Why are there TWO high tides per day?

We all learn that the tides are caused by the Moon pulling on the Earth's oceans. So how come there's two high tides per day but we only have one Moon?

00:00 - Introduction
00:35 - One Moon, Two High Tides?
00:54 - Is it the Sun?
01:07 - Spring Tides
01:16 - Neap Tides
01:42 - The Earth-Moon Barycentre (centre of mass)
02:45 - Planetary wobbles
03:09 - Detecting exoplanets with star wobbles
03:20 - Wobbles create centrifugal force
04:29 - A lunar day of 24h50m
04:56 - Brilliant
06:06 - Bloopers

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📚 Note, USA & Canada hardback out 1st November 2022

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👩🏽‍💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford (Christ Church). I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.

[lorenat]

Whoa!!!! 😲 The beloved BARYCENTRE!! ❤️😎 Thank you Dr Becky!!!

[cybermonkeys]

Spaghettification in “linear” free-fall motion or Spaghettification in “orbital” free-fall motion? hmm… (get you with the long words and caps-locks!) I don’t know what the Earth is thinking!! but, I think I’ll stick with Dr Becky’s explanation - thank you very much - As I have a liking to staying alive! It’s called the tidal force, why is this so difficult for you to understand?

[tomasnemec]

Thank you Dr. Becky - I could not get my head around this for years and this is the 1st time someone actually explained it so clearly! I love your videos.

[KathleenGuillot]

I love the way you explain concepts. The questions you ask and the way you link all the components to clarify the explanation. This video is awesome but uses Southern Hemisphere orbits(clockwise). Could you also make one for Northern Hemisphere with counterclockwise graphics? The students of Georgia, USA would be very grateful!

[wavydaveyparker]

This makes total sense.
It’s extremely difficult to visualise the effects of gravity in a four-dimensional manifold, using a two-dimensional rubber sheet, in a three-dimensional time space. All you achieve is a demonstration of gravity, using gravity! Which, rather defeats the objective.
The gravitational and centrifugal effects of motion around a barycentre, both alleviate the gravimetric tensor of the earth’s own gravitational attraction, and allows water to flow in sideways from the adjoining areas to equalise the system…and, we call this movement of vast quantities of water…tides. All perfectly sensible. Thank you, Dr Becky.

[somewherenorthofstarbase]

Interestingly, the solid ground beneath our feet also rises and falls in tides, though this is much harder to see. One way scientists have observed it is through pressure variations in oil bodies deep beneath the surface, directly attributable to the Moon. And in many high-energy accelerators in which sub-atomic particles travel at near-light speeds in tunnels beneath the ground, we need to apply corrections to the magnetic fields that guide them to compensate for the physical distortion of the ground as the Moon passes overhead. In all of these systems, we see tides

[RussellChapman]

Grew up in a small village on the coast. Tides can be massive. On the fridge was a tide table so we didn't get caught out by the the tide when going to Mersea Island. The Strood can really catch you.

[wavydaveyparker]

It may surprise a lot of people that the centrifugal “effect” has something to do with tides, but it’s really just “Inertia” in action, and all the matter in the Universe is accelerating away from each other, at an ever increasing rate (?) because, the gravity is unable to negate the expansion.

[maximumkillmtg]

Thank you. Not only does this answer my question, it also does so in a way that I can understand well enough to explain it to my children.

[prasenjitpatel]

thanks a lot for this wonderful and simple explanation
lots of videos have uselessly complicated things

[jeffmckeown]

I'm sure many have said this already. I've tried wrapping my head around this reading, watching videos, discussing with others, thought experiments...a phenom woman in STEM explains it in 3 minutes.
Now to have her explain literally everything else.

Thank you. This was driving me bonkers.

[oortcloud]

The funniest part of this excellent explanation is: 06:55 Here you go!

_"You say either, and I say eyether._
_You say neither and I say nyther._
_Either, eyether, neither, nyther..."_
*Let's call the whole thing off!* ~ written by George Gershwin and Ira Gershwin.

You're so funny Dr Becky. Thanks for that appraisal of reference frames, it was hilarious! 😄

[wavydaveyparker]

The two tides are of approximately equal height on both sides of the globe shaped planet, because in the *equilibrium* of tides theory, the centrifugal effect is the same across the whole earth, whilst the moon’s gravity diminishes with distance by the inverse square. 👇 Thank you.

[wavydaveyparker]

This explanation is much better than others, because it’s simplified to a point that is correct and easy to understand. The tides are essentially caused by the inertial motion of planets and oceans around a common centre of gravity, in a non-uniform gravitational field. I see no reason to mention the SpaceTime extravaganza here, as it only deals with a misleading static scenario of tidal forces and makes no mention of the tidal effects produced by orbital motion around the sun.

[wavydaveyparker]

Dear Dr Becky, this video is exceptional and I sincerely apologise for not arriving earlier - _sorry!_ - seeing as I’m having trouble making a video myself, to correctly explain tides to a younger audience and have only produced something to dispel the totally incorrect assumption that the Earth moves directly towards the Moon in a straight line filling up bulges. This seems like the perfect place to post my boring lengthy comment on the matter…so here goes! Enjoy:

When Isaac Newton first proposed the theory of Gravitation in his _‘Principia’, _ around 300 years ago! He unfortunately, neglected to give us the mechanism by which _‘gravity’_ worked! However, he did provide us with some very beautiful and simple equations, to correctly calculate the movement of Planets. His problem started with his acceptance of _‘absolute time’_ and distinguishing the difference between _‘gravitational’_ and _‘inertial’_ mass, which he considered to be equivalent, but being the genius he undoubtedly was, he agonised over that conclusion and knew it might not be completely accurate.

It wasn’t until Albert Einstein around 200 years later, that we were finally given the more complete answer to gravity, when he told us that the natural state of things is _‘free fall’_ ... and the _‘mass’_ of an object had the potential to curve the space around itself, within the confines of his proposed idea of _‘spacetime’, _ which he presented in his brilliant theory of General Relativity.... _I hope that helped?_ 🙃

As for the tides... Well, they’re not that difficult to understand really? Think of it like this... At the very centre of the Earth, there is a centripetal ‘force’ of gravity pulling it towards the Moon... And there is also a centrifugal ‘force’ _(I use the term loosely, because it’s a ‘apparent’ force in an inertial frame, but the Earth-Moon system is definitely a rotating, non-inertial frame and it exists)_ due to the inertia of the moving objects wanting to continue in straight lines!

These two effects ‘balance’ according to Newton’s Third Law and the orbits continue, because the ‘net’ forces at there centres are zero. The centres of the Earth and Moon are indeed moving on spacetime paths, and don’t experience a ‘net’ acceleration, because they are in free-fall around each other, but still under the influence of a non-uniform gravitational field.

When we work out the tides, we use the ‘tidal force’ equations, which effect an object from the centre outwards.

Therefore, ‘No!’ pulling, or tugging away from oceans or moving towards the moon filling up bulges is necessary. Just the knowledge to realise that the Earth-Moon system operates like any two-body problem.

Of course, we then have to correctly apply the method of resolving these associated acceleration vectors into there respective components and recognise the buildup of sideways pressure in the underlying water column, which is what actually drives the tidal systems, but that’s not my particular area of expertise?

Take care and thank you so much for making this video.

Kind regards, Wavy 👋.

[Kram_Farkel]

I have watched dozens videos that try to explain this phenomenon and this is by far the easiest to understand. Your students are in great hands Dr. Becky.

[scottnorton]

You are seriously a world treasure. Thank you for giving us these amazing concepts in ways that we can understand!

[skidmo]

Dr. Becky - nailing the explanation on tides in a simple to comprehend video. It's like you're super smart or something.

[roscothefirst]

I love your pronouncing "Wobble" it sounded like Wooble
I grew up right beside the highest tides in the famous Bay of Fundy, and had my own fishing skiff at the age of twelve -
your explanation is much better than anything else I've heard.
And I later worked as a geologist at Bedford Institute of Oceanography in Nova Scotia
The funnel shape and the shallower (shoaling) depth towards land accentuates the tidal bulge in the Bay of Fundy
One unusual effect is that during ebb flow you can catch a temporary eddy close to shore that actually carries you in the opposite direction to the tidal currents in the main channel- handy in a row boat -very localized by the current jetties or promomtories such as reefs - fly fishermen use the same technique in rivers

[augustkatana]

The Bloopers are adorable.
May the force be with you!!