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February 2006: Perigean Spring Tide


The Moon, our closest celestial neighbor and the Queen of the Night Sky, is often thought to influence mood swings in humans and all of Earth's creatures. The symbol of romance and its inherent fickleness, of waxing and waning fortune, and the manic nature of life, the Moon is seen by many to have a rhythmic yet discordant relationship with our planet Earth. What more aptly symbolizes the emotional ebb and flow than the swelling and falling ocean tides?
Our rotating Earth turns full circle under the Moon in 24 hours and 50 minutes, this being the length of the lunar day, which itself is somewhat variable. The tides don't repeat at the same hour, but some 50 minutes later daily. As the Earth turns, there are generally two high tides and two low tides (semi-diurnal tides) in this 24 hour and 50 minute period. But due to numerous complications, unequal high and low tides are not out of the ordinary -- some coastal areas in essence getting one high tide and one low tide in a lunar day (diurnal tides).
The Moon is hardly the only player in the enormously intricate drama. Though the Moon's tidal influence is more than double the Sun's, it's really the Moon's phase -- or its changing position relative to the Earth and Sun -- that determines the severity of the tides. The Moon may be Earth's paramour, but the Sun triangulates the relationship, this interplay between the Earth, Moon and Sun giving rise to two neaps tides and two spring tides every month.

Neap & Spring Tides

Twice a month -- at waxing quarter and waning quarter Moon -- neap tides bring an even keel to this sometimes tempestuous tidal triangle. At neap tide, the Moon and Sun are at a right angle to Earth, so the Sun and Moon neutralize one another's excesses to bring relative calm. High tides don't climb particularly high at neap tide and low tides don't fall too low. Such an alignment finds a minimal variation in ocean levels, a metaphor of peace and tranquility.
Twice a month -- at New Moon and Full Moon -- the tidal influences of the Moon and Sun combine to give rise to spring tides. At spring tide, the Moon and Sun line up with Earth, and the Sun and Moon accentuate one another's volatility. High tides climb exceptionally high whereas low tides plunge exceptionally low. Variation in ocean levels reaches an extreme. Spring tide, perhaps, is an allegory of double-edged potential, of opportunity fraught with danger.

Perigean Spring Tide

Spring tides accompany the February Full Moon near mid month, and the New Moon toward the month's end. Spring tides are far from being equal, however, with their intensities depending on the closeness of the Moon.
Because the Moon doesn't orbit the Earth in a perfect circle -- but rather an oblong ellipse -- the Moon snuggles up closest to the Earth for the month at perigee, and stands most distant at apogee. Even stable couples -- or perhaps stable couples in particular -- have their moments of intimacy and retreat.
Spring tide goes through the gamut in February. The Full Moon on February 12 (Eastern Standard Time) swings rather close to apogee, so the distant Moon tempers the buoyancy of February's first spring tide. Yet, the New Moon on February 27 (EST) nearly aligns with perigee, the exceptionally close Moon energizing February's second production.
In fact, at 356,884 kilometers* -- or 221,757 miles -- February's perigee is the closest of the year, ensuing a foray that's called a proxigean spring tide. At apogee on February 13 (Eastern Standard Time), the Moon stands at 406,278 kilometers* -- 252,500 miles -- away.
It may be interesting to note that 7 lunar months later, in September, there's another perigean and another apogean spring tide. The shoe is on the other foot this time around, though, with the September 7 Full Moon (not the September 22 New Moon) ushering in the second and final perigean spring tide of 2006.

Perigean Spring Tide Cycles

I learned about perigean spring tide cycles on page 186 of John Edwin Brown's book Sun, Moon and Standing Stones. 14 returns to New Moon equal 15 returns to perigee, this period being a bit more than 413 days. For me, it's easier to think of this time period as 1 year 1 month and 18 days. So New Moon and perigee next realign on April 17, 2007, whereas Full Moon and perigee coincide again on October 25, 2007 (Eastern Standard Time).
A lunar month refers to the time period between sucessive New Moons or Full Moons, a mean period of 29.53059 days. An anomalistic month refers to the period between successive perigees or apogees, a mean period of 27.55455 days. Hence,
14 x 29.53059 days = 413.428 days
15 x 27.55455 days = 413.318 days
Perigees and apogees recur on or near the same calendar dates every four years.

A Parting Word

I wish to drive home the point that all of the above concerns astronomical reasons for the tides. It is well to keep in mind that there can be a lag of perhaps a few days between the Moon's four major phases and the arrival of neap and spring tides. A whole host of factors influences the tides, some of which include wind and barometric pressure, topography and latitude.

copyright 2006 by Bruce McClure


* page 399 of Jean Meeus' Astronomical Tables Of The Sun, Moon and Planets

January 2006 Feature * March 2006 Feature


For Your Information. . .
Moon Phase Almanac
Moon's Perigee & Apogee Dates
Home of the World's Highest Tides -- Bay of Fundy, Canada
Ocean Tides explained -- includes Illustrations & World Map
Tidal Predictions -- National Ocean Service
Tidal Predictions -- University of South Carolina
Old Farmer's Almanac Tidal Glossary

Tide Predictions Calculator -- Old Farmer's Almanac

When a New Moon aligns with perigee, the next New Moon returns sooner than 29 days 12 hours and 44 minutes later, the mean period between successive New Moons.