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 are the result of the difference in the gravitational forces exerted by the Moon and the Sun on a different location on the Earth. This happens 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 position 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. Seawater as a whole has an enormous 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 hefty object, then by ‘logic’ it should be able to affect lighter objects with a more significant force. This is wrong because gravitational force is directly proportional to mass. The larger the mass, then the more significant the gravitational force.

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

All oceans in the world are connected. Water can flow freely between them. When a location is experiencing high tide, the additional volume of water flows 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 higher than the Moon. Why is that the Moon has more significant effects on tides than the Sun?

The Sun is much farther away, so that the difference in its gravitational force on the different parts 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 only 9.74 × 10⁷ m/s².

Why is tide 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 most significant contribution to tide is Venus, but again, its effects are minimal.

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, read these Wikipedia articles: Earth tide dan Atmospheric tide.

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

Let’s consider the following facts:

  • The Mediterranean Sea has a higher salinity than seas in general, but it has much smaller tides.
  • Not all lakes are freshwater lakes. Some lakes even have a much higher salinity than the average sea water.
  • If the Moon has an electromagnetic effect, it will have 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 a river that experiences tide is only the inlet that is close to the sea. If the riverbed 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 the tide.

But there are lakes that experience tides!

First, there are water bodies whose official name is ‘lake,’ but it is 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 phenomena 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 the 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 precisely at the time the Moon is directly above!

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