When Are Tides the Strongest? Equinox Tides Explained
In short: tides are strongest around the equinoxes, near 20 March and 22 September. At those dates the Sun lies above the equator, roughly in line with the Moon's orbit. When a new or full moon falls at the same time, the Sun, the Moon and the Earth line up, their pulls add together and the spring tides reach the highest tide coefficients of the year.
The essentials
| Question | Short answer |
|---|---|
| What is an equinox? | A moment when day and night are the same length (around 20 March and 22 September) |
| Why are tides stronger? | The Sun above the equator lines up with the Moon's orbit, so their pulls reinforce each other |
| When is the effect greatest? | When Moon, Earth and Sun are aligned (new or full moon) |
| When are tides weaker? | Near the solstices (summer and winter), when the Earth is tilted towards or away from the Sun |
| Where can you watch them? | Saint-Malo and the bay of Mont-Saint-Michel, among the biggest tides in Europe |
Equinox tides in a few words
Equinox tides are the biggest of the year: spectacular, but sometimes dangerous.
Tides are a complex phenomenon that raises plenty of questions. In an earlier article, we explained why there are high tides and low tides. In some places the water moves by a few tens of centimetres, in others by many metres! And tides become particularly impressive around the equinox.
As these periods approach, people living on the coast feel a mix of excitement and apprehension. Big tides make for a magical sight, but their force and speed can be dangerous: it is better to watch them from a distance, and always check the tide times before heading out onto the foreshore.
What is an equinox?
An equinox is a moment of the year when daytime is as long as night-time.
This phenomenon comes from the position of the Earth relative to the Sun: at the equinoxes, the Sun is directly above the equator. It happens twice a year, with the spring equinox around 20 March and the autumn equinox around 22 September, depending on the year. But what does it have to do with tides?
When are tides the strongest? The role of the Sun and the Moon
At the equinoxes, the Sun and the Moon pull along the same line, and alignment makes tides stronger still.
You may already know that the pull of the Moon and the Sun on the Earth is what causes tides. Around the equinoxes, the Sun sits above the equator, close to the plane in which the Moon orbits the Earth. Their pulls then work along the same line over the oceans, and the tides are amplified: these are the big tides.
Add the alignment of the Moon with the Earth and the Sun (at new moon and full moon) and the tides get even stronger. The water is pulled by both bodies in the same direction, and the tide coefficients climb to their highest values of the year.
By contrast, around the summer and winter solstices (the longest and shortest days), tides are weaker: the Earth is tilted relative to the Sun, so the pull along the equator is lower and the tide coefficients fall.
What do equinox tides look like on our coasts?
On the coast, an equinox means a very large tidal range: the landscape changes radically within a few hours.
Let's leave space and come back down to Earth! Around the equinox, the coastal landscape changes quickly because of a large tidal range, meaning a bigger difference in height between low tide and high tide than usual. In the bay of Mont-Saint-Michel, the average range is between 8 and 9 m, but it can reach around 13 m with high coefficients such as those of the equinox tides.
These very low big tides uncover stretches of sand as far as the eye can see, a favourite moment for shellfish gathering and walks along the sea. But beware of the incoming tide, which rises very fast and can flood the foreshore. To admire the strongest equinox tides, head to Saint-Malo and the bay of Mont-Saint-Michel, which have some of the biggest tides in Europe.
Tides still have plenty of secrets to reveal!
Keep exploring: riptides and how to swim safely.
Never miss a big tide
Frequently asked questions
When are tides the strongest?
Around the equinoxes, near 20 March and 22 September, especially when a new or full moon coincides with them: the Sun, the Moon and the Earth are then aligned and the tide coefficients are at their highest.
Why are equinox tides stronger?
At the equinoxes the Sun is above the equator, close to the plane of the Moon's orbit, so the pulls of the two bodies reinforce each other. Combined with a new or full moon, this produces big tides.
What is the tide coefficient?
An index used mainly in France that measures the size of a tide. The higher it is, the bigger the difference between high tide and low tide (the tidal range). It reaches its highest values around the equinoxes.
Why are tides weaker at the solstices?
At the summer and winter solstices the Earth is tilted relative to the Sun, so the Sun and the Moon pull less along the same line and the tide coefficients are lower.
Where can you see the biggest tides in France?
In Saint-Malo and the bay of Mont-Saint-Michel, which have some of the biggest tides in Europe, with a tidal range that can reach around 13 metres.
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