Earth's rotation, with the camera locked to the sky instead of the ground | Hacker News Reader

Earth's rotation, with the camera locked to the sky instead of the ground

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399 pointsbehnamoh51 comments

Straight to the source:

https://artuniverse.eu/gallery/190705-rotation24h

How is this actually accomplished, though?

The camera was on an equatorial telescope mount.

"An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis, the polar axis, parallel to the Earth's axis of rotation."

An equatorial mount keeps the camera pointing in a fixed celestial direction by rotating at the same rate as the Earth in the plane of the equator. It's essential for any long exposure of the sky where you don't want star trails, the unusual thing here is having the horizon in frame.

    Mount Sky-Watcher Star Adventurer
    Tripod Manfrotto MT055XPRO3

The tripod is a good, heavy, stable one. https://www.manfrotto.com/us-en/055-aluminum-3-section-tripo...

The head is https://www.skywatcherusa.com/collections/star-adventurer/pr... which is a bit more fancy than the old school equatorial mounts.... but

    Star Adventurer 2i multi-function equatorial tracking mount with built-in Wi-Fi control

It's still an equatorial mount.

https://astrobackyard.com/equatorial-telescope-mount/

https://en.wikipedia.org/wiki/Equatorial_mount

You adjust the base to the latitude that you are at so that the mount can rotate to match the Earth's rotation on one axis, and then the other axises can track ascension and declination - which remain constant in the sky.

This differs from the altazimuth mount ( https://en.wikipedia.org/wiki/Altazimuth_mount ) where a celestial object is that that location now, but moves.

Sirius is at 06h 45m 08s RA and -16° 42’ 57”... but as I type this, as seen from Greenwich, United Kingdom it would be at -27.4° altitude (below the horizon), 277.5° azimuth - and that coordinate system moves.

Instead of turning with the earth, it stays "in place". Meaning it turns to the opposite of the Earth's rotation.

It is easier to understand if you imagine that the observer is at the south pole looking up, instead of thinking of them on the equator going around. In reality, they are somewhere between the two extremes.

You could also get the same effect with a regular tripod, time-lapse photography and a rotation effect. The sky would look normal though, not like in this. https://en.wikipedia.org/wiki/Time-lapse_photography

Sibling comments say/explain how it is done 'properly', but surely you could just 'lock to the ground' and then animate it flipping 'in post'? It's not like we need the fancy camera mounts to know how the rotation goes?

Wouldn't the equatorial mount prevent star trails? I guess you could compensate with a shorter exposure time, and perform the rotation in post. I assume it's only possible to do from some angles of the sky.

The linked source names the mount used, the Sky-Watcher Star Adventurer, but yes, you could replicate this in post, except that the frames have exposure times of up to 45 seconds, which would give star trails within individual frames.

This gave me a bunch of insight into the workings of those old survival tricks of using analog watch hands to find cardinal directions:

- Since the video starts at solar noon, the sun is always at 12:00 relative to the video.

- Swapping reference frames, if you had a clock face with 24 hours on it, and aligned the "12:00" to point at the equator, the "0:00-12:00" axis would define a longitudinal plane through the Earth, and the hour hand would define a second plane that would intersect with the sun--the hour hand would "follow" the sun.

- Conversely, if you pointed your clock's hour hand at the sun, you would know your "12:00" would be due north/south (depending on hemisphere).

- The same is true for conventional 12-hour watches and clocks, but you must find the "half-way" mark between your hour hand and noon, because the hour hand is moving at twice the speed relative to the hypothetical 24-hour clock.

Neat!

> Conversely, if you pointed your clock's hour hand at the sun, you would know your "12:00" would be due north/south (depending on hemisphere).

Not quite. In the southern hemisphere the sun moves 'backwards', so you have to flip the clock backwards, or instead point the 12:00 mark at the sun and then the hour hand shows north/south.

Just a word of caution for anyone that would want to use thos in an actual survival situation. If you're really lost, the best thing to do is stay put and wait for someone to find you. If you have to move, knowing which direction is North is pretty useless unless you know enough about where you are and the surounding area. And even then, in such situations, topographical landmarks will likely be more useful.

Not to be a nay sayer, it can be fun to do, and perhaps even useful in some situations. But probably not something that's going to save your life, no matter how many time Bear Grillis used it.

Also featured on NASAs APOD: https://apod.nasa.gov/apod/ap200701.html

Its a classic effect, there are many examples, for example: https://www.youtube.com/watch?v=w8OK7M2_hUg ... I wonder where this idea got started

Once you start looking there are all sorts of interesting ones, like this one showing different perspectives/projections https://www.youtube.com/watch?v=JmCNNHQ86NE

This is a good illustration of how unintuitive heliocentrism really was, specially before we had a theory of gravity: it really made more sense to assume that the heavens were populated by ethereal bodies floating around a heavy, immobile Earth.

A bunch more examples here, which show how your latitude changes how the rotation looks:

https://www.youtube.com/watch?v=72hBReBQRAg

and a few more:

https://www.youtube.com/watch?v=1zJ9FnQXmJI

https://www.youtube.com/watch?v=zRTJ5ISmVXE

There's a reddish haze near the ground (which rotates with the earth). What is that? Light pollution?

Does anyone live in an area that has breathtaking views of the night sky? Able to see satellites? Do you sit outside each evening to marvel at how beautiful it is?

During two separate trips, I had the pleasure to be in an area that had zero light pollution. So amazing. Two of my favorite experiences.

I'm in the Murchison quiet zone in Western Australia. [1]

There's zero (0 , !!ZERO!!) ambient manmade light at night *, ourselves and our few neighbours have no outside lights, there is no city light popping up over the horizon, and there is very little radio frequency noise.

> Able to see satellites?

* Way too many - the freaking low orbit Starlink constellations smear trails across the sky as they reflect sunlight over the horizon back here onto the ground at night. There are moves being made to make them less reflective and to have ALL satellites turn off transmissions when passing overhead here .. but that's still largely an agreement in principle and still needs full implementation.

> Do you sit outside each evening to marvel at how beautiful it is

All the time, it's a past time that locally goes back several tens of thousand of years.

Growing up I was in a town further north but equally isolated, we watched movies at night on a big outdoor screen and often could see meteor showers up behind (say) Midnight Express and other movies.

[1] https://www.atnf.csiro.au/projects/askap/ARQZWA.html

Do a barrel roll!

Reminds me of when I would make myself dizzy by pretending the sky was down and the earth up, and it felt like the only thing keeping me from falling trillions of lightyears through the inky void was that I was stuck to the ceiling of my home planet.

Love watching the shadows, it’s like they are being pulled down by gravity

I get the impression that this shows a sort of 'double-dip' sunset where, just before it goes completely dark, it brightens briefly (and the converse at dawn.) It seems to be there when I step through the frames. Can someone say if this is an illusion (or a personal delusion on my part), a meteorological effect, a camera sensitivity-adjusting effect...?

It doesn't seem to change in intensity, but in what is illuminated. While high, sun itself appears bright, when dropping lower the things lighted from the side, and at the end scattered light at the horizon.

Thank you for your reply. I have to admit that I was paying attention to the sky, rather than the surface. The effect I am referring to can be seen in the transition from the 0:05 and 0:06 time in the video, and I see that what I perceive as a brightening of the sky is also accompanied by a brightening of the nearest structure. If I am not mistaken, the sun is already below the horizon during this sequence.

Of course, if this is just a delusion on my part, what I have written will make no sense to you!

I see what you see but I'm not sure whether it's a real effect or just an artifact of the photography. There must be some adjustment of exposure to have both the daytime and the nighttime well exposed, so it may just be the changing exposure that makes the brightness of the horizon non-monotonic.

I wish there was a way to experience this first hand, but it gets a bit boring looking at the same point in space for 24 hours.

Even watching the video, I still feel like its the sky that moves, not us. Its wrong, but I can't shake that feeling.

The 21st century version of a Foucault pendulum [0]

[0] https://en.wikipedia.org/wiki/Foucault_pendulum

Heres a song I suggest a listen to when contemplating the universe https://youtu.be/9mvZ-FjdgLw

I can't decide whether I'm relieved or disappointed that you didn't take advantage of the fantastic rick-rolling opportunity.

Ah, Black Hole Sun.

Is that the Southern hemisphere? It looks foreign to me.

The source[0] mentioned in other comments says it is in Namibia. Good eye!

[0] - https://artuniverse.eu/gallery/190705-rotation24h

Yes, I think those are the two Magellanic Clouds visible to the right of the center of the image.

You can also quite clearly see the Southern Cross (Crux) and the pointers (Alpha & Beta Centauri) about one quarter of the way from the left edge.

Train your mind and imagine how this video would look on the north pole and south pole! Guess such footage already exists on YouTube.

I tried and it's easy to think of looking straight 'up' along the rotation axis. While keeping the horizon in the focus is not so easy.

Yes. But you can also point your camera in directions other than straight up and get different paths.

For example if you point it towards the horizon the camera will rotate 360 degrees and it will record a full view of all the land around the camera.

If you're on the equator and you point the camera "up" towards the azimuth, then after 6 hours it will be pointed towards the horizon on the west. After 6 hours the camera will be pointing straight down to the ground etc.

But if you're in the equator and point the camera to the horizon towards the north, then the camera will roll and you'll see the ground rotate around similar to this video

Flat earthers will just say its the Photoshop AI.

Interesting. Maybe they will say that the flat Earth is doing a barrel roll?

Sheesh, it's no wonder the flat-earthers have such an easy time baiting "scientists".

Very interesting content but... why can I not download it? Whats a crappy website that tries to force me to stay with them

That peeved me too. I wanted to save something cool.

yt-dlp works on the artuniverse page

Works fine for me. Have you tried Firefox?

That's really your user agent's fault, not the site. I can right click on the video and click "Save As" without issue.

You’re right!

Direct link to the video:

https://files.mastodon.social/media_attachments/files/110/53...

I share this complaint with nearly every podcast site. We all know the episode is just an MP3, but sites generally refuse to have a clear download link, and often just push you onto pointless sites like Apple Podcasts or Spotify. As someone who uses a very locked down browser, podcasts should be one of the most easily accessible forms of media and yet they all seem to go to a lot of trouble to hide the actual files from users.

Podcasts without RSS are not podcasts.

Ask Apple. Other platforms let you save whatever you want.

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