The topic of tide come up frequently in the flat-Earth community. Let’s examine some of the ‘issues’ they brought up about tides in Q&A format.

What causes tides?

Tides is caused by the difference in gravitational forces exerted by the Moon and the Sun on different location on the Earth. This can happen because the distance to the Moon or the Sun is not the same on different parts of the Earth.

A location on Earth closer to the Moon will experience larger gravitational forces from the Moon. And conversely, a location farther from the Moon will experience smaller gravitational forces from the Moon.

If the Moon’s gravitational force is so strong that it can pull sea water, why can’t it pull anything else like a ship or a human until it pulls it to the moon?

Even sea water itself does not fly to the Moon. The gravitational forces exerted by the Earth itself is far greater than the gravitational forces from the Moon, so everything stays on the Earth.

Gravitational forces are directly proportional to mass. Sea water as a whole has a very large mass, and the resulting gravitational force is much larger than objects like a ship or a human.

This happens to be one the most common misconception about gravity within the flat-Earth community. The idea is that if gravity can attract a very heavy object, then by ‘logic’ it should be able to attract lighter objects with a larger force. This is wrong because gravitational force is directly proportional to mass. The larger the mass, then the larger the gravitational force.

Why only sea water is affected by tide, not water in lakes or rivers?

All oceans in the world is connected to each other. Water can flow freely between them. When a location is experiencing high tide, the additional volume of water is shipped from another location that is experiencing low tide.

In lakes, water will experience greater lunar gravitational forces under a full moon., but no additional volume of water can flow to the lake, so tide at the scale experienced by oceans does not occur.

Tide can happen in lakes, but on a much smaller scale. The Great Lakes experience tide less than 5 cm in height.

Why is that the part of the Earth that is facing away from the Moon is also experiencing high tide?

The gravitational force exerted by the Moon is weakest at the side facing away from the Moon. But keep in mind the direction of the force is the opposite. At this position, the Moon is ‘under’ us, and thus its gravitational force points downwards.

On the side of the Earth facing away from the Moon, the gravitational force of the Moon exerted on that location is weaker than the Earth’s average. So the resulting tidal force has an upwards direction, resulting in a high tide.

The gravitational force exerted by the Sun is greater than the Moon. Why is that the Moon has greater effects on tides than the Sun?

The Sun is farther away, and the difference in its gravitational force on different part of the Earth is smaller. Conversely, the Moon is much closer, and the difference of Moon’s gravitational forces felt by different parts of the Earth is more pronounced.

The difference of the lunar gravitational acceleration between the farthest and closest position from the Moon is 2.47 × 10⁶ m/s². While the same difference with respect to the Sun is 9.74 × 10⁷ m/s².

Why tide is only affected by the Moon and the Sun?

Other celestial bodies have gravitational effects on the Earth, but because of their distances and smaller mass, the effect is insignificant.

Other than the Moon and the Sun, the celestial body that has the greatest contribution to tide is Venus, but again, its effects is very small.

If oceans experience tides, then other parts of the Earth should also experience tides. But we never see this in reality.

Earth’s atmosphere and crust also have tides. For more details please read these Wikipedia articles: Earth tide dan Atmospheric tide.

Tide only affects sea water. Freshwater does not experience tide. So, we can conclude the cause of tide is salinity of water and electromagnetic effects of the Moon!

Let’s consider the following facts:

  • The Mediterranean Sea has higher salinity than seas in general, but it has much smaller tides.
  • Not all lakes are freshwater lakes. Some lakes even have much higher salinity than the average sea water.
  • If the Moon has an electromagnetic effect, it will have a more pronounced effects on ferromagnetic objects than non ferromagnetic ones, but in reality it does not.

Thus we can rule out electromagnetism and salinity as the cause of tides.

Some rivers also have tides. So, tides can happen anywhere!

Part of river that experiences tide is only the estuary that is close to the sea. If the river bed is lower than sea level, then it will be affected by tide.

If the river bed is higher than sea level, it will not be affected by tide.

But there are lakes that experience tides!

First, there are water bodies whose official name is ‘lake’, but it is in fact part of the ocean. An example is Lake Maracaibo, Venezuela. This ‘lake’ is connected to the sea through a strait that is 5.5 km wide.

Second, the term ‘tide’ is sometimes used for other phenomenon that have nothing to do with the lunar or solar cycle. For example, the rising of the surface of a lake as a result of increased river flow or melting glaciers is sometimes called ‘tide’ in some culture. In this article, we only consider tides that are caused by lunar and solar cycle.

Third, some lakes do experience very small but measurable tides. The largest lakes in the world have tides, but on a much smaller scale, no more than 4 cm in height.

The highest tide in my location happens not exactly at the time the Moon is directly above!

Tide has phase shift. Gravitational forces arrive almost instantly. But water takes time to move. So, there is delay between the cause and the effect. Different places have different tide patterns and phase shifts.