Wikipedia:Reference desk/Archives/Science/2021 January 29

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January 29

baseball in space

If the inhabitants of a Stanford torus or something similar formed a baseball team, would the Coriolis effect have any sort of effect on the ball? For example, if I were standing on the habitable surface of my rotating space station, and threw the ball straight up (up meaning from my perspective experiencing the pseudo-gravity of the rotating space station) would it seem to me to curve away in an unusual manner?

Duomillia (talk) 01:19, 29 January 2021 (UTC)

It will rotate around the space station in the opposite direction, this is then the centripetal acceleration of the ball in the rotating frame, so you have a fictitious centripetal force in the direction of the rotation axis. This fictitious centripetal force is the sum of the fictitious centrifugal force, which is equal in magnitude as the centripetal force but it points away from the rotation axis, and the fictitious Coriolis force which has a magnitude of twice the centripetal force and points in the direction of the rotation axis. Count Iblis (talk) 02:12, 29 January 2021 (UTC)

So the short answer is "Yes". The effect is very asymmetric in one direction. Given a hollow spinning disk instead of a torus, a ball thrown vertically up in the air at just the right speed (there is actually an infinite number of speeds that will do), neglecting air forces (assume a vacuum), will make a loop and return to its point of departure. The initial deviation of the ball's curve from the vertical is in the direction the space station is rotating. To an outsider, the ball's trajectory is a straight line through the centre of rotation. For an inside observer, stationary in the rotating frame, the trajectory is part of a two-armed Archimedean spiral.  --Lambiam 10:31, 29 January 2021 (UTC)

What are the lines in these satellite images?

I see a series of faint but visible neatly parallel lines in the sea in the following satellite images of the same area: false-color and natural color. What do those lines represent? StellarHalo (talk) 15:20, 29 January 2021 (UTC)

It could be the imaging error explained in the 'Landsat 7 Scan Line Corrector (SLC) Failure' section of this page, although i'm not sure. Zindor (talk) 17:10, 29 January 2021 (UTC)

According to the USGS Landsat 7 page, the SLC failed on May 31, 2003, and affects all subsequent data. But the image in question is from September 22, 2002, more than half a year before the failure happened. Therefore, I don't think we're seeing that particular artifact here. --Amble (talk) 17:43, 29 January 2021 (UTC)

The features I'm seeing look like ocean waves. --Amble (talk) 17:58, 29 January 2021 (UTC)

I'm not sure whether I'm seeing the same (artefact?) as the OP but both images have two versions on Commons. The first version of the false-colour image was lower resolution but doesn't seem to have the same striations, so I wonder if this is caused by the .jpg rather than the camera. where did you get the higher resolution version from, StellarHalo, as you were the one who uploaded it? Mike Turnbull (talk) 18:11, 29 January 2021 (UTC)

The higher resolution version I uploaded is from https://avax.news/pictures/44776 as stated in the source information. StellarHalo (talk) 18:19, 29 January 2021 (UTC)

Where exactly are the lines? I can only see what looks like natural patterns, that to me look like sediment plumes. PaleCloudedWhite (talk) 18:37, 29 January 2021 (UTC)

I am not talking about the sediment plume directly coming out just above the delta. The lines I am referring to a series of curved ones that are found all over the ocean area of the image. Admittedly they are not easy to see but they form a pattern. I am trying to get the false-color image featured on Wikimedia Commons and someone there asked me about it. StellarHalo (talk) 18:58, 29 January 2021 (UTC)

I can't see anything except sediment plumes and deposits, so either my eyes can't see what others see, or the person who commented at the FPC page is mistaken when they said that sediment plumes "wouldn't be so neatly parallel". PaleCloudedWhite (talk) 19:14, 29 January 2021 (UTC)

Both pictures have the pattern, though I find it a lot easier to see in the natural colour image. For example, in the extreme top-left corner, the false colour pic is pretty murky while the same area in the natural colour pic shows distinct banding of, well, blue-green and green-blue, I guess. I don't think they're waves; if they were, I think we'd see them affected by the land form and I don't think that's happening. Instead, they're all at about 40 degrees off of straight up and down (sorry, I don't have a protractor handy). To me it looks like a Moiré pattern. Matt Deres (talk) 19:53, 29 January 2021 (UTC)

In the true-color image, I see what seems to be the same pattern -- in the same direction -- over some parts of the river and lakes. Probably not ocean waves, then. --Amble (talk) 21:11, 29 January 2021 (UTC)

I don't want to speculate, but I would emphasize that the provenance of an image on the internet - and the chain of image-processing-operations that have been performed to transform scientific data into a viewable image file - is almost impossible to prove. The artifacts may be real (radiometric artifacts, physical terrain, ocean waves, sea surface variations); or they may be a processing artifact (algorithmic; compression; intentional or ununintentional; ... we just can't know. This is the science of photogrammetry and radiometry. Anyone who pretends to tell you a simple answer that explains why any individual pixel is brighter or darker in your "photograph" isn't thinking about the problem hard enough - and if they extend that answer to explain why you see a pattern of dark- or bright- ... well, they're basically just repeating one wrong/incomplete answer a million times.

If the answer matters - if you're using the image for a scientific data processing operation - the procedure you'd want to use is:

• Find the exact source data that was used to produce this image - in this case, you're looking at an image composed from Landsat 7 data, on or around September 22, 2002. You can pull up the exact orbit. For what it's worth, the SLC device was on and operational at that time (you can verify using the metadata for any of the Level 1 or Level 2 products).
• Learn everything about the way that source data is transformed - in this case, the Landsat Product Generation System software.
• Run that software yourself - and verify the effect is visible in the pixels rendered on your screen. And then... conduct radiometric and algorithmic analysis to determine where that digital signal originates, and then conduct some earth science and optical physics to explain what physical process corresponds to the digital signal that is measured.

If satellite imaging was easy, satellite imaging scientists wouldn't need to ask rocket scientists for help getting their cameras in to position.

Nimur (talk) 20:49, 29 January 2021 (UTC)

It looks consistent with Landsat coherent noise: [1], [2]. From the abstract of the second link, which is a journal article: "A coherent pattern of system noise observable on all visible wavebands of Landsat Thematic Mapper images over homogeneous surfaces such as waterbodies is regarded as serious enough to impair visual interpretation and affect image analysis results." --Amble (talk) 21:24, 29 January 2021 (UTC)

(ec) Since none of the waviness is visible over land, I think we can exclude digital artifacts as explanations. The patterns I see are compatible with waves. The parallel strands do each extend lengthwise over only about four to six wavelengths; then the pattern is interrupted as if by interference. Think of an arrangement of frankfurters (wieners). The points of interruption, where complimentary waves appear to cancel each other, follow a pattern of roughly straight lines roughly orthogonal to the waves. These could be wave patterns in the sediment.  --Lambiam 21:27, 29 January 2021 (UTC)

When I posted the above, I had not yet seen the refs to the USGS page and the article in the International Journal of Remote Sensing. So apparently the issue does not manifest over land, which invalidates the conclusion I drew in the first sentence.  --Lambiam 21:34, 29 January 2021 (UTC)

Interesting. I work in hydrographic surveying, and know about and can recognise all sorts of artefacts in sonar imagery that others might e.g. misinterprate as the lost city of Atlantis. But I know next to nothing about Landsat images and the sort of artefacts you get in them, and would have misinterpreted those as "almost certainly waves on the sea surface, or possibly large-scale ripples on the seabed". Iapetus (talk) 17:45, 30 January 2021 (UTC)