Great Tidal Bores
The tides are what make our shoreline endlessly fascinating. The main factor driving this is the moon. Our cycle of tides happen because the moon's gravity tugs at the ocean creating a bulge of water that's pulled away from the Earth towards the moon. But, that's not the whole story. It's the spin of the Earth that's responsible for this daily cycle of tides. As the Earth rotates underneath the moon, It also rotates under this tidal bulge. And when that water hits land, it creates our tides. In Britain, we have some pretty big tides, but to see the largest tides of all, you need to cross the Atlantic.
It's now September the 29th, We've come to the eastern coast of Canada, the best place - and today is the best day - to witness one of nature's great events. This is the Bay of Fundy, a massive stretch of water, at high tide. But, at low tide all the water has gone. Leaving just enough time to reach those islands before it returns.
The Bay of Fundy is famous for having the greatest tidal range anywhere in the world. And, today it's due to be the biggest tide of 2011.It's a 3 mile walk from the high tide mark to the sea at low tide.
At it's peak the water advances at 10 metres/minute. As the tide turns, 115 billion tonnes of water flow into the bay. The Bay of Fundy has the highest tides in the world because of it's shape. The water is funnelled up a channel that gradually narrows. The reason why the tides are at their biggest at this time of year is down to the interaction between the Earth's spin and a very particular orbital alignment. The highest tides happen when the gravity of the moon and the sun work together.
Bay of Fundy
But, around the equinoxes, something special happens. At this time of year, both the sun and the moon are tracking along the equator. And, as they pass over the centre of our planet, we spin right through the middle of a mammoth tidal bulge, giving us the biggest tides of the year.
Geographical peculiarities cause different tidal phenomena in different parts of the world at this time of the year. Where big tides pour up river valleys they often result in a tidal bore.
In Britain, the Severn bore is one example. Even bigger is the Amazon tidal bore. But, biggest of all is in southeast China. where crowds gather, often at great personal risk. To see the bore arrive.
Riding the Tidal Bore
Today's tides are pretty impressive, but they were actually much, much bigger in the past. The evidence for that is just off the coast of Bermuda. understanding how the tides have changed gives us a surprising insight into the history of the Earth's spin.