Anyone who has ever been to the ocean has witnessed high tide and low tide. What causes this phenomenon? How does the phase of the Moon affect the size of the tides? I've outlined some of the basic facts below. Complications such as the slowing of the Earth's rotation, fluid distribution, coastline topography and tidal acceleration are left out of this general discussion.
Basic Tides
The Moon has mass and therefore exerts a gravitational force on another object proportional to the inverse square of the distance between the two objects. This essentially means the closer you are to the Moon the larger the force of gravity. When the Moon is directly overhead it exerts the strongest gravitational force at the location directly underneath. At the same time, the gravitational force it has on the other side of the Earth(nearly 13,000km further away) will be at its weakest. This gravitational force causes fluids (atmosphere and water) to seemingly be pulled towards the Moon when it is directly overhead causing a high tide. Low tide occurs at the location on Earth 90° from the Moon.
**It is interesting to note a complication that high tide does not occur directly under the Moon due to the lag in the water flow. This phenomenon causes the recession of the Moon and the slowing of the Earth's rotation.**
The Bay of Fundy in Nova Scotia is known to have the largest tides in the World due to its geometry and water distribution. An image is shown below.
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| Bay of Fundy, Low Tide (left), High Tide (right) |
Second High Tide in a Day
So why is there also high tide on the other side of the Earth at the same time, I thought the force was weaker there? That is a good question. There are many explanations in cyberspace that are significantly confusing. I'll give it a try. The basic idea is that the gravitational force on other parts of the Earth are stronger than at the point furthest from the Moon. This causes the Earth to seemingly pull away from the regions of lesser force causing it to bulge out at either end.
How Phase Affects Magnitude of High Tide
Let's get into something easier to understand -- how the phase of the Moon affects the size of the tides. The Moon is not the only strong gravitational source in the Earth's vicinity. The Sun is much larger than the Moon but much further away. If you work out the calculations, the tidal force from the Sun is about 50% that of the Moon.
During a New Moon the Sun and Moon are on the same side of the Earth and add their gravity to create large tides. This is what is called a spring tide. Don't confuse spring tide with the season it is supposed to mean that the tide springs ahead more at this time. The same thing occurs when there is a Full Moon whereby the Sun and Moon are again in a line except on opposite sides of the Earth.
When the Moon is at First Quarter or Last Quarter it is at the point in its orbit 90° away from the Sun. This has the Sun and Moon pulling in different directions cancelling out each other's effects. High tides will be noticeably lower and it is called a neap tide.
An image borrowed from Wikipedia describes this process.
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| Spring and Neap Tides |
Perigean Tides
Since the Moon orbits the Earth in an ellipse it can be closer or further from the Earth at a given time in its orbit. This is the same phenomenon that causes what are known as 'Supermoons' -- the perigee (closest approach) aligns with a Full Moon. We discussed earlier than the force of gravity is dependent on the distance between the two objects. When a Full Moon or New Moon occur at perigee there will be higher high tides and lower low tides. Alternatively, when a Full Moon or New Moon occur at apogee (furthest approach) the tides will be smaller.
Other Considerations
Since the Earth rotates in the same direction the Moon orbits, the time between high tides is about 12.5 hours. It takes the Moon approximately 24.5-25 hours to be at the same spot above the Earth. This is the same idea as the minute hand on a watch meeting up with the hour hand every one hour five minutes and twenty seven seconds.
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