By: Chief Meteorologist Eric FisherBy Eric Fisher

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Disclaimer: I feel like everything on the planet now has a name and is built up way too much. Case in point: Supermoons. I don’t think I ever even heard them in discussion until the past few years. Now it feels like every full moon is an event you need to buy a ticket to see! I’m on record with loving all things in the sky, so I’ve got no beef with the moon. Whether super or not, it’s a wondrous sight. No doubt that will be the case at the end of this weekend. And even a curmudgeon such as myself can appreciate that this one may in fact be at least a little bit special.

What exactly is a ‘supermoon’? According to an enlightening article by Sky and Telescope:

“The term “supermoon” was first coined by astrologer Richard Nolle to describe the full or new Moon when it’s less than 223,000 miles (359,000 km) from Earth. That’s about 6% closer than the average Earth-Moon distance, which is 239,000 miles.”

That’s a fairly loose definition, and it happens several times a year. And generally you can’t notice the difference with the naked eye. This time, you might have a shot. The moon will be full at the closest distance to earth since January 26, 1948 (the range of distance in its orbit goes from 225,623 miles at perigee to 252,088 miles at apogee). This will allow the moon to appear 14% larger and 30% brighter than a full moon that occurs at apogee, the farther point from earth in its orbit. And such a close pass during the full phase won’t happen again until the year 2034.

compare Closest Supermoon in 68 Years To Light Up The Skies And Bring King Tides

The best time to take advantage of this apparent size change is when the moon is on the horizon. There’s an optical illusion that takes place when you view it low in the sky, as your mind processes it as larger versus when it is higher up in the sky. It helps to have elements in the foreground, such as trees, buildings, lighthouses, etc. This changes the way your brain perceives the size, in comparison to when it’s high up in the openness of space. Plus, pretty pictures and Instagram posts.


So when can you expect it to appear? Technically, the full moon takes place Monday morning. But it will have already set here in eastern New England. So I’d say the best time to view will be Sunday night or just before dawn on Monday. Sunday’s moonrise is at 4:14pm in the Boston area, with the moonset taking place at 6:17am on Monday  morning. Lucky us, the forecast is for mainly clear skies! So the night should be bathed in bright beautiful moonlight.

There’s a bit of a trade-off that will occur thanks to this celestial light show. While the moon is great and all, there’s also a fantastic meteor shower on the calendar. The Leonids, typically one of the best shows in any given year, peak November 16-17th. But a big bright moon is going to drown out a lot of it for us, as it will be up all night. So even if we get clear skies, it will be difficult to see a good portion of the meteors streaking across the sky.

One more impact of such a close full moon is the tides, which anyone near the coast will certainly start to notice this weekend. ‘King Tides,’ aka perigean spring tides, will take place Monday through Thursday. These happen when things line up just right, typically a few times per year. The moon has to be at perigee, its closest point of approach to earth. Plus, the sun is also aligned with the moon. The added gravitational pull allows high tides to be higher and low tides to be lower. Typically, the difference is a few inches compared to a typical high tide.

 Perigean Spring Tide infographic.

Source: NOAA

How will next week stack up? Around Boston, the highest high tide will be Tuesday morning at 12.5′. For reference, this is a couple of inches higher than the tides we saw back in October which inundated some low-lying areas and zones typically prone to flooding. Compared to the highest tide in July, it’s nearly a foot higher. So there is a definitely a noticeable difference.

Minor flooding is a given. Any onshore flow can exacerbate the flooding during a King Tide, so we’re keeping an eye on the pattern next week. There are fairly good odds that a storm system is going to be nearby in the Tuesday-Wednesday time frame. Wherever there’s an onshore component to the wind, flooding will be more of an issue. Keep in mind that there may be some road closures (I’m looking at you, Morrissey Boulevard) and we’ll update the forecast as we get closer.

Cws36SCUcAABoC8 Closest Supermoon in 68 Years To Light Up The Skies And Bring King Tides

Source: National Weather Service in Taunton, MA

These extra high tides are also a good time to think about sea level rise. Even though they are only a few inches higher, everyone notices and ‘nuisance flooding’ pops up. The water levels in coastal Massachusetts have risen about 4″ since 1950. It doesn’t sound like much, but if a King Tide is a few inches higher than normal, it shows us what the typical high tide will look like by mid-century if the rise continues as is expected by many in climate circles. Almost a glimpse into the future, and a good time to think about adaptations that may be needed in the years to come.

Flooding in Gloucester due to King Tides. (Photo courtesy Donna Ardizzoni)

Flooding in Gloucester due to King Tides. (Photo courtesy Donna Ardizzoni)

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