and Sun are
both high (Sun at 23N, Moon at 28S). So that adds up to a maximal set of
astronomical factors. The tidal range at Cape Hatteras will be over four feet on
June 22 compared with a range of about two feet on June 15 (the preceding neap
tide).
These things often get exaggerated. The main thing here is perigee spring
tides. But they occur often. Compared to other dates with perigee spring tides,
the tides on June 22 will have an additional inch or so of predicted range
at Hatteras (out of four feet). The natural daily variation from
meteorological factors such as low barometric pressure is larger than this additional inch
from the astronomical factors so it's not really an observable difference.
In areas with diurnal or mixed tides, the difference may be more noticeable.
If you want to run some comparisons for different dates, I've written a
small tide calculator web page (I've mentioned it before, I think). It's designed
small so that it fits onto my cell phone's display. You can calculate the
tides for a set of locations that happen to interest me personally. If there's
interest I can add others. You can access it at http://fer1.com. I mention my
calculator primarily because I can answer questions about how it calculates
things (having written it and all...). There is also the widely-available
XTide program (http://www.flaterco.com/xtide/) which makes larger graphs and
accesses a large set of tidal harmonics. It has the small disadvantage of using
old pre-calculated tables for the astronomical factors which limits its time
range.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
