Tides are a fact of life for our readers on the ocean coasts and have been important to all coastal inhabitants who have become fishing and seafaring peoples. Their cause was a mystery until the application of Newton’s law of gravity almost 400 years ago. The oceanic tides are the manifestation of two independent “heartbeats” of our solar system (the sun and our moon) and display the force that keeps our universe together—yet they’re revealed to any patent observer standing by the seashore.
The sun is 400 times more distant than our moon, but it is 27 million times more massive, so its gravitational influence is still about 175 times greater. But our tides are caused by the “difference” in the gravitational attraction between one side of the Earth and the diametrically opposite side. The moon has the dominant effect because, compared with the sun’s attraction, it is much closer to us and its relative attraction to the far side of the Earth is much weaker than on the near side. The moon pulls on a litre of Earth-bound water with only 34 micro-newtons, but that is enough to raise tides that are over a metre in height.
The moon pulls on our Earth, but the water on the moon-facing hemisphere is pulled more strongly than the water on the far hemisphere. With respect to the centre of the Earth, water flows toward the moon on the near hemisphere and is pushed away on the far hemisphere. The results are the two tidal bulges on each side of the Earth. We experience the rise and fall of the tides as the Earth rotates “under” those tidal bulges.
All ocean ports have different tides, so these tables cannot be used for St. John’s, Nanaimo or Iqaluit. Landforms get in the way of a steady movement of water, but that rhythmic movement can cause exceptionally high tides, such as those in the Bay of Fundy. The amplitude of the tides is also caused by the volumes of water involved; therefore, inland lakes don’t show significant tidal activity. The tides on the Great Lakes are only a few centimetres, which is masked by water levels raised by wave action, as well as storms that cause the sloshing effect called “seiches,” which can be tens of centimetres high.
As the moon slowly orbits the Earth, it passes overhead about 50 minutes later each day, causing maximum tide to occur 50 minutes later. The heights of the tides are not always the same, because the combined pull of the moon and the sun change with the angle between them in the sky, which also manifests in the lunar phases.
The figure below shows the slowly evolving pattern of maximum and minimum tides that results from the gravitational pull of the moon and sun as they go in and out of phase each month (Halifax, 2016). On the left is the height of the tides in metres. The swing between high and low tides occurs about twice a day. The month-long wave-like variations are due to the sun and moon’s gravitational forces drifting in and out of phase. We have been told that these patterns are lost in our many pages of tide tables, so we thought we should put them into a more revealing context.
Halifax, 2016. Even though the moon is 400,000 km and the sun is 150 million km away from us, tides demonstrate the physical impact of these celestial bodies on the Earth. We cannot reach out to touch them, but they can touch us.
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One of Canada’s foremost writers and educators on astronomical topics, the Almanac has benefited from Robert’s expertise since its inception. Robert is passionate about reducing light pollution and promoting science literacy. He has been an astronomy instructor for our astronauts and he ensures that our section on sunrise and sunset, stargazing, and celestial events is so detailed and extensive it is almost like its own almanac.
