A Great King Tide

I found myself writing this on the day of the August’s full Moon, named a Sturgeon Moon because August is associated with the abundance of sturgeon fish in the rivers of North America. It was also the third of four full moons in a single season, making it a Blue Moon and because it passed our planet by less than 224,791 miles, it was also a Supermoon. I had great plans to photograph this Sturgeon Blue Supermoon, but it was not to be. The day wasn’t blessed with clear skies, but instead we endured rain and thick low cloud. Forever the optimist, I began making plans for photographing September’s full Moon after remembering this image of last year’s one.

Harvest Moon Reflections - Ardtoe Pier, Kentra Bay, Ardnamurchan

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”.

Sunset Mirror II - Sàilean nan Cuileag, Salen, Ardnamurchan

​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.