Seems like you hear more about “supermoons” than you used to. So, exactly what are they, and just what makes them so “super”?
Well, a supermoon used to be called a perigean full moon. Perigee is the point in the moon’s monthly orbit around the Earth when it is closest to Earth. So a supermoon (perigean full moon) is said to occur when perigee coincides with the moon’s full phase. Last week, this occurred on April 7, and the earth-moon distance was about 222,000 miles. Next week, on April 20, at its farthest point, called apogee, it will be about 30,000 miles farther away, around 252,000 miles.
By the way, a lot of people are surprised to learn that the International Space Station is “only” 250 miles above the Earth. So, roughly, the space station is only one-one thousandth of the way to the moon. I’ll bet that will surprise a lot of folks.
Speaking of the Space Station, Chris Cassidy of York rocketed to the Space Station last Thursday, joining the crew which includes Caribou native, Dr. Jessica Meir. Dr. Meir is scheduled to return to Earth on April 17.
Back to the moon now. As seen in the accompanying photo, there is indeed a difference in size, as viewed from Earth, when comparing a full moon at perigee (closest) and a full moon at apogee (farthest). These two shots were taken with the same equipment and at the same lunar elevation (moon at the same height in the sky in each shot). While the supermoon is noticeably larger than the full moon at apogee, the casual observer may have a hard time seeing the size difference, though it is noticeable at moonrise and moonset.
The perigean full moon is about 14 percent larger to an observer on Earth, and appears about 30 percent brighter under clear skies in areas away from ambient light than the full moon at apogee.
Another aspect of a supermoon is the tidal impact. Sometimes tides during a perigean full moon are referred to as “spring tides,” not because they occur in spring, but rather because they seem to “spring up,” meaning the difference between high and low tide is greatest at this time. The opposite of a spring tide is a neap tide. A neap tide has a much smaller range between high and low tide than a spring tide does.
As an example of the influence of the perigean full moon last week, the tidal range in Eastport was about 25 vertical feet. So imagine, there you are at low tide, tinkering away on a project, and six hours later, the water level is not 25 feet farther up the shore, but rather 25 feet higher (two basketball hoops, plus 5 more feet). The lowest tidal range at Eastport this month is about 14 feet. So on supermoon day, the high tide was 11 vertical feet higher. That’s a lot. The take-away is that you don’t want a coastal storm during a perigean full moon.
When there is a coastal flood threat, the timing of the strongest onshore winds, relative to high or low tide is of paramount importance. It can be the difference between minor and major damage to coastal structures.
Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Virginia native, he has been chief meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.
