The Bible informs us that God created the moon on Day 4 of Creation Week (Genesis 1:14–19). Some of its functions are briefly described: it provides light and helps us plan out our calendars. But the moon has another important function: it helps create tides, the daily rise and fall of water along our shorelines. But how do tides work? This is a mystery that baffles people today, as it did people in the ancient past. Yet, it is not that hard to understand, if you think about it.
Why do we have tides?
The forces that create ocean tides are outside our normal daily experience and so people sometimes struggle to explain them. Galileo (AD 1564–1642) famously erred by claiming that the earth’s rotation and orbit caused tides. However, as people in his day pointed out, this would cause only one high tide per day. Instead there are two along most coastlines in the world. However, centuries before Galileo, the medieval Anglo-Saxon monk and historian, the Venerable Bede (AD 673–735), correctly worked out that the moon was responsible. Yet Galileo rejected lunar attraction as too mystical. This was not an easy thing to understand, because even if the tides are higher at a full moon, they are also high at the new moon. Also, the maximum height is usually not when the moon is straight overhead.
How the moon causes tides
Tides are caused by gravity. The closer two things are, the stronger the force of gravity between them, and the faster they accelerate toward one another (see box below). We’ll explain one step at a time how this causes tides.
First, imagine that the earth were sitting still in space. What would happen? Due to gravity, it would immediately begin to fall toward the sun. But gravitational attraction depends on distance. Thus, the near side of the earth would be attracted to the sun more strongly than the far side. But the earth is solid. Even if one side wanted to accelerate toward the sun faster than the other, the whole mass would move as one and the earth would not stretch into an egg shape.1
The oceans, however, are not solid. They are free to react to the sun’s gravitational pull. The ocean on the near side would literally fall toward the sun faster than the earth and the ocean on the far side would literally fall more slowly than the earth.
Of course, water does not stretch and cannot lift off the bottom of the ocean, so why does the water bulge up? The water that adds to the bulges comes from the places on Earth where the net gravitational forces are near-parallel to the earth’s surface (Figure 1). Here, the water ‘slides sideways’, just a little, as it ‘falls’ faster toward the sun on the near side, or ‘falls’ more slowly than the rest of the earth on the far side.
But the earth is not falling into the sun, right? Actually, it is. But it is also moving sideways at the same time, resulting in a nearly circular orbit. However, our forward orbital movement does not negate the force of gravity from the sun. Points on the earth’s surface react to the sun’s gravity in the same way as if we were falling straight toward it. So, there should be two tidal bulges on the earth caused by the sun’s gravity (there are).
We can explain lunar tides in exactly the same way. The moon and the earth are also constantly attracting each other. Again, the water on the near side of the earth falls toward the moon faster than the water on the far side. Interestingly, the lunar tide is over twice as high as the solar tide because, even though the sun is much more massive, the moon is much closer (see box below).
This is what causes oceanic tides, as first explained by Sir Isaac Newton in 1687. However, tides are more complex than in this simple illustration, because as the water bulges move across the oceans they run into shallower water (which increases their speed and often deflects them in different directions) and irregular coastlines (which create complex patterns of water flow). The final result is that some places in the ocean experience regular, twice-daily high tides, some places experience once-daily high tides, some places experience extreme tides, and some places have no tides at all.
Also, the sun and moon are not in phase. Solar tides occur twice every day, 12 hours apart, as the earth spins. But since the moon is orbiting the earth, it takes 24 hours and 50 minutes for the moon to get to the same place in the sky as it was the day before. Thus, the solar and lunar tides happen at different frequencies. When the solar and lunar tides are lined up, either at a full moon or a new moon, we experience above average spring tides. They occur every two weeks, not just in the Spring! When the lunar high tide happens at the same time as the solar low tide, and vice versa, the two tides cancel each other out. The smaller tidal effect caused by the sun is subtracted from the larger tidal effect caused by the moon. At these times we have below average ‘neap’ tides.
There is an amazing design aspect to this system. The moon is just the right size and just the right distance to produce tides that gently flush our coastlands. If the tides were much stronger, the earth would experience twice-daily tsunamis that would rip apart rocks and destroy any living thing even within great distances from the shore. If the tides were much smaller, bays and estuaries would be stagnant and inhospitable to life. The earth-moon system is unique in our solar system. It defies the possibility that it has arisen by chance. Thus, the presence of tides points to intelligent design, by a loving God, who gave us a wonderful place to live.
The gravitational force between two objects is given by F = Gm₁m₂/R², where G is the gravitational constant, m1 and m2 are the masses of the objects, and R is the distance between their centres of mass.2 Look at the formula and you can see why it is called an inverse square law. For example, if the distance between two objects doubles, the force decreases fourfold (because (2R)² = 4R².
But this means the gravitational force differs from one place to another. The difference between the gravitational force on opposite sides of any object is called the ‘tidal’ force. The tidal force drops off with R³—an inverse cube law—double the distance and the tidal force decreases eightfold [because (2R)³ = 8R³].3
Consider: The average earth–sun distance is 149.6 million km. The earth’s equatorial diameter is only 12,756 km. Thus, the tidal force, the difference in gravitational attraction of the sun on opposite sides of the earth (0.0017%), is only a small fraction of the total gravitational force. But the moon is only 384,500 km away. There is a 6.7% difference in the moon’s gravity across the diameter of the earth. So, even though the sun is 27 million times more massive than the moon, and even though the gravitational force from the sun is about 175 times that of the moon, the tidal force from the moon is greater. The inverse cube law means that the mass of the sun is no match for the closeness of the moon, thus the moon produces a tide 2.2 times that of the sun.
Understanding gravity and tides also helps us avoid irrational fears. For instance, from time to time we hear about people being afraid of major planetary alignments. But the inverse cube law for tides means we can totally ignore sensationalist claims about planets lining up and causing massive disasters. E.g. the tidal effect of the largest planet, Jupiter, on the earth is only 0.008% that of the sun, while the effect of the closest planet, Venus, is only 0.003%. So, even if every planet were lined up with the sun and moon, there would be no noticeable effect.
References and notes
- Actually, the earth does stretch a little (about 10cm). We can measure this with sensitive instruments. Return to text.
- DeYoung, D., Gravity: the mystery force, Creation 22(3):40–44, 2000. Return to text
- This can be determined using either algebra or calculus, by the way. We’ll leave it up to you to figure it out. Hint, what is Gm₁m₂/R² – Gm₁m₂/(R+r)²? But don’t forget that the radius of the earth (r) is negligible compared to the distance (R) from the earth to the sun (13,750r) or moon (60r). Return to text