For those located near the coasts, it's impossible to miss the influence of planetary bodies (including the moon) upon the Earth. Twice a day, the sea level rises and falls. In some places, such as Canada's east coast Bay of Fundy, those changes can be quite dramatic. Here, liquid flowing on a spherical Earth, moving under a changing gravitational potential combines with the shallowness of the sea-floor to create a low-tide line kilometers from the high-tide line. As consistent as the system may seem, this "moon-driven [...] timeless circuit of invasion and retreat" may soon combine with longer-term effects of our changing climate, as PhD Candidate Giang Nguyen describes below.
By Giang Nguyen
Our lives are defined by astronomical cycles. How fast the Earth spins about its axis dictates our daily cycle. The angle between Earth’s spin axis with respect to its orbital plane tilts either the North Pole or South Pole towards the sun. How fast the tilt alternates between the North and the South directs our habits from season to season, year to year. These effects are the most apparent manifestations of astronomical forcing in our lives, but there are weaker forces at play that affect us in more subtle ways.
The Moon has its own cycles as well. The phases of the moon have often been used to keep track of time and many cultures use lunar calendars to this day. However, neither the Earth’s rotation nor the Moon’s orbit are in sync with the Earth’s orbit around the sun; we have to subtract and add days year to year to keep dates consistent with the seasons. The moon with its crescent shape is an important icon for many people across the world and indeed it does have its astronomical effect on us.
The moon’s gravity, along with the sun’s, exert tidal forces on Earth. These tidal forces shift water in our oceans from one place to another. You can imagine water sloshing back and forth in a bathtub and this is essentially what is happening in our ocean between the continental coasts. The movement of water will resemble standing waves where each node (point that experiences minimal amplitude change) is called an amphidromic point. Tidal forcing, along with the Coriolis effect, create huge hydrodynamical systems around these amphidromic points making them very important when trying to understand tides.
Seas that are somewhat enclosed by land such as the Gulf of Mexico and the Mediterranean Sea experience small tides. Regions that experience strong tides include northern Québec in Canada and France’s Bay of Biscay. These are the places where tides play a significant role in day to day lives, all resulting from how the moon orbits around Earth. Tidal effects are even strong enough to affect large climate systems [1].
The moon’s orbital plane around Earth does not line up with the Earth’s orbital plane around the sun. The wobble in the Moon’s orbit follows an 18.6 year cycle, called lunar standstills, and can translate to significant tidal effects. High-tide floods are already a reoccurring problem in many coastal regions. During periods in the cycle where tides are amplified, the risk of tide floods are predicted to be higher than ever. The next period of amplified tides is expected to arrive in the mid-2030s [2]. When this time comes, the Earth is predicted to be warmer and sea levels higher. Therefore, it is now more important than ever to continue to study the moon and tides and see how they affect the climate and weather around us.
Although our lives are mainly shaped around the sun, the moon still has its own special effects on Earth. The slight deviation in the moon’s declination as seen on Earth reveals a decade long oscillation that has significant implications on tidal forcing. This, in turn, may lead to climatic feedback that may redefine our everyday lives. Astronomical cycles remain a subject of great interest and I hope we get to learn more about it resultant from our relentless exploration of Space.
References:
[1] Lin, J., & Qian, T. (2019). Switch between el nino and la nina is caused by subsurface ocean waves likely driven by lunar tidal forcing. Scientific reports, 9(1), 1-10.
[2] Rasmussen, C. (2021). Study Projects a Surge in Coastal Flooding, Starting in 2030s. NASA JPL, https://www.nasa.gov/feature/jpl/study-projects-a-surge-in-coastal-flooding-starting-in-2030s
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