Sun Rotates in 27 Days: How Do They Know?

[engineer_comp_geek]

Moderator Note

@Exapno_Mapcase

I mocked the OP for poisoning the well. Asking the plain question “How is the sun’s rotation measured?” would get a straight answer with no snark. The OP chose to frame the question in a way similar to the way Creationists or birthers ask questions. There’s no place in GQ for questions that start with the premise that all scientists are idiots. That can be argued in several other forums.

The OP seemed to have a fundamental misunderstanding of how stationary sunspots are on the surface of the sun. I don’t see any implication in the OP that scientists are idiots. This makes your mocking very off-topic for GQ. Please do not do this again in this forum.

[Pleonast]

@Jinx

And, surely the sun’s surface is not solid, so is there a stationary point to measure from?

This is a useful question. The term “surface” is not by itself very meaningful for a fluid body. A more precise answer will depend on what’s important to you. Are you concerned about where the photons (which wavelengths?) you see are coming from? How about the density gradient? Or the heat or temperature? Magnetic fields? Neutrino flux? Etc.

[Really_Not_All_That_Bright]

@Little_Nemo

Keep in mind you don’t actually have to watch a particular sunspot make a complete circuit in order to know how fast the sun is rotating. The sun is rotating at a pretty constant speed. So if you track the movement of a sun spot for a few days, you can figure out how fast the sun is rotating.

I think we can both agree that the Sun rotates at a more-or-less constant speed, but that’s still an assumption unless you do track a sunspot all the way around (which has apparently been done).

[Little_Nemo]

@Really_Not_All_That_Bright

I think we can both agree that the Sun rotates at a more-or-less constant speed, but that’s still an assumption unless you do track a sunspot all the way around (which has apparently been done).

Yes, you can go wrong if you collect too little data and then make assumptions about what it means. If you just measured the movement of one sun spot and then assumed it was fixed in place on the surface of the sun, you might get a false impression that the sun was moving when in fact the sun spot wasn’t fixed and it was the sun spot that was moving.

Suppose an alien race was studying the Earth and was trying to decide if the Earth moved or was stationary. But their technology doesn’t let them directly observe the planet. They can only observe objects on the planet.

If the first object they chose to examine was an commercial jet liner, they would observe its pattern of moving in different directions at different speeds. If they assumed the plane was fixed in place on the Earth, they would get a strange impression of how the Earth moved.

But as they continued with different objects, they observe the movement of things like buildings and trees and bridges and telephone poles (alongside the movement of things like cars and boats and elephants). At some point, they would figure out there was a pattern. Some things moved in erratic and random ways. Other things moved at constant speeds in a single direction and all those speeds and directions were connected. The aliens would then figure out that some things were moving independently on the surface of Earth and some things were attached to the surface and were only moving with the Earth’s movement. And knowing this, they could figure out what the Earth’s movement was.

Scientists use this same logic to studying the Sun. They observe the movement of lots of sun spots and figure out if there’s a pattern to it that indicates whether it’s the sun spots moving or the Sun itself that’s moving.

[Chronos]

Even aliens observing nothing but commercial jetliners could eventually deduce the correct rotational speed for the Earth, by averaging many jetliners together.

[scr4]

@Really_Not_All_That_Bright

I think we can both agree that the Sun rotates at a more-or-less constant speed, but that’s still an assumption unless you do track a sunspot all the way around (which has apparently been done).

All it takes is to observe the Sun from many angles - i.e. as it rotates, and/or as the Earth orbits around it - and note that the sunspots are moving across the face of the Sun at roughly the same speed at all times.

[watchwolf49]

So, it seems the Sun rotates at different speeds depending on latitude … do we know why it does? … that’s not the case for Earth am I’m not sure if Jupiter shows this behavior …

Top notch question from the OP … they came to the right place to get the StraightDope …

[Chronos]

The Earth doesn’t rotate at different rates depending on latitude, because it’s solid. Any fluid body, though, will show that behavior, including the gas giant planets.

[watchwolf49]

@Chronos

The Earth doesn’t rotate at different rates depending on latitude, because it’s solid. Any fluid body, though, will show that behavior, including the gas giant planets.

Let me get back to dropping cats and I’ll get back to you with a more intelligent question …

[Jinx]

Thanks all. No, not doubting today’s instruments, etc. This is question that has personally bugged me long before the internet. To those who say in jest (or maybe not) that I am doubting the holy Wiki, no…it’s something I’ve wanted to know for a long time how Galileo (the discoverer of sunspots) could have known [without me knowing how long they last…nor thinking to Google the life of sunspots]…yatta, yatta, yatta!

[scr4]

@Chronos

The Earth doesn’t rotate at different rates depending on latitude, because it’s solid. Any fluid body, though, will show that behavior, including the gas giant planets.

The core of the earth, however, is liquid and does rotate at different rates.