How Do Tides Work

After years of living in a beach town, I've gotten into the habit of checking the tide table every morning. Partly because I love tide pools so I always want to visit the beach at the lowest possible tide, but also because many of Santa Barbara's beaches don't even exist at high tide!

Observing the tide data every day, I started asking myself how much I actually understood about tides. I mean, hopefully we all remember from grade school that tides are caused by the moon. But honestly, do you remember WHY? It was a little fuzzy for me, so I decided to refresh my memory.


High tides and low tides 101

So what you probably remembered from high school was right, the moon's gravitational pull on the Earth causes the tides. Specifically, it causes three things to happen:

  1. The water on the side of the Earth facing the moon (sub-moon water) will be pulled toward the moon, causing a bulge, i.e. there's a high tide on the side of the Earth facing the moon. 
  2. The water on the side of the Earth facing away from the moon (antipode water) will also bulge, causing concurrent high tides on opposite ends of the Earth. Why?* Because the gravitational pull of a mass on another becomes weaker as distance increases. This means the sub-moon water get pulled away from the center of the Earth, but the center of the Earth also gets pulled away from the antipode water. Basically, the whole thing is getting stretched out like silly putty. But since the force is stronger closest to the mmon, the sub-moon water will experience a greater high tide than the antipode water.
  3. Everywhere not experiencing a high tide is experiencing low tide.

Because the Earth completes a full spin of its axis in about 24 hours, we can expect two high tides per day. But because the moon is also traveling around the Earth in 28 days, every day the moon's a little further along on its path than it was at the same time the day before. So the high tide doesn't happen exactly every 12 hours...more like 12 hours and 25 minutes. If you are trying to, say, go to the beach every day to observe tide pools at low tide, you'll have to delay your beach trip by about 50 minutes from day to day.


But wait, that's not all!

The sun also has a gravitational pull on Earth, obviously, and therefore the sun also affects the tides. When the sun and the moon align with Earth in the straight line during a new or full moon, the sun magnifies the effect of the moon, causing a larger tidal range (higher high tides and lower low tides). This is called a spring tide, which is uselessly confusing because it has nothing to do with the spring season but rather with the action of springing toward the moon.

Conversely, when the moon and the sun are pulling at different angles during a first and third quarter moon, their powers start to cancel each other out, causing smaller tidal range (lower high tides and higher low tides). This is called a neap tide.

What else can affect the tides?

Because the sun and the moon are moving in very predictable patterns, tides heights can be also predicted consistently. However, weather systems can also play a factor. Storms, winds, and low or high atmospheric pressure can all cause tides to become higher or lower than expected.

The distance of the moon to the Earth, which changes slightly over time due to its elliptical path (remember hearing about the "super moon"?), can cause a minor change to the tidal range.

Seasons also play a part in tides. As the Earth's tilt causes the sun and the moon's "declination" to vary through the year (with the sun shining more directly at the equators in March and September, in the northern hemisphere in June, and in the southern hemisphere in December), the tides will also follow suit.

*Tidal controversy, oh my!

Many oceanic websites will explain that the bulge of antipode water is due to the centrifugal force of the Moon-Earth system having a center of mass that is not the center of the Earth. While it is true that the center of mass of the Moon-Earth system is off-center of the Earth, physicists argue that it is not the reason for the antipode water bulge. After reading a few math-laden scientific papers, I chose to side with the physicists at JPL lab, who explained the whole thing very clearly.

Further Reading

The Moon Effect on Ocean Tides,

What Causes High Tide and Low Tide? How Stuff Works.

Why is There a Tidal Bulge Opposite the Moon? Jet Propulsion Lab.