^ First quarter fe Light from Sun

Waxing gibbous Waxing crescent

Moon 6 PM Moon

Moon New Moon

Waning gibbous 6 AM Waning crescent Moon Moon

Ik Third quarter Moon

Light from Sun

Figure 1.6 Correct explanation for the phases of the Moon. Except at full moon, we see part of the Moon's dark side. The amount we see determines the phase. The drawings of the Earth and the Moon are as they would appear from space. The photographs are views of the Moon seen from the Earth. Note that the same craters and other lunar features are visible throughout the cycle of phases.

If you have looked at the Moon often enough, you have probably noticed that the dark side is often slightly visible along with the bright side. Some people attribute this to light emitted by the Moon itself from a glowing molten surface, or to starlight or sunlight scattered by the Moon's atmosphere. In fact, the stars are too dim to make the dark side of the Moon visible from Earth. Its surface is solid and it does not glow on its own. Its atmosphere is almost nonexistent, so it can't scatter light. The correct reason the dark side is slightly visible is that some of the sunlight striking the Earth scatters back into space and strikes the Moon's surface. Some of this light, called Earthshine, reflects off the dark side facing us and comes back.

The Tides of March

The other common area of confusion about the Moon is its relationship to the tides on Earth. Among the incorrect beliefs about tides are the following: they are due to winds on the ocean; the Moon does not cause the tides; tides are just due to the Moon; the height of high tide doesn't change throughout the year; there is only one high tide and one low tide on the Earth at any time; there is a high tide at the point directly between the Earth and the Moon; and the spring tide only occurs in the spring of each year.

Anyone who has spent time at the ocean quickly becomes aware of the tides. I first learned about them as a child living in New London, Connecticut. We often went to Ocean Beach during the summer, and I built countless sand castles, many of which were washed away by the rising tide. The ocean rises. It settles. We observe a predictable cycle of tides. Observations such as these will lead us to their correct explanation.

Do Winds Cause the Tides?

If the tides were due to winds on the ocean, then when the winds die, the water should stop changing height. The rising and lowering of the ocean is almost always imperceptibly slow. If you go down to the beach on a calm day and fail to see any change in the ocean height while you watch it, you might well conclude that the lack of wind has stopped the tides. You can change that belief by setting up a beach chair and umbrella and watching the calm waters for a few hours. They will change height regardless of the wind strength. Also, the fact that the tides cycle reliably should put the wind-tide relationship in doubt. If you believe that winds cause tides, then if the wind is calm for a day, you shouldn't see normal cycles of tides. But you will.

Observations often help correct beliefs about the natural world. The problem is that we often don't know our beliefs are erroneous, so we don't bother to test them with observations. Worse, incorrectly interpreting observations can lead to additional incorrect beliefs. I believe the major source of confusion about winds and tides comes from the reality that winds do cause ocean waves. Go to the beach on a windy day and you will see waves run farther up the beach than on a calm day. This occurs because higher winds generate higher waves.

To be fair, extreme winds can actually change the heights of the tides. During hurricanes the height of the high tide increases due to lower air pressure above the water. This change is called a storm surge, but it only occurs noticeably during severe storms.

Does the Moon Alone Cause the Tides?

Sailors have known for millennia that the height of the tide is related to the location of the Moon. For example, high tide occurs just after the Moon passes its highest point in the sky each day. This strongly suggests that there is a physical relationship between the Moon and the tides. If the Moon were the only cause of the tides, then the tides would be the same height every time the Moon was at the same place over the Earth. In other words, each day when the Moon was just past being highest in our sky, we would have a high tide of the same height as on all previous days. However, observations reveal that the height of the high tides varies throughout the cycle of lunar phases.

Well, you might argue, then perhaps the Moon's orbit around the Earth isn't circular. If that were the case, when the Moon is closer to the Earth, the Moon's gravity would be stronger, making high tides higher than they would be when the Moon is farther away. Indeed, observations reveal that the Moon's orbit is elliptical. Newton's law of gravitation explains that the closer two bodies are, the greater the tidal forces between them. When the Moon is closest to the Earth and high in our sky, the high tides are often higher than average.

However, further observations reveal that sometimes when the Moon is closest to us, the high tides are lower than usual, and when the Moon is farthest from the Earth, the high tides can be even higher. These last results are at odds with the belief that the Moon is the only cause of the tides.

Are There Only One High and One Low Tide on Earth at Any Time?

I went to Mount Desert Island on the coast of Maine to see the tides for myself. I arrived shortly before the Moon came up on the eastern horizon one morning7 and noted the height of the tide on one of the pilings in Southwest Harbor. I stayed there for about an hour, drinking coffee and watching the harbor activity. During that time the tide first went down slightly and then began climbing as the Moon rose. It was brisk that morning, but the sky was clear and the day looked promising.

I spent the rest of the morning walking on various carriage trails in Acadia National Park, enjoying the breathtaking landscapes. As lunchtime neared, I returned to Southwest Harbor and had a clam basket and side order of onion rings at Beal's Lobster Pier. As the Moon passed its highest point,8 I interrupted my lunch several times to again measure the height of the tide on the same piling. The tide rose to its highest level and then started back down during that period. The time interval between the low point that morning and the high point was roughly 6 hours and 15 minutes.

7 Keep in mind that the Moon rises at all different times of day and night, depending on the phase it is in.

8 Contrary to common belief, the Moon is visible during the day. Indeed, it is visible as often during the day as during the night.

I spent the afternoon in Acadia National Park, going up Cadillac Mountain, sitting by Thunder Hole, lying on Sand Beach. The day passed all too quickly, and at around five o'clock, I went back to Beal's for a lobster dinner and to again watch the tide, which was lowest at around 6 p.m., about 6 hours and 15 minutes after high tide. By the time I left the island at about 8 p.m., the tide was rising.

But wait a minute. I just experienced two low tides in one day and, all things being equal, there should be a high tide before the next low tide the following morning. That means two low tides and two high tides each day, not one.

There is another way of getting to this point: at the same time I saw the Moon rising in the east that morning, someone on the other side of the world was seeing it setting in the west. Since I experienced a low tide when the Moon was rising, it seems plausible that there would be a low tide on the opposite side of the Earth from me at the same time. This is consistent with the fact that I witnessed another low tide as the Moon set. Likewise, when I experience a high tide just after the time the Moon is highest, someone on the opposite side of the world must also experience one. That means there is a high tide when the Moon is high in the sky and one when it is high over the opposite side of the Earth.


Many people believe that the spring tide only occurs in the spring. The fact is, the spring tide occurs roughly every two weeks throughout the year. The name doesn't derive from the season. Rather, it comes from the German word springen, which means "to spring up." Spring tides are when the water "springs up" the most—the days of the highest high tides and lowest low tides.

Causes of Tides

Tides result from centrifugal and gravitational forces acting on the Earth. By far the most important sources of these tide-generating forces are our Moon and the Sun. Consider the case of the Moon. It and the Earth actually orbit a common point, called their center of

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