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Comment by Stratoscope | Hacker News Reader

I learned something the other day that blew my mind. We are now able to take pictures of the far side of the Sun.

How? We could put a telescope in Earth orbit 180 degrees opposite from us. But we don't have one there yet.

We use sound waves.

Sound? In the Sun? Don't you need air for that?

No, any gas will do (solids or liquids too). The Sun is a big ball of very dense gas, perfect for propagating sound waves.

So we image the near side and analyze distortions caused by the sound waves traveling through the Sun. The resulting images of the far side are low resolution, but they are good enough to "see" things like coronal mass ejections and the giant sunspot that rotated into view last week.

https://www.google.com/search?q=solar+sound+waves

https://www.cora.nwra.com/~werne/eos/text/solar_waves_main.h...

https://www.cora.nwra.com/~werne/eos/text/farside.html

https://www.space.com/thanksgiving-sun-sunspot-treat-2020

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That blew my mind, too, when one of the astrophysicists at the university I was attending grad school at told me this. He also had a telescope set up with a solar filter so people could look at the sun. It was moderately active at the time, and I saw this cluster of sunspots that looked quite a lot like the Hawaiian islands. He told me the biggest one was much, much larger than the diameter of the Earth, which also blew my mind.

We're actually kind of lucky to have a fairly typical, main sequence star right in our back yard to study.

There is the anthropic principle regarding that last: if the star wasn't typical and stable, life wouldn't have developed here to observe it.

The anthropic principle doesn't need it to be typically stable though, right? It could be exceptionally stable, in a particularly evil universe, and we'd most likely crop up in a nice little stable oasis.

If it were unstable during the 4 billion year history of life it could wipe life out and we wouldn't be here to observe it. But it depends on the magnitude of "unstable".

Our sun is exceptionally stable - it has no close encounters with other stars that might disrupt planetary orbits, which does happen in galactic nuclei or globular clusters. We are in that nice little stable oasis.

We may not have a satellite at 180 degrees, but we have STEREO A and B at 120 and 240, used along with the SDO to give us a pretty full view of what’s going on

Can we take pictures of the sun at night?

No you have to wait for the sun to come up.

The sum total effect of small quips, when people upvote them, is the degradation of forums to reddit as people seek easy points.

While not "directly", we can look at the sun through the Earth: http://strangepaths.com/the-sun-seen-through-the-earth-in-ne...

Wow, that's definitely amazing.

This reminded me that the way humanity knows about the interior structure of the Earth is also amazing -- and one important source of information is looking at propagation and reflections of earthquake vibrations!

Physics is amazing. Reminds me of non-line-of-sight optical imaging which makes perfect sense scientifically but still boggles my mind. [1]

[1]: https://news.ycombinator.com/item?id=20813557

Physics is the language that never ceases to blow my mind.

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