Most coastal zones on Earth experience two high and two low tides per day, known as semidiurnal tides. However, some areas feature a diurnal pattern where only one tidal cycle occurs in each lunar day. A part of this is due to continental interference and the interruption it provokes on the flow of waters across the globe. The other cause is related to the planet’s spin axis and the fact that its tilt (in relation to the moon) makes it so that the moon’s gravitational forces are not centered in the Equator and some regions only experience a one high tide per lunar day.
As the moon rotates around our planet, strengthening its gravitational force over various points on the surface of the Earth, it sets off protuberances on our oceans, and coastlines around the world experience the phenomenon of tides. However, the Earth also rotates on its axis, producing centrifugal force and dictating different tidal patterns
Since the moon completes a full lap around our planet every 24 hours and 50 minutes – and considering that every time it is above or directly below a given place on the Earth’s surface that spot will experience a high tide – it would be expected that two high tides and two low tides should happen each day, everywhere. Despite that being the case in most places (where semidiurnal tides occur), some areas only experience a single tidal cycle per day and are thus known for featuring a diurnal pattern.
Diurnal tides occur due to a series of factors, among which continental interference and the Earth’s spin axis are the most important to be considered. In the first condition, if our planet was a perfect sphere covered by an ocean of even depths, there would be two high tides of same amplitudes in each lunar day. But the fact that the Earth has a very uneven surface, with landmasses separating oceans in irregular ways, makes it so that some areas feature complex tidal patterns that differ greatly in period. With regards to the angle of our planet’s spin axis, it is tilted 28.5 degrees off of the moon’s orbit, placing it almost in the plane of the Earth’s orbit around the Sun. Hence, the moon’s gravitational force – and the subsequent water bulges created by its pull – are not centered in the Equator, so some regions only witness one high tide per lunar day.
Ryan is the founder of Tideschart.com. Originally from New Zealand, Ryan has traveled to more than 20 countries and has combined more than 10 years of development experience with his passion for the ocean.