Tuesday, October 30, 2012

Spring Tide, Neap Tide

Almost as if the media had just discovered that there are such a things as tides, every third report on Hurricane Sandy yesterday had some person knee-deep in water reporting that things are a lot worse because it is now Full Moon. So how much of a difference does that make?

Well, first of all, the difference would be the same even if Sandy had arrived at a period of New Moon. In both cases, Sun-earth-moon, or Sun-moon-earth are aligned. This means that the gravitational pull of both the moon and the sun are acting on the ocean directly from the same angle. By night the moon, by day the sun, pulls the water toward itself, and the tides are then higher, the ebbs lower. This is known as “spring tide” because the waters sort of leap higher in the direction of these bodies relative to the earth.

Conversely, at neap tides, when the moon is in its quarter phases and at 90° to the sun as seen from the earth, the magnetic pull of the sun and moon are opposed, hence the tides are lower. But by how much? Enough to justify the media in underlining that difference in its reportage?

Well, Rehoboth Beach, Delaware (once quite near where we used to spend weekends by the seashore when we lived in Virginia) was much in the news yesterday. So, with the help of the Old Farmers Almanac (link), I looked up the higher of two daily tides for October 22 (first quarter, a neap tide day) and October 30 (full moon, spring tide day):

The Higher of Two Daily Tides at Rehoboth Beach, DE (in feet)
Moon Phase
October 22, 2012
First Quarter
October 30, 2012
Neap and spring tide difference this month:

Well, there is a difference of 6.24 inches. In my case that reaches to just above my ankles. But the fullness of the moon is a rather indirect way of signaling a rise in the sea level by six inches for about a minute or so.

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