Date: July 2nd, 2012
Title: Tides, Moons and Other Bodies
Podcasters: Nik Whitehead
Description:The Moon has a massive effect on the planet Earth, and this can most easily be seen in the movement of water that we know as the tides. Although these seem to come in and out twice a day, the actual pattern of the tides is far more complex and depends on more than the Moon alone. Water is not the only thing that feels the effect of tides, as satellites, planets and even galaxies are prone to tidal effects.
Bio: Nik is a lecturer in computer science at Swansea Metropolitan University in south Wales… but computer science is not her passion. She has a Bachelors degree in astronomy and astrophysics then took her Masters and Doctoral degrees in computer science when she realised that there are not enough jobs in astronomy to go around. What she’d really like to be when she grows up is either the navigator of the starship Enterprise or maybe a space traffic controller. In the meantime she’s working on visualisation tools for astronomical data.
Today’s Sponsor: “This episode of 365 days of astronomy was sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.”
Hello! Welcome to the 365 Days of Astronomy podcast for July 2nd. I’m Nik Whitehead, talking to you from South Wales in Great Britain, where I teach computer science at Swansea Metropolitan University. Although computing is my day job, astronomy is my first love, and I’m delighted to be able to contribute again to the 365 Days of Astronomy series. Today I’m going to talk about tides – what they are, how they’re formed, and how they appear not just on Earth but also on other planets and even in other galaxies.
Anyone who has lived near to or visited the coast is likely to have seen the action of the tide – although water comes in and goes out in individual waves, over a period of time the water level climbs higher and drops lower in a regular manner. At its simplest, a tide is simply a long-period wave that moves across the oceans as a result of the gravitational attraction of the Moon and other celestial bodies. When the crest of the wave reaches the shore we experience a high tide, and when the trough of the wave arrives we have a low tide. The difference between the two is called the tidal range, and this varies across the globe depending on the shape of the local coastlines.
This giant wave is caused by the gravitational attraction of the Moon on the Earth. As the Moon orbits the Earth, the side facing towards the Moon is attracted towards it. Land, being solid, doesn’t move very much – although it does move – but the liquid oceans move a lot more. This means that there is a bulge of water pointing towards the Moon. On the other side of the Earth, pointing away from the Moon, the gravitational pull of the Moon is much smaller, with the result that a second bulge forms pointing away from the Moon. These bulges stay fixed in position relative to the Moon while the Earth rotates underneath them. The land moves into the tide, rather than the tide moving into the land.
Having two bulges means that any point on the Earth will move through them twice in every tidal lunar day. A tidal lunar day is defined as the length of time between moonrises at a particular point on the Earth’s surface. This period is 24 hours and 50 minutes, slightly longer than a solar day because by the time the Earth has completed one rotation the Moon has moved further along in its orbit so the Earth has to catch up. This means that high tides occur 12 hours and 25 minutes apart, and that there are 6 hours and 12 1/2 minutes between high tide and low tide in any given location.
Most coastal areas on Earth have two roughly-equal high tides and two roughly-equal low tides a day, and so these are called semidiurnal tidal cycles. In some places, such as the western coast of north America, the two high tides are different heights, and this is known as a mixed semidiurnal tide. In a small number of places, such as the Gulf of Mexico and the south-western tip of Australia, there is only one high tide and one low tide a day. This is a diurnal tide, and is caused by the complex current patterns set up by the different coastlines and continental shelves across the globe.
In most places with semidiurnal tides you can estimate the height of the tide if you know the time and heights of high water and low water using a rule of thumb called the ‘rule of twelfths’. This assumes that the rate of flow of the tide starts at zero at low or high tide, increases smoothly to a maximum half-way between high and low tides, then decreases smoothly to zero again. It also assumes that the interval between high and low tides is approximately six hours, which is close enough for a rough calculation like this. The rule states that the first hour after low tide one twelfth of the water will come in, so the height of the water will rise by a twelfth. In the next hour two twelfths will come in, so together with the twelfth that came in in the first hour the water level will be three twelfths above low tide level. In the third hour another three twelfths comes in, bringing the tide up to halfway. In the fourth hour another three twelfths comes in, then in the fifth hour only two twelfths and in the sixth hour the final one twelfth comes in and the water level will reach high tide. So if low water was four hours ago and the tidal range is 6 metres then the water level will be approximately nine twelfths of 6 metres, or 4.5 metres above the low water mark. The tide goes out at the same rate it comes in so the rule of twelfths can be used to work out time from high tide just as easily as it can from low tide.
Now the Moon isn’t the only celestial body that raises tidal bulges on the Earth. The Sun, although it is a lot further away, is large enough to have a gravitational effect of just under half of that of the Moon and so this has a big effect on the size of the tides. This means that when the Earth, Moon and Sun are in a line – a condition known as syzygy – the gravitational attraction of the Sun reinforces that of the Moon and the tides are higher. These tides, close to the full moon and the new moon, are known as spring tides. Similarly, when the Moon is at first or third quarter and the Sun and Moon are separated by 90 degrees in the sky, the two tidal forces partially cancel each other out and the tides are smaller. These small tides are known as neap tides. Spring tides are higher than the average high tide and lower than the average low tide, while neap tides are not as extreme as the average tides.
Two other factors can affect the size of the tidal range – how close the Moon (or the Sun) is to the Earth, and how far above or below the equator it is. When the Moon is at perigee, its closest point to the Earth, the tidal range is larger because the gravitational attraction is greater, and when it is at apogee the range is lower. Every 7 1/2 lunar months perigee coincides with either a full moon or a new moon, and these perigean spring tides have the largest tidal ranges of all. The tide known as the highest astronomical tide occurs when both the Sun and Moon are closest to the Earth. The second factor is that the Sun and Moon move relative to the Earth’s equator, and so the tidal bulges move north and south of the equator with them, producing higher tides at higher lattitudes.
In fact, the two bulges of water don’t exactly line up on the line between the centres of the Earth and the Moon, but are a little bit ahead of the line due to them having angular momentum imparted by the Earth’s rotation. This has a couple of interesting effects. First of all, because the bulge has a small but measurable gravitational effect on the Moon – it pulls the Moon forward in its orbit. This gives the Moon more orbital energy, making it recede from the Earth by a few centimetres a year. So one side-effect of the tides that we see at the coast is that in just under 1.4 billion years the Earth will see its last total solar eclipse as the apparent size of the Moon becomes too small to cover the sun completely.
Secondly, as the water moves over the Earth there is energy lost to friction between the sea and the land. This energy comes from the Earth’s angular momentum and thus gradually slows down the Earth’s rotation, making the day longer. This change is very slow, but eventually the Earth will slow down enough that the tidal bulge will line up exactly between the Earth and the Moon. At this point the Earth will have slowed down enough that it will become tidally locked with the Moon. The rotational period of the Earth will match the orbital period of the Moon and the two bodies will keep the same points on the surface facing each other. The time needed for this to happen, though, is such that the Sun will have become a red giant and engulfed both the Moon and the Earth by that point.
The Moon is already tidally locked with the Earth. The Earth’s gravitational effect on the Moon is much more than that of the Moon on the Earth, enough to cause bulges to form in the Moon. The Moon’s rotation was slowed down by the tides caused by the Earth until the Moon’s rotational period matched its orbital period and so it now keeps the same part of the surface facing the Earth.
I mentioned earlier that the Moon’s gravitational effect on the Earth produces tides on land as well as in the water. Because rock is far stiffer than water, and thus more difficult to move, these Earth tides are a lot smaller in height than water tides. At the equator they can be up to 55cm when the Sun and Moon are lined up. This may sound like a small amount, but it is enough that GPS systems have to take it into account, as do astronomical observations made by very long baseline interferometry systems. The researchers at CERN also have to take account of Earth tides as the movement of the rocks slightly changes the circumference of the large particle accelerator rings that they use for their experiments.
Probably the best-known example of tidal forces causing movement in rock is that of Jupiter’s moon Io, the most volcanically active object in the solar system. Io is tidally locked to Jupiter, so the tidal bulges caused by the gas giant always remain in the same places on Io’s surface. If Io was Jupiter’s only moon then the tidal heating that causes Io’s volcanoes wouldn’t happen. The moons Ganymede and Europa also have an effect on Io, altering its orbit sufficiently that the tidal bulge could be as much as 100m higher when Io is at its closest approach to Jupiter than it is when Io is at the furthest point in its orbit. This constant stretching due to the varying tidal pull causes Io’s mantle and core to melt, providing magma for the moon’s 400 or so active volcanoes.
Any large enough body can create tidal effects – moons, planets, stars or even galaxies. The universe is full of unusual structures that are the result of tidal interactions between galaxies. Take the antenna galaxies in the constellation of Corvus, NGC4038 and NGC4039, two interacting galaxies with long tails of stars and dust that have been thrown out of their parent galaxies by tidal forces as the galaxies approached each other.
So next time you’re walking by the beach, maybe letting the waves sweep back and forth across your feet, why not take a moment to think about how the same force that makes the waves rise and fall in tides has the power to create volcanoes and to reshape galaxies?
End of podcast:
365 Days of Astronomy
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