UPDATE OCTOBER 29, 2012 2:30 p.m. EDT (18:30 UTC) The center of Hurricane Sandy is expected to make landfall this evening along or just south of the southern New Jersey coast around 7-9 p.m. EDT (23 UTC – 1 UTC on October 30). The storm is speeding up, so it could make landfall as early as 5 or 6 p.m. EDT. High tide is going to occur at nearly the same time as landfall. Sandy is striking the U.S. on the same day as this month’s full moon. Already people are calling Hurricane Sandy a Frankenstorm, in part because it’s happening around Halloween, while costumed partiers are moving about their cities, and partly because it is a monster storm. What’s more, high and low tides are at their most extreme at full moon time, which will no doubt exacerbate any flooding or storm surges that do occur from Sandy. I can’t explain Sandy to you (I’ll leave that to my colleague Matt Daniel), but I did want to take a bit of time to talk about the moon and the tides as we wait to see how Sandy affects millions of people in the U.S. East and Great Lakes region, with a sure impact in major cities including Boston, New York, Washington, Baltimore and Philadelphia.
Of course, we all know the moon is primarily responsible for the rising and falling of ocean tides. In most places, but not everywhere, there are two high tides and two low tides a day. That’s true of the U.S. East Coast, where Sandy is having its effect. That effect can’t be predicted precisely because – for any particular spot on Earth’s surface – the height of the tides and their fluctuation in time depends not only on the moon, but also on the sun – and also on the shape of the specific beach, the larger coastline, the angle of the seabed leading up to land, and the prevailing ocean currents and winds.
The difference in height between high and low waters varies as the moon waxes and wanes from new to full and back to new again. The higher tides are called spring tides (nothing to do with season of spring). The lower tides are called neap tides. It’s a spring tides that’s happening around this weekend, as Sandy is sweeping closer to U.S. shores.
Spring tides. Full moon this month comes on October 29 at 19:49 Universal Time (or 2:49 p.m. Central Daylight Time in North America). Around each new moon and full moon, the sun, Earth, and moon arrange themselves more or less along a line in space. Then the moon’s pull on the tides increases, because the gravity of the sun reinforces the moon’s gravity. In fact, the height of the average solar tide is about 50% the average lunar tide. Thus, at new moon or full moon, the tide’s range is at its maximum. This is the spring tide: the highest (and lowest) tide. Spring tides are not named for the season. This is spring in the sense of jump, burst forth, rise. So spring tides bring the most extreme high and low tides every month, and they happen around full and new moon.
It’s when a spring tide coincides with a time of heavy winds and rain – flooding due to a weather extreme – that the most extreme flooding occurs. That’s the case this weekend. There is one small bit of luck. That is, the moon is not near perigee, or closest to Earth for the month. In fact, the moon will be at apogee- farthest from Earth for the month – on November 1. A full moon at perigee during a destructive hurricane would be bad news indeed. A full moon at apogee is slightly less bad, but still bad.
Neap tides. Too bad Sandy didn’t strike at a neap tide. These sorts of tides occur halfway between each new and full moon – at the first quarter and last quarter moon phase – when the sun and moon are at right angles as seen from Earth. Then the sun’s gravity is working against the gravity of the moon, as the moon pulls on the sea. This is the neap tide: the tide’s range is at its minimum. Neap tides happen approximately twice a month, once around first quarter moon and once around last quarter moon. There’s about a seven-day interval between spring tides and neap tides.
Two high tides and two low tides each day. If the moon is primarily responsible for the tides, why are there two high tides and two low tides each day in most places, for example, the U.S. eastern seaboard? It seems as if there should just be one. If you picture the part of Earth closest to the moon, it’s easy to see that the ocean is drawn toward the moon. That’s because gravity depends in part on how close two objects are. But then why – on the opposite side of Earth – is there another tidal bulge, in the direction opposite the moon? It seems counterintuitive, until you realize that this second bulge happens at the part of Earth where the moon’s gravity is pulling the least.
Earth spins once every 24 hours. So a given location on Earth will pass “through” both bulges of water each day. Of course, the bulges don’t stay fixed in time. They move at the slow rate of about 13.1 degrees per day – the same rate as the monthly motion of the moon relative to the stars. Other factors, including the shape of coastlines, etc., also influence the time of the tides, which is why people who live near coastlines like to have a good tide almanac.
Bottom line: As Hurricane Sandy barrels up the U.S. East Coast this weekend, with a strike to the U.S. mainland expected Monday night, October 28, you might hear that its effects will be exacerbated by the fact that the moon is full. This post explains a few details about that awful coincidence and gives you a short lesson on the tides.