Come for the Whales, Stay for the Tides

June 14, 2023

By: Madison Malloy

Sitting at the base of the Bay of Fundy, St. Andrews is no stranger to large tides. The Bay of Fundy sees tides up to 16 metres high, and here in St. Andrews, our tides can rise as high as 8.5 metres (Government of Canada, 2023)! In order to appreciate what causes this phenomenon, we must understand how the Sun and Moon influence tides, as well as the special characteristics of the Bay of Fundy.

How frequent are tides? (National Oceanic and Atmospheric Administration [NOAA], 2013)

The largest tidal force is the gravitational pull of the Moon on the Earth, which pulls the water towards the Moon, forming a bulge. On the other side of the earth, a bulge of approximately equal size is formed from inertia, the opposing force to gravity (NOAA, 2005). In the image above, this is represented by the high tides on either side of the equator, and low tides at both of the poles.

We must also consider the secondary tidal force of the Sun on the Earth. Despite the Sun being around 400 times bigger than the Moon, it has a lesser impact on the tides because it is much, much farther away (Britannica, n.d). The gravitational pull from the Sun creates a smaller bulge in addition to the one from the Moon. Instead of thinking of the tides as going up and down, you can think of the Earth as rotating in and out of these stationary bulges every day.

Spring vs. neap tides (Bay of Fundy, n.d)

Have you ever wondered why the phase of the Moon impacts how high the tides are? It comes down to the position of the Moon relative to the Sun. As seen above, when the Moon, Sun, and Earth are in line, a full or new Moon is observed, and the bulges from the Sun and Moon amplify to make the highest tidal range. This is known as a spring tide, and these are seen twice each lunar month (NOAA, 2014). Also twice each lunar month are neap tides, which happen when the Sun and Moon are perpendicular to each other. This causes the bulges to have a dampening impact, resulting in the lowest tidal range.

Why does the timing of the tides change every day?

First, we need to think about how the Earth rotates once every 24 hours and the Moon rotates around the Earth once every 28 days. Because both the Earth and Moon are rotating, the Earth needs to move a bit extra, around 50 minutes every day, in order to catch up to the Moon. This means that in places with only one high tide a day (diurnal tides), like the poles, the tides come every 24 hours and 50 minutes instead of every 24 hours. Here in St. Andrews, we get two approximately equal-sized tides every day (semidiurnal tides) which come every 12 hours and 26 minutes.

The difference between high tide and low tide outside of the Jolly Breeze Whale Adventures office, St. Andrews

In The Bay of Fundy, home to the highest tides in the world, there are two main features that influence its size. The first factor, the shape of the bay, allows the water to be funneled up and down, creating high tidal levels. Tidal resonance, the process of water being flushed in and out of the bay at perfect timing, allows incoming water to be pushed to high levels. These two factors result in breathtaking tides. We get lower tides at the base of the Bay of Fundy than at the head of the bay, but tides an average of 5 metres high can still be observed here, which tower over the global average of 1 metre (Time and Date, n.d).

Whether you come to St. Andrews for the tides, whales, or lobster rolls, one thing goes without question: St. Andrews would not be the same without the powerful influence of the tides.

 

References

Encyclopædia Britannica, inc. (n.d.). Understand the relative size of the Sun, the Moon, and other Solar System objects. Encyclopædia Britannica. Understand the relative size of the Sun, the Moon, and other solar system objects

Parks Canada Agency, G. of C. (2023, March 17). Tides in Fundy National Park. Fundy National Park. Tides in Fundy National Park

Spring vs. Neap Tides. Bay of Fundy. (n.d.). Spring vs. Neap Tides – Bay of Fundy

US Department of Commerce, N. O. and A. A. (2005, December 1). Tides and water levels, gravity, inertia, and the two bulges, nos education offering. Gravity, Inertia, and the Two Bulges – Tides and water levels: NOAA’s National Ocean Service Education. Gravity, Inertia, and the Two Bulges – Tides and water levels: NOAA’s National Ocean Service Education

US Department of Commerce, N. O. and A. A. (2013, June 1). How frequent are tides?. NOAA’s National Ocean Service. How frequent are tides?

US Department of Commerce, N. O. and A. A. (2014, August 1). Why do we have spring tides in the fall?. NOAA’s National Ocean Service. What are spring and neap tides?

What causes ocean tides?. The Moon Causes Tides on Earth. (n.d.). What Causes Tides? – Moon