How Tidal Forces Shape Planets, Moons, and Black Holes

Name: Tides: Crash Course Astronomy #8
Uploaded: 2015-03-05T22:00:01+00:00
Duration: 9 min 46 s
Channel: CrashCourse
Description: Summary and key takeaways on Tides: Crash Course Astronomy #8: Summary & Key Takeaways, covering Fundamentals of Gravity and Tides Gravity weakens with

CrashCourse

Mar 05, 2015

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9 min video

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2 min read

YouTube video ID: KlWpFLfLFBI

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Gravity weakens with distance and is measured from an object’s center of mass. The tidal force is the change in gravitational pull over a distance, and its strength depends on three factors: the primary body’s gravity, the diameter of the affected object, and the distance between them. The primary effect of a tidal force is to stretch the object.

The Earth‑Moon Tidal System

The Moon pulls harder on the side of the Earth that faces it and less on the far side, creating two tidal bulges. Earth’s rotation carries these bulges around, producing two high tides and two low tides each day. Even the solid Earth stretches, moving up and down by roughly 30 centimeters daily. As the speaker notes, “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.”

Long‑Term Effects of Tidal Interaction

Because Earth’s spin sweeps the bulges forward, the bulges exert a forward gravitational tug on the Moon. This tug accelerates the Moon into a higher orbit, causing it to recede at a rate of a few centimeters per year—“roughly the same speed your fingernails grow.” The same friction that pushes the Moon outward also slows Earth’s rotation, gradually lengthening the day. Over time, tidal locking can occur, where an object’s rotation matches its orbital period, leaving it permanently facing one side toward its host.

Solar Influence and Tidal Variations

The Sun’s tidal force on Earth is about half that of the Moon’s. When solar and lunar forces align during New and Full Moons, spring tides produce especially high and low water levels. When the forces are at right angles during quarter moons, neap tides produce milder variations. A particularly strong event, the proxigean tide, happens when the Moon is at its closest approach during a New or Full Moon.

Universal Tidal Phenomena

Tidal forces operate far beyond Earth’s oceans. In binary star systems, they can slow stellar spin and increase the separation between the stars. Exoplanets orbiting close to their stars may become tidally locked, always showing the same face to the star. Near black holes, extreme tidal forces stretch objects into long, thin strings—a process astronomers call “spaghettification.” As the lecture emphasizes, “Tides are a subtle but inexorable force that have literally shaped most objects in the Universe.”

Takeaways

Tidal force arises from the variation of gravity across an object and depends on the primary body's gravity, the object's size, and the distance between them.
The Moon creates two tidal bulges on Earth, and Earth's rotation turns these bulges into the twice‑daily high and low tides we observe.
Earth’s rotating bulges pull the Moon forward, causing the Moon to recede a few centimeters each year while simultaneously slowing Earth’s spin and lengthening the day.
Solar tides are about half as strong as lunar tides, producing spring tides when aligned and neap tides when at right angles, with especially high tides during proxigean events.
Across the cosmos, tidal forces can lock planetary rotations, widen binary star separations, and near black holes stretch matter into spaghettified strings.

Frequently Asked Questions

Why does the Moon move away from Earth over time?

The Moon recedes because Earth’s rotation drags the tidal bulges ahead of the Moon, and the forward‑shifted bulges pull the Moon forward gravitationally. This extra forward pull adds orbital energy, raising the Moon’s orbit by a few centimeters each year.

What is spaghettification and how does it relate to tidal forces?

Spaghettification is the extreme stretching of an object near a black hole caused by intense tidal forces. The gravity gradient pulls more strongly on the side nearer the black hole, elongating the object into a thin string as the forces become overwhelmingly differential.

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Rotating Earth Moon Orbital Model Kit Recommended

A physical model helps visualize the Earth-Moon system, tidal bulges, and orbital mechanics described in the lecture.

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Astrophysics For People In A Hurry Book

This book provides accessible explanations of complex space phenomena like gravity, black holes, and orbital dynamics.

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High Quality Tabletop Orrery Model

An orrery demonstrates the movement of celestial bodies, helping to visualize orbital periods and tidal locking concepts.

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Full Transcript YouTube

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.

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The Earth‑Moon Tidal System

Long‑Term Effects of Tidal Interaction

Solar Influence and Tidal Variations

Universal Tidal Phenomena

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