Tides: Crash Course Astronomy #8

Summary

The video explores the concept of tides and their significance in the universe. It explains how tides, influenced by gravity, shape objects and create oceanic movements on Earth. The script delves into tidal forces, their impact on celestial bodies like the Moon and Earth, and the phenomena of tidal locking and spaghettification near black holes. The interaction between the Moon, Earth, and Sun's gravitational forces results in complex tidal patterns, such as spring and neap tides. Overall, tides play a crucial role in the dynamics of astronomical objects and their interactions.

Takeaways

• 🌊 Tides are not just about the ocean's daily ebb and flow but are a fundamental force in the universe that shapes celestial bodies.
• 🌌 Tidal forces originate from gravity, which weakens with distance and is measured from the center of mass of an object, not its surface.
• 🔍 The difference in gravitational pull on different parts of an object due to distance is what causes tidal forces, which stretch the object.
• 🌕 The Moon's gravity, though weaker than Earth's, significantly affects Earth due to Earth's size and the distance between them, causing tidal bulges.
• 🏞️ Earth experiences two high tides and two low tides daily because of its rotation and the tidal forces exerted by the Moon.
• 🌀 The Earth's rotation causes the tidal bulges to lag, slightly ahead of the Earth-Moon line, influencing the Moon's orbit and causing it to recede from Earth.
• 🌅 The Moon's gravity pulls on Earth's tidal bulge, which in turn affects the Moon's orbit and Earth's rotation, making days longer over time.
• 🌑 The Moon is tidally locked to Earth, showing only one face due to the synchronization of its spin with its orbit around Earth.
• ☀️ While the Sun has a stronger gravitational pull than the Moon, its greater distance means its tidal force on Earth is about half that of the Moon's.
• 🌐 Spring tides occur when the Sun and Moon's tidal forces align during new or full moons, causing higher high tides and lower low tides.
• 🌊 Neap tides happen when the Moon is at first quarter, with the Sun and Moon's tidal forces partially cancelling each other out, resulting in less pronounced tidal changes.

Q & A

What is the primary force responsible for tides in the universe?

The primary force responsible for tides in the universe is gravity, which weakens with distance and acts on the center of mass of an object.

How does the difference in gravitational force experienced by different parts of an object create tidal forces?

Tidal forces are created when the gravitational force on one end of an object is stronger than on the other, causing the object to stretch and deform.

Why does the Earth experience two high tides and two low tides each day?

The Earth experiences two high tides and two low tides each day because of its rotation and the tidal bulges caused by the Moon's gravity, which are on opposite sides of the Earth.

How does the Earth's rotation affect the position of the tidal bulges?

The Earth's rotation causes the tidal bulges to lag behind the Earth-Moon line, as the water cannot respond instantly to the tidal force, resulting in the bulges being slightly ahead of the direct line to the Moon.

What is the phenomenon where the Moon always shows one face to the Earth?

The phenomenon where the Moon always shows one face to the Earth is called tidal locking, which occurs when the Moon's spin and orbital period match due to the tidal forces from the Earth.

Why is the Moon slowly moving away from the Earth?

The Moon is slowly moving away from the Earth because the tidal bulge on Earth exerts a gravitational pull on the Moon, causing it to accelerate and move to a higher orbit.

What causes the variation in the height of tides, such as spring tides and neap tides?

The variation in the height of tides is caused by the relative positions of the Earth, Moon, and Sun. Spring tides occur when the Sun and Moon's tidal forces align, while neap tides occur when they oppose each other.

How does the distance between two celestial bodies affect the strength of tidal forces between them?

The strength of tidal forces between two celestial bodies decreases with the increase in distance between them, as the gravitational force weakens with distance.

What is the term used to describe the stretching effect of gravity near a black hole?

The term used to describe the stretching effect of gravity near a black hole is spaghettification, where objects are pulled into long, thin strings.

How does the Earth's tidal bulge affect the Moon's orbit?

The Earth's tidal bulge exerts a gravitational pull on the Moon, causing it to accelerate and move forward in its orbit, which in turn causes the Moon to gradually recede from the Earth.

What is the effect of tidal forces on the Earth's rotation and the Moon's orbit?

Tidal forces have slowed the Earth's rotation, making the day slightly longer, and have also caused the Moon to move to a higher orbit, increasing its distance from the Earth.

Outlines

00:00

🌊 Tidal Forces and Their Impact on the Universe

This paragraph delves into the concept of tides beyond the oceanic phenomenon, explaining them as a universal force shaped by gravity. The script clarifies that gravity's strength diminishes with distance and is measured from the center of mass of an object. It uses the example of a person standing to illustrate the minute difference in gravitational pull on the head versus the feet due to height. The concept of tidal force is introduced, highlighting its dependence on the strength of gravity, the size of the affected object, and the distance between the two objects. The paragraph explains how tidal forces stretch objects, using Earth's interaction with the Moon as an example to describe how the Earth becomes slightly football-shaped due to the gravitational pull, leading to two tidal bulges. It also touches on the daily occurrence of two high tides and two low tides due to Earth's rotation and the ocean's response to tidal forces.

05:04

🌕 Lunar Influence and Tidal Locking

The second paragraph explores the Moon's gravitational effect on Earth, including how the Moon's gravity pulls Earth's tidal bulge, causing the Moon to recede from Earth at a rate comparable to the growth of fingernails. It discusses the impact of this tidal interaction on Earth's rotation, making days longer over time. The script then shifts perspective to the Moon, explaining the tidal bulges it experiences due to Earth's gravity and how these forces have slowed the Moon's spin, leading to tidal locking. This phenomenon results in the Moon always presenting the same face to Earth. The paragraph also addresses the Sun's role in Earth's tides, noting that despite the Sun's greater mass, its greater distance results in a tidal force only about half that of the Moon's. It describes how the alignment of the Sun and Moon during new and full moons results in spring tides, while a 90° angle between them leads to neap tides. The paragraph concludes with a broader view of tides in the universe, mentioning their role in the dynamics of star systems, planetary orbits, and the extreme effects near a black hole, known as spaghettification.

Keywords

💡Tides

Tides are the periodic rise and fall of sea levels caused by the gravitational forces exerted by celestial bodies, primarily the Moon and the Sun, and the rotation of the Earth. In the script, tides are not only the familiar oceanic phenomenon but also a broader astronomical concept that shapes objects in the Universe. The video discusses how tides affect the Earth, causing it to bulge and leading to the daily cycle of high and low tides.

💡Gravity

Gravity is a natural force that attracts two objects with mass towards each other. The script explains that gravity weakens with distance and is measured from the center of mass of an object. It is the fundamental force behind tides, as it causes the differential pull that leads to the stretching and subsequent tidal effects on Earth and other celestial bodies.

💡Center of Mass

The center of mass is the average position of all the mass in an object, which is considered when calculating gravitational forces. In the script, it is used to illustrate how gravity affects different parts of an object, such as a person standing up, where the head is slightly farther from the Earth's center than the feet, resulting in a minute difference in gravitational pull.

💡Tidal Force

Tidal force is the differential gravitational pull that stretches an object, such as Earth, due to the varying gravitational forces acting on different parts of the object. The script describes how tidal forces are responsible for the deformation of the Earth, creating bulges that result in high and low tides.

💡Moon

The Moon is Earth's natural satellite and plays a significant role in the creation of tides due to its gravitational influence. The script explains how the Moon's gravity causes one side of the Earth to be pulled more strongly than the other, leading to the formation of tidal bulges and the tidal locking of the Moon, which always shows one face to Earth.

💡High Tide

High tide is the period when the sea level is at its highest, occurring when the gravitational forces of the Moon and the Sun are aligned, causing the water to bulge. The script describes how high tides happen on opposite sides of the Earth due to the tidal forces, and how the Earth's rotation leads to two high tides and two low tides each day.

💡Low Tide

Low tide is the period when the sea level is at its lowest, occurring when the gravitational forces are weakest, pulling water away from certain areas. The script explains that low tides are a result of the tidal forces being at their weakest, which happens between the high tide bulges.

💡Tidal Locking

Tidal locking is a phenomenon where a celestial body's rotation period matches its orbital period around another body, resulting in one face always being presented towards the other body. The script describes how the Moon is tidally locked to Earth, showing only one face due to the gravitational interactions over time.

💡Spring Tide

Spring tide occurs when the Earth, Moon, and Sun are aligned, resulting in the highest high tides and the lowest low tides. The script explains that spring tides happen during the new moon and full moon phases when the tidal forces of the Moon and Sun reinforce each other.

💡Neap Tide

Neap tide occurs when the gravitational forces of the Moon and Sun are at right angles to each other, resulting in less pronounced high and low tides. The script describes neap tides as happening during the first quarter and third quarter moon phases, when the tidal bulge from the Moon is 90° from the Sun's, leading to a less significant tidal effect.

💡Spaghettification

Spaghettification is a term used by astronomers to describe the extreme tidal forces near a black hole, which can stretch objects into long, thin shapes. The script humorously introduces this term to illustrate the universal nature of tides and their potential effects in environments with extremely strong gravitational forces.

Highlights

Tides are a fundamental force in the universe, shaping most objects, and are driven by gravity.

Gravity's strength is measured from the center of mass of an object, not its surface.

Tidal force is the difference in gravitational pull experienced across an object due to distance from the gravitational source.

The taller an object, the more pronounced the tidal force experienced due to gravity's distance dependency.

Tidal forces cause an object to stretch, creating a bulge on the side facing the gravitational source and another on the opposite side.

The Moon's gravity causes Earth to stretch, resulting in two tidal bulges, one towards and one away from the Moon.

The Earth's rotation leads to two high tides and two low tides each day due to the tidal bulges.

The solid Earth also experiences tidal bulging, though to a lesser extent than water.

The Earth's tidal bulge exerts a gravitational pull on the Moon, causing the Moon to recede from the Earth at a rate of a few centimeters per year.

The Moon's gravity slows the Earth's rotation, gradually lengthening the day.

The Moon is tidally locked to the Earth, showing only one face due to the synchronization of its spin and orbital period.

The Sun's tidal force on Earth is about half that of the Moon's, despite being more massive, due to its greater distance.

Spring tides occur when the Sun and Moon's tidal forces align during New or Full Moon, causing higher high tides and lower low tides.

Neap tides happen when the Moon is at first quarter, resulting in less pronounced tidal differences due to the tidal forces being out of phase.

The Moon's elliptical orbit can intensify spring tides when it is closest to Earth during New or Full Moon, a phenomenon known as the proxigean tide.

Tidal forces are universal, affecting celestial bodies in various ways, including tidal locking and spaghettification near black holes.

Tides are a result of the changing gravitational force over distance and have significant effects on Earth's rotation, the Moon's orbit, and the synchronization of the Moon's rotation and orbit.

Transcripts

Y’know, if Shakespeare had been an astronomer, he’d have said that “there is a tide in

the affairs of the Universe, and on such a full sea are we now afloat.”

He would’ve been right. You might just think of tides as the ocean going in and out every

day, but in fact what astronomers call tides are a subtle but inexorable force that have

literally shaped most objects in the Universe.

And to understand tides, we start with gravity.

Gravity is a force, and it weakens with distance. An important thing to note is that we measure

gravity from the center of mass of an object, not its surface. One way to think of the center

of mass of an object is the average position in an object of all its mass. For an evenly

distributed sphere, that’s it’s center.

Right now, unless you’re an astronaut, you’re about 6400 kilometers from the center of the

Earth. If you stand up, your head is a couple of meters farther away from the Earth’s

center than your feet. Since gravity weakens with distance, the force of Earth’s gravity

on your head is an eensy weensy bit less than it is on your feet. How much less? A mere

0.00005%. And that’s way too small for you to ever notice.

But what if you were taller? Well, the taller you are, the farther your head is from the

Earth’s center, and the weaker force it will feel. If you were, say, about 300 kilometers

tall, the force of gravity would drop by about 10% at your head. That probably would be enough

to notice, if you weren’t dying from asphyxiation and, y’know, being 300 kilometers tall.

This change in the force of gravity over distance is what astronomers call the tidal force.

When you have a massive object affecting another object with its gravity, its tidal force depends

on several factors. For one thing, it depends on how strong the gravity is from the first

object; the stronger the force of gravity, the stronger stronger the tidal force will be on the affected object.

It also depends on how wide the affected object is. The wider it is, the more the force of

gravity from the first object changes across it, and the bigger the tidal force.

Finally, it depends on how far the affected object is from the first object. The farther

away the affected object is, the lower the tidal force will be. Tides depend on gravity,

and if gravity is weaker, so is the tidal force.

The overall effect of the tidal force is to stretch an object. You’re applying a stronger

force on one end than you are on the other, so you’re pulling harder on one end. That’ll

stretch it! And this is where tidal forces become very important.

Look at the Moon. It has gravity, but much less than the Earth because it’s less massive.

It’s 380,000 kilometers away, so the gravitational force it has on you is pretty small. And you’re

pretty small compared to that distance, just a couple of meters long from head to feet.

But the Earth is big! It’s nearly 13,000 kilometers across. That means the side of

the Earth facing the Moon is about 13,000 kilometers closer to the Moon than the other

side of the Earth. This is a pretty big distance, enough for tides to become important. The

side of the Earth facing the Moon is pulled harder by the Moon than the other side of

the Earth, so the Earth stretches. It becomes ever so slightly football-shaped, like a sphere

with two bulges, one pointing toward the Moon, and one pointing away.

This is probably the weirdest thing about tidal forces. You might expect only one bulge,

on the side of the Earth facing the Moon. But remember, we measure gravity from the

centers of objects. The side of the Earth facing the Moon feels a stronger pull toward

the Moon than the Earth’s center, so it’s pulled away from the center.

But the side facing away from the Moon feels a weaker force toward the Moon than the Earth’s

center. This means the center of the Earth is being pulled away from the far side. This

is exactly the same as if the far side is being pulled away from the center, and that’s

why you get two bulges on opposite sides of the Earth.

The tidal force is therefore strongest on the sides of the Earth facing toward and away

from the Moon, and weakest halfway in between them on each side.

A lot of the Earth is covered in water, and water responds to this changing force, this

stretching. The water bulges up where the tidal force is strongest, on opposite sides

of the Earth. If there’s a beach on one of those spots, the water will cover it, and

we say it’s high tide. If a beach is where the tidal force is low, the water’s been

pulled away from it, and it’s low tide.

But wait a second: The Earth is spinning! If you’re on the part of the Earth facing

the Moon, you’re at high tide. Six hours later, a quarter of a day, the Earth’s rotation

has swept you around to the spot where it’s low tide. Six hours after that you’re at

high tide again, and then another six hours later you’re at low tide for the second

time that day. Finally, a day after you started, you’re back at high tide once more.

And that’s why we have two high tides and two low tides every day. Very generally speaking,

the ocean tide causes the sea level to rise and fall by a meter or two, every day.

Incidentally, the solid Earth can bulge as well. It’s not as fluid as water, but it

can move. The tidal force stretches the solid Earth by about 30 centimeters. If you just

sit in your house all day, you move up and down by about that much...twice!

Like the saying goes, a rising tide lifts all… surfaces.

The Earth’s spin has another effect. Lag in the water flow means the water can’t

respond instantly to the tidal force from the Moon. The Earth’s spin actually sweeps

the bulges forward a bit along the Earth. So picture this: the bulge nearest the Moon

is actually a bit ahead of the Earth-Moon line.

That bulge has mass; not a lot, but some. Since it has mass, it has gravity, and that

pulls on the Moon. It pulls the Moon forward in its orbit a bit, like pulling on a dog’s

leash, accelerating it. The Moon responds to this tug by going into a higher orbit:

The Moon is actually moving away from the Earth! The rate of recession of the moon has

been measured and it’s something like a few centimeters per year, roughly the same

speed your fingernails grow.

Now get this: the Moon has gravity. Just as the bulge is pulling the Moon ahead, the Moon

is pulling the bulge back, slowing it down. Because of friction with the rest of the Earth,

this slowing of the bulge is actually slowing the rotation of the Earth itself, making the

day longer. The effect is small, but again it’s measurable.

OK, let’s get a little change of perspective. Everything I’ve said about the Moon’s

tidal effect on the Earth works the other way, too. The Moon feels tides from the Earth,

and they’re pretty strong because the Earth is more massive and has more gravity than

the Moon. Just like Earth, there are two tidal bulges on the Moon; one facing the Earth and one facing away.

Long ago, the Moon was closer to the Earth, and spinning rapidly. The Moon’s tidal bulges

didn’t align with the Earth, and the Earth’s gravity tugged on them, slowing the Moon’s

spin and moving it farther away. As it moved farther away, the time it took to orbit once

around the Earth increased: Its orbital period got longer. Eventually, the lengthening rotation

of the Moon matched how long it took to go around the Earth. When that happened, the

axis of the bulges pointed right at the Earth.

That’s why the Moon only shows one face to us! It spins once per month, and goes around

us once per month. If it didn’t spin at all, over that month we’d see the entire

lunar surface. But since it does spin once per orbit, we only ever see one face.

This is called tidal locking, and it’s worked on nearly every big moon in the solar system;

tides from their home planet have matched their spin and orbital period. These moons

all show the same face toward their planet!

Now wait a second. If the Moon has gravity, which causes tides, and is the root cause

behind all these shenanigans, what about the Sun? It’s even bigger than the Moon!

Tides depends on the gravity from an object, and your distance from it. The Sun is far

more massive than the Moon, but much farther away. These two effects largely cancel each

other out, and when you do the math, you find the Sun’s tidal force on the Earth is just

about half that of the Moon’s. The way the Sun’s tidal force and the Moon’s tidal

force interact on Earth depends on their geometry, which changes as the Moon orbits us.

At new Moon, the Earth, Moon, and Sun are in a line. The Moon’s tidal force aligns

with the Sun’s, reinforcing it. This means we get an extra high high tide and an extra

low low tide on Earth. We call this the spring tide.

When the Moon is at first quarter, the tidal bulge from the Moon is 90° around from the

Sun’s; high tide from the Moon overlaps low tide from the Sun. We get a slightly lower

high tide, and a slightly higher low tide. We call those neap tides.

The pattern repeats when the Moon is full; the Moon, Earth, and Sun fall along a line

again, and we get spring tides. A week later the Moon has moved around, and we get neap tides again.

Not only that, the Moon orbits the Earth on an ellipse. When it’s closest to us we feel

a stronger effect. If that also happens at New or Full Moon, we get an added kick to

the spring tides. This is called the proxigean tide, and can lead to flooding in low-lying areas.

Unless you live on the coast, I bet you had no idea tides were so complex!

Tides are universal; they work wherever there’s gravity. If two stars orbit each other, each

raises a tide in the other. Just like the Earth and Moon, that can slow their spin and

increase their separation. Many planets orbiting other stars may be tidally locked to those

stars. Near a black hole, where the gravity is incredibly intense, the tides are so strong

they would pull you like taffy into a long, thin string. Astronomers call this effect…

spaghettification. No, seriously, that’s what we call it!

Today you learned that tides are due to the changing force of gravity over distance. The

strength of the tidal force from an object depends on the gravity of the object, and

the size of and distance to the second object. Tides raise two bulges in an object, creating

two high tides and two low tides per day on Earth. Tides have slowed the Earth’s rotation,

moved the Moon away from the Earth, and locked the Moon’s rotation and orbit so that the

Moon always has one side facing us.

So. Tide goes in. Tide goes out. It turns out, I can explain that. Now you can too.

Crash Course is produced in association with PBS Digital Studios. This episode was written

by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr.

Michelle Thaller. It was co-directed by Nicholas Jenkins and Nicole Sweeney, and the graphics

team is Thought Café.