Saturday’s ‘supermoon’ will cause exceptional high tides

‘Supermoon’ seen from Canary Wharf, 2014 (credit: Matt Buck)

‘Supermoon’ is a term widely used to refer to when either a full or new moon coincides with the closest approach the Moon has to Earth in its elliptical orbit. It is not an astronomical term (where the correct phrase is the Scrabble-winning “perigee-syzygy”) but this astronomical coincidence results in a larger apparent size of the Moon as seen from Earth as well as higher than average tides. The solar eclipse of 2015, observed on Friday 20 March, occurred one day after a ‘supermoon’, and because of ocean inertia the largest tides will be experienced on Saturday 21 March.

Kevin Horsburgh, from the National Oceanography Centre (NOC) said "Obviously, the eclipse and the tide are linked. For an eclipse to take place, the sun, the Earth and the moon need to be in a straight line, which is also an essential condition for high tides”.

On Friday, the moon’s shadow will cross on Earth’s surface at 0741 GMT in the eastern central Atlantic, according to Britain’s Nautical Almanac Office. By 0913 GMT, seen from a point about 700 kilometres south of Greenland, the sun's face will be completely obscured. This "path of totality" will follow a 5,800-kilometre curve across the farther north Atlantic, into the Arctic Ocean. It will cross land in the Faroe Islands, a Danish archipelago halfway between Iceland and Norway, and the Norwegian island group of Svalbard.

The celestial phenomenon on Saturday will result in major tides, most perceptible in Canada’s Bay of Fundy, on the French Atlantic coast, in the Channel and the North Sea. But these will not be the biggest tides of 2015 – for most places around the UK those will occur around 29 or 30 September. Details of the highest tides each year can be found on the website of the National Tidal and Sea Level Facility (NTSLF) see: www.ntslf.org/tides/predictions.

Tide generating forces are also enhanced when the Sun and the moon are directly overhead at the equator. For the Sun this happens on or around 21 March or September (the equinoxes) which is why tides around the UK are always higher at these times of year.

But it is important to remember that stormy weather has a greater impact than year-to-year variability in tides. Storm surges, due to low pressure and high winds, can raise sea levels by up to four metres around the UK coastline as seen most recently during the stormy winter of 2013–14.