Old Moon and the Sea
April 19, 2012
Photographer: Till Credner
Summary Author: Till Credner
The photo sequence above shows the synchronous motion of the tide and the Moon as observed in Penobscot Bay near Eagle Island Maine on August 22, 2008. In this part of Maine, the tide and Moon are quite in phase. Click here to see the time-lapse sequence, which covers about 25 hours. You can see the tide coming and going, especially after nightfall. The Moon rises and then nearly simultaneously the bay water rises as well. It's pretty obvious why our Moon is the main cause of the tides.
In the simple Newtonian gravity model of the tides, high tides are in the direction of the Moon but on the opposite side of the earth, whereas low tides are at a right angle to the Moon. So high tide occurs when the Moon is high in the sky (observer at point C in the illustration - for someone on the Equator) and below the horizon (observer at A). Low tide is when the moon rises (B) and sets at the horizon (D).
This is just a simple illustration. The Earth isn't completely covered by oceans of course, so the high tide can't turn around the Earth freely as the Earth rotates -- the continents get in the way of the water. Instead, each ocean swashes back and forth like water in a bathtub. Or it goes in circles in each ocean basin like in a glass of wine. Many more factors influence the action of the tides including the Sun, tidal friction, the Moon’s declination and even local weather effects. Thus, for most locations by the seashore, the time of high tide is considerably off the culmination of the Moon (point C). It just turns out that Penobscot Bay more or less fits well with the simplest tidal model. The Moon and ocean are rising simultaneously, connected by the invisible force of gravity.
Photo details: Nikon D3 camera; f/10; exposure from 1-30 seconds.