By Jenny Neyman
Credence Clearwater Revival’s John Fogerty might talk about a bad moon rising last week. But luckily for Kenai, though a particularly close new moon caused extra-high tidal ranges, there were no hurricanes ablowin’ at the time, thus avoiding any rivers overlowin’, much less any rage and ruin to the unstable bluffs along the river mouth.
Cook Inlet was awash in its highest tides and widest tidal ranges of the year last week. Anchorage on Wednesday saw a high tide of 33.1 feet at 7:48 p.m., followed by a low tide of minus 5.2 feet at 2:55 a.m. Thursday, for a tidal range of 38.3 feet. At Homer, a high tide of 22.3 feet at 2:55 p.m. Wednesday was followed by a low of minus 4.9 at 9:16 p.m., for a range of 27.2 feet. And at the mouth of the Kenai River, a high tide of 24.9 feet at 4:42 p.m. Wednesday was followed by a low of minus 4.5 at 11:31 p.m., for a rage of 29.4 feet.
The high highs and low lows were created by a conjunction of factors having to do with the position of the moon. A little Earth science 101 refresher, here — tides are essentially long-period waves rolling around the planet as the ocean is affected by the gravitational pull of the moon and sun. Twice each month, tidal ranges are a little larger than average as the Earth, sun and moon are nearly in alignment. As seen from Earth, that’s either a full moon — when Earth is between the moon and the sun — or a new moon — when the moon is between Earth and the sun. In both cases the gravitational pull from the sun and moon combine and tug a little harder at Earth’s oceans, making high tides a littler higher, and low tides a little lower.
These are confusingly named “spring tides,” which have nothing to do with the season of spring, but come from the idea of “springing forth.” Spring tides happen twice every 28-day lunar month, all year-round.
In addition to spring tides, the moon also exerts a little extra pull on our oceans when it is nearest to the Earth. That’s called perigee, or a “super moon,” and it also results in slightly higher tidal ranges. Three or four times a year, a new or full moon will coincide with a super moon. This month’s new moon was Jan. 20, and the lunar (pear-eh-gee) happened about a day and seven hours later, on Jan. 21.
All together, that’s called a perigean spring tide, and that combination of factors causes even higher tidal ranges than either factor alone.
That’s what happened last week. Along most coasts around the globe, the effect to tidal ranges is only a couple of inches. But since Cook Inlet already sees some of the highest tidal ranges on the planet, the effect of a perigean spring tide can be 6 inches or more beyond average.