This morning the moon was a beautiful crescent low in the morning sky. It is moving towards a rendezvous with the sun on Sunday morning where they will line up to present Tahiti and her islands with a rare treat - a solar eclipse. Tahiti Expeditions is working with Betchart Expeditions hosting a group of eclipse enthusiasts from the Planetary Society, including astronomer Jim Bell. On Friday we will be heading out to a remote island in the Tuamotu Archipelago called Anaa, which is one of only four islands here that will get to see the total eclipse. Tahiti and all the other islands will see about 98% totality though, which is still a spectacular sight.
But there is another phenomenon created by the interaction of the sun and the moon that we experience all the time - although here it is tweaked a bit. Many visitors to Tahiti and her islands have noticed that the tides here have a pretty minimal range. In fact the total range from the highest highs to the lowest lows is about 70 centimeters, or a little over 2 feet. An obvious response to this that visitors also notice is that people build their houses very close to the water. Interestingly, this is not the only thing that is distinct about the tides here, they are also regular every day – highs at about 12 noon and midnight, and lows at 6am and 6pm. So what is going on?
The tides on earth are caused by the gravitational pull of the moon and the sun. Since the moon is a lot closer it exerts a stronger pull on the tides than the sun. Our “day” is 24 hours because that is how long it takes for the earth to do a complete rotation on its axis in relation to the sun. To do the same thing with the moon takes 24 hours 50 minutes and 28 seconds. So the time of the tides following the pull of the moon advances by about 50 minutes every day, and this is the way it is over most of the earth.
Near Tahiti though the lunar tide is cancelled because of an interesting phenomena called an amphidrome, or amphidromic point. As the lunar tidal swell passes around the earth and through the ocean basins, it gets broken up into lots of smaller swells, bouncing off continents and islands along the way, and it also gets affected by the Coriolis Force which bends it north and south. The ultimate effect is that there are places where a crest of the lunar swell meets a trough of the lunar swell, and cancels the tide out completely and these are the amphidromes. So with a lunar amphidrome located here we just have a solar tide, which is weaker, thus lower in amplitude, and locked into our normal 24 hour solar day. As you move away from Tahiti, into the Tuamotu Archipelago for instance or to the Marquesas, you get away from this amphidrome and the lunar tides begin to play a stronger role in the tidal cycle.