Last night’s nearly full moon finds a break in the clouds as it shines over Lake Superior. The small ring of color centered the moon is called a corona. Photo: Bob King

Yikes! Normally I’d be thrilled to announce that tonight’s the Hunter’s Moon, the full moon named after the hunting season that traditionally begins after the harvest. While the moon will rise around sunset and shine the night long in Aries the Ram, it offers little solace to the people living along the east coast of the U.S. Never mind that the moon will be hidden beneath rolls of clouds spun out by Hurricane Sandy. The real concern is tides. Full and new moon are when tides are highest.

Hurricane Sandy bears down on the U.S. Atlantic coast in this satellite photo taken yesterday. Credit: NOAA/NASA

Tides are a normal occurrence along the Eastern Seaboard but those from the full moon are 20 percent higher than normal tides. That wouldn’t necessarily be a bad thing except for timing. The combined influence of the moon and sun today and tomorrow will magnify the hurricane’s expected storm surge adding insult to injury. Water is expected to rise 4-8 feet along the New Jersey to Connecticut coasts leading to massive flooding and beach erosion.

“A gravitational lag usually causes the highest tides to come a day or two after every full moon,” says Joe Rao, a meteorologist with News 12 TV (Long Island) and astronomy enthusiast. That could mean Tuesday might be the worse day.

The moon’s gravitational pull creates tidal bulges of water on both sides of the Earth. Credit: Mike Guidry, University of Tenn. (csep10.phys.utk.edu/astr161/)

Tides have been a fact of life ever since the moon was fashioned from the debris created when a Mars-sized asteroid sideswiped Earth some 4 billion years ago. The gravitational pull of the moon and sun cause the tides. In spite of its much smaller size, the moon’s the stronger puller because it’s so much closer than the sun.

The moon’s gravity causes the oceans to bulge in its direction. A second bulge is raised on the opposite side of the globe because the moon pulls the Earth toward it and away from the water on the farside. Since the planet is rotating while this is happening, there are two high tides and two low tides a day. While the physics of tides is more complicated than I’ve described that gives us a basic understanding.

The strongest high tides happen at new and full moon when the sun, Earth and moon are all in a line. The weakest tides occur at the quarter moons. Credit: Mike  Guidry, University of Tenn. (csep10.phys.utk.edu/astr161/)

When the moon and sun line up on either or the same side of Earth, their combined gravity  create high or spring tides (‘spring’ here derives from the German verb ‘springen’ - to jump). Either lineup has approximately the same strength.

The lowest high tides, called neap tides, occur during quarter moons when the sun and moon are at right angles to one another. The sun’s gravitational force partially cancels out the moon’s and the difference between high and low tide reaches a minimum.

Hurricane Sandy photographed by moonlight early Sunday morning. Lights from cities near and along the East Coast are visible. Credit: NASA/Suomi NPP

So twice a month – at full and new moon – the sun re-enforces the moon’s pull to create spring tides, and twice a month at the quarters, the sun thwarts the moon’s pull and we get neap tides.

The highest of the high tides – called perigean spring tides – happen when the new or full moon is at perigee or closest to Earth. Its gravitational pull is felt most strongly at those times. We can be grateful that the Hunter’s Moon is only a few days from apogee, its most distant point from Earth, otherwise the news for the East Coast would be even worse. Let’s hope everyone gets through this big storm safely.