Sky Matters: How tides are created goes on above our heads

The sun and the moon's gravitational forces will determine how high a tide is and when it will come in
Sky Matters: How tides are created goes on above our heads
An early morning tide at  Robert's Cove, Co. Cork. Tides are created by gravitational pull from our solar system. Picture: David Creedon/Anzenberger

The recent fine weather has had many of us taking a close look at tide tables as we eagerly anticipated the perfect dip in the sea (notwithstanding the shock of the cold, character-building water). 

Tides are the result of the gravitational pull of the Sun and the Moon. They would still occur without the Moon because the Sun’s gravitational pull would still exist, but they would be smaller, about a third the size of the tides we are familiar with today. And they would occur at exactly the same time each day. 

In reality, the Moon does exist and it orbits the Earth approximately every 28 days. The net effect is that the Moon returns to the same place in the sky every 24 hours and 50 minutes, meaning coastal areas experience two high and two low tides every 24 hours and 50 minutes. This is why the times of high/low tides slide on a daily basis. 

In addition, when the Moon is on the same side of the Earth as the Sun their combined gravity causes a larger tide height, whilst the opposite is true when they are on the opposite sides of the Earth to one another. 

Local geography also influences the exact times of the tides. In some places, the flow of water is unrestricted, in others less so. 

It’s no wonder that predicting the navigability of challenging waterways by sailors requires deep knowledge!

Technically, the heights of tides are also influenced by the planets. The largest influence comes from the planet Venus, with an impact that is 10,000 times less than that of the Sun and Moon (pretty insubstantial, all things considered). 

The planet Jupiter, the largest in the solar system, exerts an influence some 10 times less than Venus because it is so distant, and the effect becomes smaller for all the other planets. Every now and again the planets align reasonably well and tales of combined influence have led to bizarre "theories" of woe for our planet. 

Based on the strength of their combined gravitational effects and our knowledge of the recurrences of planetary alignments (which we know to have happened millions of times over the 4.5 billion year history of our planet), we can dismiss these outright.

Tides are not restricted to water. The Earth’s crust and atmosphere change height in unison with the ocean’s tides. The height of the ground beneath your feet changes by up to 50cm per day, but as everything rises and falls together you don’t notice it. And planetary tides are not restricted to the Earth. 

On Jupiter’s moon Io, the volcanic activity there is driven in part by a phenomenon called “tidal friction” in which the constant gravitational tug of war between Jupiter and one of its other moons, Europa, causes Io to heat up.

The same gravity that produces tides is also responsible for attracting comets into the inner solar system from their lair in the icy outer reaches far beyond Jupiter. Unexpectedly, we have seen this in the welcome appearance of comet Neowise which may remain visible beneath the Plough during August. 

Another manifestation of comets will grace our skies when the annual Perseid meteor shower peaks on the night of 12th August. The Perseids are one of the most reliable meteor showers, characterised by fast meteors (lasting around 1 second) and persistent trains (lasting several seconds) and best viewed with just your eyes. 

They are the result of the Earth colliding with dust particles trailed by Comet Swift-Tuttle. This is a large comet, much larger than that which heralded the demise of the dinosaurs. It returns every 133 years, and it left a refreshed trail of debris during its last pass around the Sun in 1992. 

Happily for us, the predictability of gravity means we can not only understand the tides, but plot the orbits of comets. And we can rest easy in the knowledge that it doesn’t have our name on it as a destination!

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