Perigee: Supermoon and Supertides?

Hey everybody! There is a supermoon and lunar eclipse happening THIS weekend! On Sunday, September 27th (for those in the Eastern Hemisphere it’s in the wee hours of the morning on September 28th) just as the sun starts to set there will be a lunar eclipse of the largest full moon of the year! (NASA has more details)

shutterstock_171118934This super-sized full moon is caused by the proximity of the moon to Earth – the closer the moon the bigger it looks. Since the moon’s orbit around our planet is not a perfect circle, there is a point when the moon comes closer to the Earth – perigee is what we call that closest point.


Not only do we have a perigee full moon – or supermoon – there is also a lunar eclipse. A lunar eclipse is when the moon travels through the Earth’s shadow, which blocks the sun and causes the moon to glow red. A full lunar eclipse is often called a “blood moon”.

Lunar_eclipse_sideview_v01Separately, these events happen a few times a year, but put ’em together and what have you got….. (some Disney fanatics might say bibbidi-bobbidy-boo) but I’m gonna go with super blood-red full moon! Aka an event that last happened in 1982 and won’t happen again until 2033.

So why the question of Supertides?

Ocean tides are driven by the pull of the moon and the sun. Just like the Earth’s gravity pulls at the moon, keeping it in orbit, the moon’s gravity pulls on the Earth. This pull ‘stretches’ our planet, but since the Earth itself is pretty rigid the oceans do most of this stretching. Water is pulled to the side of the planet closest to the moon, causing a bulge that we see as high tide. As the moon orbits the Earth, the bulge of water follows and in it’s wake is low tide.

94579-004-2DF826ECInterestingly though, physics causes a bulge of water on the opposite side of the Earth as well, opposite the moon. So there are two bulges of water opposite each other that follow the moon’s orbit around the Earth. Encyclopedia Britannica provides some more technical details.

The sun’s gravity has a similar effect on tides, but since it is farther away the ‘pull’ is lessened. But, when the sun’s pull and the moon’s pull align (like on a full moon) it can cause higher high tides or spring tides. The lowest low tides, when the sun and moon’s pulls interfere, are called neap tides.

So with the moon being so close to the Earth as well as aligned with the sun should we expect a supertide? Well… sort of. It turns out that the high tide on September 29th will be one of the top 10 highest high tides in the UK, but only by a meter or so. These ‘supertides’ could cause flooding, but weather and resulting storm surge has a higher impact on the coast.

The lunar eclipse does not effect the tides so don’t worry about that bit and scout out a spot to see the super blood-red full moon!!

Here’s the Supermoon Lunar Eclipse Video from NASA’s website.



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