Regular readers of this blog might recall that I listed the names and timings of the full Moons for this year in my post for December 2023 and that September’s full Moon will be the Harvest Moon because it is the closest one to the autumnal equinox. 
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All full moons rise around sunset and generally speaking, they rise about 50 minutes later each day. However, when the Moon rises close to the equinox, this reduces to about 20-25 minutes on the days before and after it is full. The resulting combination of twilight and moonlight means that there is more time when it is light enough to allow farmers to harvest their crops. Hence it being named the Harvest Moon.
 
In addition, September’s full Moon is the second of 2024’s four Supermoons with the others being the aforementioned Sturgeon Supermoon in August, then the Hunter’s Supermoon in October and finally November’s Beaver Supermoon.
 
For those who have an interest in the sea and tides, Supermoons are notable because they bring especially high spring tides that some people call king tides.
 
So, what is a supermoon and why does it create an extra-large spring tide, or king tide?
 
Well, the Moon doesn’t orbit around the Earth in a perfect circle. Instead, it has an elliptical orbit, where its distance from the Earth varies throughout the month, with the closest distance on this ellipse in any given month being called the perigee. When any full Moon comes within 90% of the perigee, or within 224,791 miles (or less) of our planet, it is called a supermoon. On these occasions it will look bigger and brighter than normal and will also exert a greater gravitational pull on the Earth than normal. This greater gravitational pull means that, just as a full moon gives us spring tides, a supermoon will give us extra-high spring tides, or king tides.
 
Given that September’s full Moon will pass by Earth at a distance of 222,131 miles, making it the second closest of the four supermoon’s, we can certainly expect these extra-high king tides. However, another thing to consider is that the full Moon is on 18 September, which is just four days before the Autumnal Equinox, so the tides will be higher than normal.
 
This is because the Sun is directly above the equator at the equinox, meaning that the Earth’s axis of rotation is not tilted towards or away from it and that it’s gravitational pull is evenly distributed across the planet. This enhances the Sun’s gravitational effect on Earth’s tides causing it to pull more directly on the Earth's equatorial regions, where the water bulges out more significantly and this gives us "equinoctial spring tides", where the tidal range—the difference between high and low tide—is at its greatest. For this reason, some people refer to these equinoctial spring tides as “great tides”.

​In Loch Sunart, the tidal range is typically just over 4 metres at most, but in September, the combination of a Supermoon and an equinox will result in a maximum tidal range of 5.3 metres. As this “Great King Tide” will give us 0.0-0.2 metres at low tide, I’m looking forward to exploring some stretches of the shoreline with the camera that are not usually accessible. Also, a high tide of 5.1-5.3 metres will occur around sunset, hopefully giving me some good opportunities to capture some colourful reflections in the submerged salt marshes that are likely to be found in certain places.
 
So, although August’s Moon photography plan didn’t work out, fingers crossed September’s will.

​This image above was taken a couple of days after the full moon of January 2018 and shows Eilean Tioram still cut off from the mainland despite the time being a few hours since the last high tide. In fact, the high tide on that day, at over 5 metres high, was good quarter of a metre higher than what is typical for a spring tide on Loch Moidart and because this extra-high tide was only a few days after a supermoon, it was what some people call a king tide.
 
So, what is a supermoon and why does it create an extra-large spring tide, or king tide?
 
Well, the Moon doesn’t orbit around the Earth in a perfect circle. Instead, it has an elliptical orbit, where its centre-to-centre distance from the Earth varies throughout the month, with its closest point to Earth on this ellipse in any given month being called the perigee. When any full Moon or new Moon comes within 90% of the perigee, or within 224,791 miles (or less) of our planet, it is called a supermoon. On these occasions a full Moon will look bigger and brighter than normal and both a full and new Moon will exert a greater gravitational pull on the Earth than normal. This greater gravitational pull means that, just as a full moon gives us spring tides, a supermoon will give us extra-high spring tides, or king tides.
 
Most of the time these king tides don’t cause too much of a problem, but shortly before this photo was taken in 2018, the supermoon of January 1-2 was closely aligned with the perigee and created especially high king tides. On the day after, Storm Eleanor hit the UK with winds of up to 100 mph (160 km/h). These high winds, combined with the extra-high tides, caused coastal flooding, hampered travel, led to loss of life and injury and left tens of thousands of homes across the UK without power.
 
Should we expect this to happen again? Well perhaps not for the remainder of this year and the whole of next year because the predicted heights of the king tides during the UK’s storm season months (typically October to February) is lower than that of January 2018. However, in 2023 and 2024, the predicted height of the king tides are at or above those of January 2018 which, if combined with the high winds of a named storm, could lead to coastal flooding of the same level as that experienced during Storm Eleanor.