Dear Editor,
I am puzzled by your report in SN of September 18, of Dr Luncheon’s noting that “more spring tides have been forecast for November, and as a result the overtopping might recur.” Isn’t he aware that there are two spring tides every month, including October? The November spring tides will not be as high as those in October.
The first spring tide for this month was very exceptional because a new moon and perigee (when the moon is nearest the earth) coincided when both the sun and moon were over our latitudes.
The second spring tide this month, occasioned by the full moon on September 23, will occur at apogee (when the moon is farthest from the earth). Therefore these tides will not be as high but they will last longer. The first spring tides of October should be higher, though not as high as what we experienced earlier this month.
I calculate that the next spring tide of comparable potential to the big one we had this month will be caused by the equinoctial full moon coinciding with perigee on March 19, 2011. The one we just had was a relatively local phenomenon, affecting the area only around our latitude and longitude and the corresponding antipode (on the other side of the earth). The big one in March next year will be more extensive; it will be experienced by equatorial latitudes around the entire earth.
There should then be a respite from these very exceptional spring tides for four years. They resume for March 19, 2015, September 27, 2015, and April 6, 2016. Thankfully, they fall in traditionally dry seasons.
The pattern of high spring tides recurs roughly every 18 years. A similar one to this year’s highest, occurred on September 7, 1975. There were three very high spring tides that year. The last one of comparable magnitude was around February 27, 2002.
Yours faithfully,
Alfred Bhulai
