The Moon and Tides - From Ancient Wonder to Celestial Mechanics

 

The Tides: From Ancient Wonder to Celestial Mechanics

by lim ju boo PhD 

Postdoctoral Astronomy (University of Oxford) 

Early Observations of the Tides

Since antiquity, the rise and fall of the sea has puzzled humankind. The ancient Greeks, keen observers of nature, noted tidal rhythms but struggled to explain them. The historian Herodotus (5th century BCE) remarked on tidal changes in the Persian Gulf. Pytheas of Massalia (around 330 BCE), a Greek explorer, was among the first to connect tides with the Moon after observing tidal cycles in Britain. Later, Posidonius (c. 135–51 BCE), a Stoic philosopher, gave more systematic descriptions, recognizing the lunar influence on the seas.

It was not until much later, with Johannes Kepler and Isaac Newton, that tides were explained as a consequence of gravitational attraction. Newton, in his Principia Mathematica (1687), provided the first quantitative theory of tides, showing how the Moon (and to a lesser extent the Sun) raised bulges on Earth.

How the Moon Raises the Tides

Tides arise because the Moon exerts a gravitational pull on Earth. The side of Earth closest to the Moon is pulled more strongly, producing a “direct” tidal bulge in the oceans. On the opposite side, the weaker pull leaves the water slightly behind, producing a “compensating” bulge.

Thus, at any given time, two tidal bulges exist on opposite sides of Earth:

1. The near-side bulge (facing the Moon), caused by the stronger lunar pull.

2. The far-side bulge (away from the Moon), caused by the relative inertia of the oceans as Earth is pulled more strongly than the distant waters.

As Earth rotates once every ~24 hours, most coastlines pass through both bulges, giving rise to two high tides and two low tides per day.

Types of Tides

1. Spring tides: Occur during full Moon and new Moon, when Sun, Moon, and Earth are aligned. The solar and lunar pulls reinforce each other, producing the highest highs and lowest lows.

2. Neap tides: Occur during first and third quarters of the Moon, when Sun and Moon pull at right angles. The result is weaker tides, with smaller differences between high and low.

Solid Earth Tides

Not only oceans respond to lunar gravity; even the solid Earth flexes. These are called solid Earth tides or body tides. The crust of the Earth rises and falls by up to 30–40 cm as the Moon passes overhead. Though invisible to the naked eye, these tides are measurable and significant in geophysics, affecting sensitive instruments, underground water tables, and even earthquake stress patterns.

Tidal Friction and Earth–Moon Evolution

The Earth–Moon system is not static. Because the tidal bulges are carried slightly ahead of the Moon by Earth’s rotation, a torque is exerted. This does two things simultaneously:

1. Slows Earth’s rotation: Friction between tidal waters and the seafloor dissipates energy, gradually lengthening Earth’s day.

2. Pushes the Moon outward: The Moon gains orbital energy and slowly recedes from Earth.

Today, laser ranging experiments (using reflectors left on the Moon by Apollo astronauts) measure the Moon’s recession at about 3.8 cm per year.

Earth’s Early Rotation and Slowing Down

When Earth first formed, some 4.5 billion years ago, its rotation was much faster. A day may have lasted only 4 to 6 hours. Over billions of years, tidal friction slowed Earth’s spin to the current 24 hours. Fossil corals and tidal rhythmites (sedimentary rock layers laid down in tidal cycles) show that 400 million years ago, days were about 22 hours long, with ~400 days in a year.

The Fate of Solar Eclipses

Solar eclipses occur because the Moon’s apparent size in the sky almost exactly matches the Sun’s. But as the Moon recedes, it appears slightly smaller. In roughly 600 million years, the Moon will be too far away to fully cover the Sun, and total solar eclipses will no longer be possible, only annular eclipses will remain.

A Balance of Cosmic Forces

The dance of Earth and Moon is a delicate interplay of gravity, rotation, and energy dissipation. Without tides, Earth’s climate and ocean circulation would be profoundly different. Without tidal friction, Earth would still spin much faster, with chaotic consequences for life.

What began as mysterious sea-level changes observed by Greek philosophers is now understood as a profound planetary mechanism shaping the very future of Earth and Moon.

Summary:

1 . Tides were noted by ancient Greeks, but only linked to the Moon around 330 BCE by Pytheas.
2. The Moon raises two bulges: a near-side tide and a far-side tide.
3. Solid Earth tides affect even the land by up to 40 cm.
4. Tidal friction slows Earth’s rotation and causes the Moon to recede at 3.8 cm/year.
5. Early Earth had 4–6 hour days; by 400 million years ago, days were 22 hours.
6. In ~600 million years, total solar eclipses will end.

The Second Version: The Restless Seas and the Reluctant Moon

Stand on a quiet shore and watch the sea. The waves lap endlessly, but if you wait long enough, you will notice something subtler: the waterline creeping upward, then falling away again. The ancient Greeks watched too, though they lacked our science. To them, the sea seemed a living thing, inhaling and exhaling in slow, great breaths.

Herodotus noted that in the Persian Gulf the sea behaved strangely, swelling and retreating at intervals. Pytheas, a Greek sailor who ventured as far as Britain around 330 BCE, was sharper. He noticed that the tides obeyed the Moon, rising higher when the Moon was full or new. He had stumbled on a truth that would take nearly two millennia to explain.

The explanation arrived with Isaac Newton in 1687. Gravity, he declared, was the culprit. The Moon pulls on Earth, and since water flows more easily than rock, the oceans bulge toward it. But curiously, they also bulge on the far side. Why? Because the Earth itself is tugged slightly more than the distant water, leaving behind a bulge of its own. And so, as Earth turns, every shoreline is swept by two high tides a day, one from the near bulge, one from the far.

The Sun joins the game, too. When the Sun and Moon pull together (at new and full Moon), we get spring tides, the highest and lowest of the cycle. When they pull at right angles (first and third quarter), we get neap tides, the meekest tides of all.

It would be comforting if tides ended there, a neat story of oceans dancing to lunar music. But the Moon’s grip is firmer than that. It does not only pull water; it pulls rock. The very crust of the Earth rises and falls, like the chest of a giant sleeper, by as much as 30–40 centimeters. These are the solid Earth tides, invisible to our eyes but measured by geophysicists with instruments of exquisite sensitivity.

Yet the most dramatic consequence of tides is not the rise and fall of seas or land. It is the slow reshaping of time itself. The tidal bulges are not neatly aligned with the Moon; Earth’s rotation drags them a little ahead. And like a hand on a spinning wheel, this drag exerts a brake. Earth’s spin slows, ever so slightly, day after day.

The energy lost must go somewhere, and it does: into the Moon’s orbit. The bulges pull the Moon forward, giving it a nudge that makes it drift away. Laser beams bounced off mirrors left on the lunar surface by Apollo astronauts measure this retreat with astonishing precision: 3.8 centimeters every year.

Think of what that means. When Earth was young, perhaps four and a half billion years ago, a day was not a leisurely 24 hours. It may have been as short as 4 to 6 hours, the planet spinning like a dancer at full whirl. Over eons, tidal friction has slowed the dance. Fossil corals tell us that 400 million years ago, the day was 22 hours long, and there were about 400 days in a year. The Earth has aged not only in rocks and mountains but in the very beat of its rotation.

And the Moon? It was once much closer, looming larger in the sky. Now it drifts steadily outward. Someday, in about 600 million years, it will be too far away to fully cover the Sun’s disk. Our descendants, if any still gaze upward, will see only annular eclipses, a blazing ring of fire around a too-small Moon. The age of total solar eclipses will be over.

So the tides are more than the breathing of the sea. They are the agents of cosmic change, sculpting the length of our day and pushing the Moon on a slow journey outward. What began as a puzzle to the Greeks has become a profound story of celestial mechanics. Each wave that falls on the shore is a whisper of that story, telling us that Earth and Moon are locked in a slow but inexorable embrace, reshaping both worlds as the ages pass.

The Third Version: The Restless Seas and the Reluctant Moon

Stand on a quiet shore and watch the sea. The waves lap endlessly, but if you wait long enough, you will notice something subtler: the waterline creeping upward, then falling away again. The ancient Greeks watched too, though they lacked our science. To them, the sea seemed a living thing, inhaling and exhaling in slow, great breaths.

Herodotus noted that in the Persian Gulf the sea behaved strangely, swelling and retreating at intervals. Pytheas, a Greek sailor who ventured as far as Britain around 330 BCE, was sharper. He noticed that the tides obeyed the Moon, rising higher when the Moon was full or new. He had stumbled on a truth that would take nearly two millennia to explain.

The explanation arrived with Isaac Newton in 1687. Gravity, he declared, was the culprit. The Moon pulls on Earth, and since water flows more easily than rock, the oceans bulge toward it. But curiously, they also bulge on the far side. Why? Because the Earth itself is tugged slightly more than the distant water, leaving behind a bulge of its own. And so, as Earth turns, every shoreline is swept by two high tides a day, one from the near bulge, one from the far.

The Sun joins the game, too. When the Sun and Moon pull together (at new and full Moon), we get spring tides, the highest and lowest of the cycle. When they pull at right angles (first and third quarter), we get neap tides, the meekest tides of all.

It would be comforting if tides ended there, a neat story of oceans dancing to lunar music. But the Moon’s grip is firmer than that. It does not only pull water; it pulls rock. The very crust of the Earth rises and falls, like the chest of a giant sleeper, by as much as 30–40 centimeters. These are the solid Earth tides, invisible to our eyes but measured by geophysicists with instruments of exquisite sensitivity.

Yet the most dramatic consequence of tides is not the rise and fall of seas or land. It is the slow reshaping of time itself. The tidal bulges are not neatly aligned with the Moon; Earth’s rotation drags them a little ahead. And like a hand on a spinning wheel, this drag exerts a brake. Earth’s spin slows, ever so slightly, day after day.

The energy lost must go somewhere, and it does: into the Moon’s orbit. The bulges pull the Moon forward, giving it a nudge that makes it drift away. Laser beams bounced off mirrors left on the lunar surface by Apollo astronauts measure this retreat with astonishing precision: 3.8 centimeters every year.

Think of what that means. When Earth was young, perhaps four and a half billion years ago, a day was no leisurely 24 hours. It may have been as short as 4–6 hours, the planet spinning like a dancer at full whirl. Over eons, tidal friction has slowed the dance. Fossil corals tell us that 400 million years ago, the day was 22 hours long, and there were about 400 days in a year. The Earth has aged not only in rocks and mountains but in the very beat of its rotation.

And the Moon? It was once much closer, looming larger in the sky. Now it drifts steadily outward. Someday, in about 600 million years, it will be too far away to fully cover the Sun’s disk. Our descendants, if any still gaze upward, will see only annular eclipses, a blazing ring of fire around a too-small Moon. The age of total solar eclipses will be over.

So the tides are more than the breathing of the sea. They are the agents of cosmic change, sculpting the length of our day and pushing the Moon on a slow journey outward. What began as a puzzle to the Greeks has become a profound story of celestial mechanics. Each wave that falls on the shore is a whisper of that story, telling us that Earth and Moon are locked in a slow but inexorable embrace, reshaping both worlds as the ages pass.

Summary for 2nd and 3rd Versions 

1. The ancient Greeks observed tides but did not fully understand them. Pytheas linked them to the Moon around 330 BCE.

2. The Moon produces two bulges on Earth, one facing it and one on the far side, causing two high tides daily.

3. Solid Earth tides cause the land to rise and fall by 30–40 cm.

4. Tidal friction slows Earth’s rotation and transfers angular momentum to the Moon, causing it to recede by 3.8 cm/year.

5. Early Earth’s day was only 4–6 hours; by 400 million years ago it was 22 hours long.

6. In ~600 million years, total solar eclipses will end as the Moon drifts too far away.

7. Tides are more than ocean rhythms; they’re a driver of planetary evolution.

References

1. Lambeck, K. (1980). The Earth’s Variable Rotation: Geophysical Causes and Consequences. Cambridge University Press.

2. Bills, B. G., & Ray, R. D. (1999). “Lunar Orbital Evolution: A Synthesis of Recent Results.” Geophysical Research Letters, 26(19), 3045–3048.

3. Williams, G. E. (2000). “Geological Constraints on the Precambrian History of Earth’s Rotation and the Moon’s Orbit.” Reviews of Geophysics, 38(1), 37–59.

4. Newton, I. (1687). Philosophiæ Naturalis Principia Mathematica. London.

5. Cartwright, D. E. (1999). Tides: A Scientific History. Cambridge University Press.

6. Dickey, J. O., et al. (1994). “Lunar Laser Ranging: A Continuing Legacy of the Apollo Program.” Science, 265(5171), 482–490.