Tides: Crash Course Astronomy #8

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.

100 thoughts on “Tides: Crash Course Astronomy #8”

1. gOdZiLlA zumagod says:

Why ?

so if tidal bulges happen because of gravitational pull (and centrifugal force respectively) of the moon and sun on earth, wont the spring tide where earth is sandwiched between sun and moon be more than the other instance ?

3. Grace Overholt says:

(ﾉ◕ヮ◕)ﾉ*:･ﾟ✧ ✧ﾟ･: *ヽ(◕ヮ◕ヽ)

4. Gui Pasc says:

I'd like to see it explained with einstein general relativity, and the curved space.

5. Sagar Jain says:

more than 7 years and the bill o'reily meme is still funny.

6. Stefan The RainbowPhoenix says:

Crash Course or PBS Space Time: who explained it better?

7. stephen li says:

man, this is complicated

8. Zachary Lin says:

9. carschmn says:

Spagetification is such a fun term!

10. mihr thebear says:

i subscribed and likeed

11. Feynstein 100 says:

Hmm but shouldn't the moon be losing energy because of all the tidal friction and falling toward the earth?

12. verdatum says:

So it appears that PBS spacetime called out this answer as not quite accurate and oversimplified. Check them out for a more complete answer, anyone who sees this. That said, this answer will probably get you full credit on the current AP exams.

13. chounry oun says:

Now i don't understand about General Relativity Theory..Can you use GRT to describe this.?

14. Christian Chen Liu says:

Most awesome topic, I was always confused about it, thanks!

15. TiMonsor says:

u said that moon was closer and moving faster ages ago, and earth was spinning faster – i guess we could use a bit more details. I know that the day used to be 19 hours, but wanted more specifics, its linked to the tides afterall.
Very good ep. nonetheless.

16. Simone Turner says:

Do you have a crash course on orbital resonance

17. Scott Anderson says:

Some places have one high tide and one low tide per day, or differing heights in their two low and high tides. This is because of the specific shapes of the coastlines.

18. geometry shanes says:

S T R E T C H

19. Will Chen says:

A bunch of ppl in the comments calling O’Reilly dumb for the “tide goes in” comment – NOT KNOWING OREILLY IS A HAVARD LAW MASTERS GRADUATE??? Did you all really think he doesn’t know 1000X more science than a typical youtuber??? He needs to say what he says to appeal to his audience

20. Rafael Molina Navas says:

On my post of yesterday only forces on sublunar and antipodal earth areas are referred to.
There is where tide-generating dynamic imbalance is the greatest.
On transverse section through earth CM, that imbalance doesn´t occur.
On the rest of closer hemisphere moon´s gravity prevails, being net tide-generating force the closer to moon, the bigger.
On the other hand, on farther hemisphere, where moon´s pull is smaller, centrifugal force prevails, being the imbalance the farther from moon the greater.
Water from all parts of the oceans tend to move and pile up on both opposite areas (50/50) …
But VERY, VERY LITTLE through squeezing (as proposed by some people) if any !! The unique area where there are some inward vertical components of net tide-generating force imbalance is where this is null or close to null, and there almost the whole of the pull vector is tangential (horizontal) …

21. Sebastian Pardo says:

Can someone explain me why at 4:15 we get high tide in the face of the earth that is farthest away from the moon? I mean, moon's gravity pulls less on that face, so to me it only makes sense if you get a low tide at that moment

22. Sebastian Pardo says:

I can't relate the fact that the earth accelerates the moon's rotation with the moon moving away from earth. Please someone help me understand!

23. Rafael Molina Navas says:

I´ve frequently seen that most people don´t fully understand the "peculiar" type of movement of the earth, in its dance with the moon: its revolving around earth-moon common center of mass (barycenter). Its location is INSIDE our planet, app. 1/3 of its radius from the surface (but please note it is not a material point).
The barycenter is the one which actually follows the elliptical orbit around the sun.
But with that movement earth doesn´t properly orbits around the moon, as many people say: earth CM follows a circular path around the barycenter, and the rest of earth material points follow identical circles, but around different geometric points, and maintaining its orientation relative to distant stars …
LOGICALLY I´m considering ONLY earth-moon dance, disregarding earth daily spinning and the orbiting of the couple around the sun, because they have nothing to do with moon-earth dynamics.
As it is certainly tough to grasp, a few days ago I managed to imagine an analogy, and sent it to another site. IT CAN BE VERY USEFUL TO MOST OF YOU:
"I suggest you put your open hand horizontally on a table. Spot the table point app. under the middle of the first bone of your middle finger (B, from barycenter). And try slowly to make your hand “revolve”, with the first knuckle of same finger (C, app. at hand center) following a circle around B .
If initially you had imagined a “moon” a couple of feet away in line with your middle finger, and that “moon” rotated around B simultaneously to hand revolving, ALWAYS in line with B and C and at side opposite to C, you would have a scenario quite similar to actual earth-moon dance.
If you carefully observe the end of any of your fingers (or any other point of your hand), you will find that THEY ALL follow equal circumferences, ALWAYS keeping their location farthest from the moon (logically, within each own path).
THAT´S, I think, A GOOD WAY to see what quoted from NOAA´s scientits work IS QUITE CORRECT:
" … since the center-of-mass of the earth is always on the opposite side of this common center of revolution from the position of the moon, the centrifugal force produced at any point in or on the earth will always be directed away from the moon”.
The NOAA work is the one I referred to here a couple of weeks ago.

24. Rafael Molina Navas says:

MY ULTIMATE GO ?
Once again I´m going to try and convey a core idea of my stand, now in a “fresh” way …
It will have three parts. I´ll post them neither on same post, nor on three different ones, but on two posts … (that way readers will have more time to “ruminate” what proposed).

A) Not to need to include a figure, please kindly IMAGINE one with two celestial objects on same “vertical” line (on your screen).
Let us imagine the lower one (M) on a fix location, and the upper one (E) moving “horizontally” from right to left, and let us consider M´s gravity starts to pull downwards E just when reaching M´s vertical (remember, M is considered somehow on a fix location).
If within certain game of relation between masses, distance and speed of E, E will start “orbiting” around M.
M´s gravity, acting as centripetal force, changes E´s speed vector direction, but it is not “able” to change its size. A “free fall” to many, but rather a “partially free fall” to me, because E´s inertia avoids a direct straight line fall.

B) Let us now imagine M, at the very moment E gets at M´s vertical, starts moving also from right to left, with same velocity as E.
Being now M´s pull on E always vertical, E will fall towards M, following a parabolic line.
Now M´S PULL IS ABLE not only to bend E´s speed vector, but also TO INCREASE ITS VERTICAL SPEED WITH AN ACCELERATION PROPORTIONAL TO M´S PULL … same way as if neither of them had an initial horizontal speed …
THAT´S WHAT I CONSIDER A "FULLY FREE” FALL …
And WE SHOULD NOT SAY E IS ORBITING AROUND M, because it will fall onto it !!

25. Rafael Molina Navas says:

MY ULTIMATE GO ? (2nd part)

C) Let us now suppose that, when E reaches M´s vertical with its initial “horizontal" speed, apart from starting M´s gravitational pull, M somehow is given an initial speed parallel to E´s, not to the left as in case B but to the right … And not just as big as E´s (as in case B) but much, much bigger (see below how much).
If with the correct values for masses, M-E distance, and initial speeds, we would have our real case, E being the Earth and M the Moon …
We know the result is that, instead of E being orbiting around M as in case A, and somehow opposite to what in case B (when E got “fully free” to fall directly onto M), now E is “forced” to bend its trajectory much more, because M´s location is changing in a sense opposite to E´s speed … E and M revolves/rotates around their common center of mass. E´s trajectory curvature is much, much bigger than when orbiting around M (case A).
For similar (but kind of opposite) reasons to case B, IT IS ERRONEOUS TO CONSIDER E “ORBITS” AROUND M, AND TO SAY THAT E IS IN A “ FREE FALL" … In cased A we had a “partially free" fall, and in B a “fully free” fall … But NOW I CONSIDER WE HAVE NO FREE FALL AT ALL, BECAUSE INITIAL SPEEDS AND MUTUAL PULLS ARE CAUSING A KIND OF DANCE OF THE COUPLE, KEEPING THEIR SEPARATION ACTUALLY CONSTANT, AS IF IT WERE A SINGLE TWO-PARTS OBJECT SPINNING AROUND ITS CENTER OF MASS …
With this and last post I´ve already delivered what “promised”: a fresh explanation of one of the main roots of my stand …
But I´ll leave for another post the elaboration of main consequences of that, as far as I can understand …

26. Rafael Molina Navas says:

MY ULTIMATE GO ? (5th part)

Before trying and convey my stand on non-inertial frames issue, let us go back to the drawing board, as if we didn´t have any education on dynamics …
Same image with Earth (E) on upper part, and vertically below the Moon (M).
Let us suppose we only know how they move, with kind of the mind of a “smart” child.
And then we learn that (as David Cooper said many times), if Earth and Moon tangential speeds somehow became null, they would begin to accelerate straightly towards each other, because of gravitational mutual attraction …
If just stopping their circular movements, they get “free” to accelerate towards each other … THOSE CIRCULAR MOVEMENTS ARE WHAT, WHEN EXISTING, WERE SOMEHOW COUNTERING THOSE STRAIGHT ACCELERATIONS NOW THEY HAVE GOT !! No other logical possibility for a “clean” child mind …
How come? Let us see …
Curiously, with a rotating (so called "non-inertial”) frame of reference, same thing happens, but let us say “virtually” …
As the frame of reference rotates with E and M, E and M don´t "rotate" anymore
relatively to that frame of reference … Centripetal forces are not required whatsoever, and gravitational pull would make them fall onto each other … But that´s far from reality …
Then physicists go on: "How can we do “real” maths with that “artificial” (the adjective is mine …) reference system ? … We have to apply a “fictitious” force, which we´ll call centrifugal force". That way WE GET BACK TO THE REAL SCENARIO … as far as dynamics is concerned.
Well, that force is certainly “fictitious”, because it has been introduced by us to work with that non-inertial frame of reference … BUT THET MEANS THAT THE REAL CIRCULAR MOVEMENTS, NOT NEEDING THE ADDITION OF ANY “ARTIFICIAL” FORCE, SOMEHOW PRODUCE AN INERTIAL EFFECT THAT KEEPS E AND M WITHOUT FALLING STRAIGHTLY ONTO EACH OTHER !! TO ME (AND NOT ONLY ME), THAT IS A REAL CENTRIFUGAL FORCE, QUITE REAL !!
In case B (“MY ULTIMATE GO ?", posted 4 days ago), as E and M were moving horizontally at same speed, Moon´s pull was able not only to bend E´s trajectory, but also to accelerate E straightly towards M with its full strength, quite a “free” acceleration. That “freedom” was made possible by the movement of the moon towards the left, with same horizontal speed as E.
The real case is dynamically quite the opposite: the movement of M in opposition to E´s speed, and with speeds which make them to revolve/rotate around the barycenter, makes impossible the decrease of E-M distance, and even any proper orbiting of E around M. It gives us a “rotating” scenario, where all forces have basically same direction as the line between centers of mass, and where the angular position doesn´t matter much (as far as Earth-Moon dynamics is concerned), as long as we accept centrifugal forces keep the separation constant …
After all, if Earth and Moon were the only celestial objects in the universe, to talk about angular position of the system would have no sense at all …
BUT, STILL, THE WOULD HAVE TO KEEP ROTATING. THAT WAY INHERENT INERTIAL FORCE, THE CENTRIFUGAL FORCE, WOULD KEEP THEM WITHOUT FOLLOWING STRAIGHTLY ONTO EACH OTHER !!
Therefore, as I´ve said many times, I find quite correct what a NOAA scientist told me:
"… to provide a basic description of the forces which create the tides. It's intended audience were the grade school children and adults of that time. It used terminology of science and forces which were common in the 1950s. Such as centrifugal force. Centrifugal force was always an "imaginary force" (not a real / measurable force). But that type of description made the concepts easier to understand and explain. That description and use of centrifugal force continued to be common practice until the 1970-80's. At that point, the terminology shifted and the TEXTBOOKS USED IN GRADE SCHOOLS WERE CHANGED TO USE A MORE MODERN TERMINOLOGY AND DESCRIPTION OF THIS “EFFECT”BEING A RESULT OF INERTIA RATHER THAN AN “IMAGINARY FORCE”".
Initially I didn´t fully grasp his point … But later I did.
The problem is that many books, dictionaries included, keep following models with ideas which are several decades out of date …

27. AsBi says:

which book is that on the table? Apollo 13

28. Navreet Kaur says:

U speak too fast it make me confused

29. Md. Ayeman Hyder says:

That WWII pun was great and terrible simultaneously.

30. Yuush says:

31. Rafael Molina Navas says:

Discussing with somebody else (who says only differential gravity counts), I think I found another simpler way to explain tides:
Total force exerted on any given particle (because of moon-earth rotation/revolving), has to be equal to the centripetal force required for its revolving, to accomplish Newton´s 2nd Motion Law). The main force “supplier” is moon´s pull AT EACH LOCATION. The other possible forces only can be exerted on each particle by surrounding ones (internal stresses), or by the very gravitational pull from the rest of our planet.
Closer to moon particles "feel" stronger pull than required centripetal force.
THAT excess (and not any "differential gravity") has to be passed onto the rest of earth through their neighbors.
On their turn, those neighbors exert opposite forces back onto considered particle (3rd Newton´s Motion Law).
At farther hemisphere interactions are a kind of mirror image of what at closer hemisphere, because the farther the particle, the bigger the deficit of moon´s pull (compared to required centripetal force).
That constitutes a complex "chain of transmission", where total force acting on each particle (leaving aside own weight) has to satisfy 2nd Newton´s Motion Law (F=mω²r) …
THAT FIELD OF FORCES, exerted DIRECTLY ON EACH PARTICLE, and the INERTIAL EFFECTS (centrifugal forces included) caused by the fact that all particles are being "forced" to revolve together, ARE ACTUALLY THE DIRECT CAUSE OF TIDES !!
Basically solid earth is stretched, and water moves towards where the two bulges build up.
None of those forces is "artificial", and all are directly felt by considered particle, what implies "natural" additions of forces, without any artificial intervention of our minds. That would be necessary to deduct moon´s gravitational pulls at locations very distant from each other (the so called “differential gravity”), what obviously material stuff can´t do !!

32. T Phas says:

Can anyone explain 2 bulges again in human language.

33. Alexandru Dragoi says:

Easy to understand tides if you don't waste time jerking over the phone with colleagues.

34. Isabel Ames says:

😛
this is billy
like this commet to
make him happy

35. Rafael Molina Navas says:

There is a very, very interesting youtube video, "What if the moon did not existed – Neil F. Comins" . Perhaps too long, due to its interactivity (between NFC and the audience).
NFC is a really eminent astronomer … Just google his name and you´ll see.
He talks about causes of tides during some 10 or 15 minutes, starting app. 06:00.
I first watched it some months ago. I had seen he clearly considers both gravity and inertial forces (due to earth-moon "dance") is what actually causes tides (not just the so called "differential gravity"). But only recently I realized he clearly also says that to consider moon orbits the earth is WRONG.
I´ve been long discussing tides on other sites, here among many others. As NFC says, there is a lot of confusion out there. One of the main roots of that confusion is precisely what mentioned. People consider centrifugal force, at least in the case of earth movement and tides (NFC says just "outward" force) is fictitious, because they say earth is in free fall in its orbit …
What I posted here three weeks ago, especially 1st and 2nd parts of my series "MY ULTIMATE GO?" is quite in line with what NFC says, though he drastically just says what mentioned, and I elaborated a three part analogy trying to explain it to anybody interested.

36. Jill Peachy says:

This video was fantastic! Great job explaining tidal forces and why we have 2 tidal bulges and tidal friction. Thanks!

37. Tommy Victor Buch says:

Could someone please send this video to Bill O'Reilly?
There must be a limit to how ignorant a journalist is allowed to be.

38. CUZZA MUS1K says:

this video sucks

39. Sean Uptown says:

I will be making a video on it, and how it's not what they think it is.

40. Oh wow says:

“What comes up comes down”

-Isac Newton

(Real quote)

41. petr shv says:

how fast does gravity travel? if an object was to spontaneously appear (i know it cant) 50 parsecs away from you, when would you be able to feel its pull?

42. petr shv says:

far in the future: ok class sttele in for astronomy the moon is one of the countless moons of jupiter

43. Math Geeks says:

The last 2 sentences were ever so close to Roche's Limit.

44. Duy Linh Chu Ha says:

For a more accurate explanation of tide, look for PBS tide on youtube.

45. Syntox says:

5:10 Can someone explain why the moon is moving away from the Earth rather than towards it?

46. Michelle Draper says:

I loved this explained tho i don't think i can remember all this! it was explained in a great way! thank you

47. Da Weather Dude says:

thanks! I learned something new!

48. edward batista says:

Now I finally understand the tide theory in my ocean engineering class!

49. corrda1993 says:

Anyone spend the whole video waiting for him to mention spaghetification?

50. EXTREME SATYA IIT JEE AIR 1 says:

Do you have any notes

51. Kebin Joy says:

So, does that mean the sun's tidal force will make the earth's one side to lock permanently to Sun one day?

Absolutely loving this series.
My only peeve is listening to an American pronounce kilometre. 😂 Just sounds odd.

53. Kitty_ cat__02 says:

What is gravity? What is the tidal force and how does it relate to gravity?

How does tidal force affect the water and beaches on Earth?

What is tidal locking and how are the moon and earth tidally locked?

What happens during a new moon and how does it affect tides? What is it called?

54. Zion Thomas says:

I just started this and I picked this cause we started doing this at school and seriously does anyone feel like this guy looks like and idiot

55. Zion Thomas says:

It not like I don't like this guy it just I swear he looks like a idiot to me and don't tell e to kill myself

56. Texas Gaming says:

Did anyone see the Enterprise E

57. Dragomir Alex says:

The thing that I didnt understand you said that the buldge on the earth is slightly ahead of the moon and it’s pulling the moon kinda like you pull a dog on the leash, so why does the moon recese instead of comming ever so close like mars moon??
And tides slow thing down does that mean that in the future the moon will come to a complete stop, and will the earth eventually come to a complete stop via the tidal forces exerted by the moon/sun? And what will happen after, do these objects simply start rotating backwards?

58. evoman1776 says:

Just found these crash course videos and I like them a lot. Very informative. My only peeve is your use of the metric system. The vast majority of people watching will be Americans…like yourself and we don't USE that system. It's annoying having to recalculate in my head the distances you're saying.

59. Mark Kearney says:

i think it is nice thanks guys

60. Joakin Perez says:

Omg I love this it explains everything perfectly

61. Wdude04 says:

guys… thE URL HAS FBI IN IT

62. Vivienne Cronk says:

This could be a better video in my opinion.

63. Phoenix Uprising says:

Battle of the Bulges can be a title for an episode in Game of Thrones.

64. Abhinav Sharma says:

Does Jupiter have tidal effect on Earth??

65. ヨーソローちゃん_17 says:

I don't understand, have to watch it again in the future

66. LiViro1 says:

8:50 My God pops up regularly in these shows.

67. Kayan Yuyutsu says:

So moon gives tides on earth and due to those tides it itself gets away from earth.. then why do we say that it will at some point lose the earth billions of years from now.. I mean if it is getting away slowly the tidal forces must also be getting fainter at the same rate…. it's like Zeno's paradox

68. tgreg63 FTW says:

how do you like live?

69. Elonzo Smith says:

sad to see at 6:23 the moon isnt shaped like an egg

its a darned shame

70. Rogan Phi says:

like ya telescope

71. Jinx _Box says:

Whz do we have 2 spring tides if when a moon is on the opposite side compared to sun gravitational forces supposed to nullify eachother )ofc moon force is 2 times rgeater so not to 0) the other part about neap ticdes shouldn't they switch too when there is 1st neap it have bigger  low tide and lower high isn;t that changes in a 2nd neap tide period ? Hope for answer

72. Nathan Taylor says:

This is a waste of a lot of time

73. Nathan Taylor says:

Tell me please how this will effect me in life as a job in future, Im gonna work as a author and a floorer

74. John Ongolla says:

This explanation was partially wrong

75. kzenias says:

Maybe I'm dumb but I've watched this at least 8 times in the past 4 years and I still don't understand

76. lisa Pecora says:

phil you made my son get 100 on a science test on tides

77. Augusto Freitas says:

I guess he didn't really explain why there are two tidal bulges on opposite sides of the Earth. One is indeed caused by the gravitational pull from the moon, but the other one (on the opposite side of the Earth) should be explained by the centrifugal force caused by the Earth's rotation around the Earth-Moon's center of mass (greater than the gravitational pull from the moon on this side). By the way, I'm an oceanographer.

78. CuteCatsMeow says:

Oof, we love science homework.

79. Luke Petterson says:

Your next series of videos should just go over each episode in more depth this is fascinating

80. Eklavya Goyal says:

"If you were say about 300 km's tall"
I died imagining that

81. Brent Granger says:

As a guy who has studied military tactics, I've always wondered what the new moon has to do with an amphibious assault like at D-Day and Incheon. Phil answered my question! Thank you!

83. C. Crespo says:

Honey that shirt no

84. HAROHAR K S says:

very super exploration

The center is being pulled from the far side? That sounds so bogus! Please tell me that’s not the accepted narrative.

86. Yours Truely says:

So why is gravity in the center not less then halfway. In the center all mass is on the outside?

87. Yours Truely says:

The earth has a molten core so this also has a tidal effect? Earth quakes?

88. kevin griffin says:

This is hella confusing 💀😡

89. james forest says:

Doesn't this cross into the geology field

90. Kevin Engstrand says:

Why can't i jump higher when its high tide if its able to sway the entire ocean one direction or the other.

91. ChrisBrengel says:

If Shakespeare had been an astronomer he would have named all the moons of Uranus.

92. Karl Bergen says:

Tidal behavior verys locally. In the Pacific coast there are two unequal tides. On Atlantic coast there are two equal tides. In the Gulf of Mexico there is only one tide. This happens because of land masses.

93. Jessica Johnson says:

wonderful explanation and images to follow; thank you!

94. davekirk100 says:

Sorry (3:02) a football is round, a rugby ball is ovoid. I think what you call 'football' is a misnomer, coz it's rarely kicked, just thrown a lot. (UK bias)

95. TBNRZen says:

tides

96. wascally wabbit says:

So gravity then

97. Taylor Garcia says:

May you do a Crash Course on Bathymetry: Mapping the Ocean Floor for Oceanography?

98. glen gravell says:

when you consider this the effect of tides – their cycles and the history of the Earth and it's cycles (24hr-365day-24000yearand others ultimately between the last cycle one lasting 1,5billion years or so
you have to consider the FACT that all life on Earth is Binary in nature AND the 24,000 Cycle represented by Astrology could be part of a binary cycle AND the disaster that wiped out the Pleistocene – whose size and world encompassing scope has ruled out being caused by many things – including asteroid and/or cometary impacts BUT it has not ruled out – another star's planet temporarily coming close enough to drag all the water over to one side of the Earth and hold it there for a few days (which is one form of disaster that has not been ruled by any of the regions of the Earth that were destroyed geological remains AND is absolutely included in some legends from areas around oceans – which describe enormous tides that they had to keep the boats at ready for – We live on a Binary Star System

99. Bruce Frizzell says:

Thank you for the many subtitles.

100. Major Tom says:

He copied from Vsauce, but he fails at delivering the bombing shenanigans that are mindblowing. He's still a good guy, but nevertheless, weaker than Vsauce by quite some hundred miles.