# Thread: How much does our sun wobble?

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## How much does our sun wobble?

Does the sun wobble more or less when the planets are aligned? Is it something we can detect?

2. Well, it's a standard problem for undergraduate astronomy students to figure out the magnitude of the Sun's motion around its center of mass with Jupiter. The answer turns out to be about 12 meters per second. Since Jupiter is by far the largest influence on the Sun's motion, you can make a rough estimate that at some times, the Sun will appear to be moving toward us at around 12 m/s, and at other times, moving away from us at around 12 m/s.

You could use the ordinary Doppler formula to compute how large a shift in the wavelength of sunlight this motion would cause. And then you could examine the properties of some standard devices for measuring wavelengths (or frequencies) of visible light, and see if those standard devices could detect these shifts.

Have fun!

3. But is that what DaCaptain is asking? To me, "wobble" means "the axis is tilting", not "the center is moving." To put it another way, "How do the planets affect the precession of the Sun's axis?"

ETA, for that matter, does the Sun's axis precess?

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Originally Posted by selden
But is that what DaCaptain is asking? To me, "wobble" means "the axis is tilting", not "the center is moving." To put it another way, "How do the planets affect the precession of the Sun's axis?"

ETA, for that matter, does the Sun's axis precess?
StupendousMan had it right, initially what I was wondering was how much the sun wobbles around the center of it's axis. How comparable is that to the wobbles of stars we are finding that have exoplanets?

Does the sun wobble along it's axis as well? Is there any way to determine if it has ever flipped it's axis?
Last edited by DaCaptain; 2018-May-08 at 02:17 AM.

5. Originally Posted by DaCaptain
Does the sun wobble along it's axis as well? Is there any way to determine if it has ever flipped it's axis?
I don't know, but probably, though probably not very much. I think it's a fallacy though to think that is something precesses a lot that it will "flip". That's probably due to the experience of seeing a top flip, but that's because it's anchored to the earth on one end. It wouldn't happen to a top if it was free floating.

And there is no evidence that the sun ever flipped. The sun rotates counterclockwise (seen from the north) as do the orbits of all the planets and the rotations of nearly all the planets.

6. Originally Posted by DaCaptain
StupendousMan had it right, initially what I was wondering was how much the sun wobbles around the center of it's axis. How comparable is that to the wobbles of stars we are finding that have exoplanets?
Most of the radial velocity variations caused by star-planet interactions which we have detected so far are much larger than the 12 m/s caused by Jupiter.
Here, take a look for yourself -- the data in the histogram below was generated just a moment ago on the NASA Exoplanet Data Archive:

https://exoplanetarchive.ipac.caltech.edu/index.html

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Originally Posted by StupendousMan
Most of the radial velocity variations caused by star-planet interactions which we have detected so far are much larger than the 12 m/s caused by Jupiter.
Here, take a look for yourself -- the data in the histogram below was generated just a moment ago on the NASA Exoplanet Data Archive:

https://exoplanetarchive.ipac.caltech.edu/index.html
Wow, that really gives you a sense of how small our solar system is compared to those we are finding. Does larger radial velocity also mean the planets are orbiting quicker? Closer to their suns?

It seems it also points out that we can most easily see the larger systems.
Last edited by DaCaptain; 2018-May-08 at 03:25 PM.

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Originally Posted by DaCaptain
Wow, that really gives you a sense of how small our solar system is compared to those we are finding. Does larger radial velocity also mean the planets are orbiting quicker? Closer to their suns?

It seems it also points out that we can most easily see the larger systems.
The radial velocity method preferably detects planets around
1. Small stars
2. with big planets
3. that has close orbits/short orbital periods
4. where the angle between the plane of the orbit and our line of sight is small.

Radial Velocity = Mass of planet * Velocity of planet (depends on mass of star and period of orbit) * sine(inclination of orbit) / mass of star

If any of three first factors is too small or the fourth too large the radial velocity can become very small, especially #4, which is 0 if the orbital plane and our line of sight is perpendicular. As we all know if you multiply any number by zero the answer is zero.

The mass of the star and the period of the orbit can be calculated with reasonable accuracy from commonly available data, the other two are almost always unknown unless there is direct evidence (e.g. the planets is also a transiting exoplanet). This means that the planet masses derived from radial velocity measurements is almost always minimum masses with no upper bound.
Last edited by glappkaeft; 2018-May-08 at 06:02 PM. Reason: Messed up plane vs LoS angle description + corrected spelling

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Also short orbital periods mean that you don't have to wait as long for good data (several orbits). This is especially important since orbital periods can easily be in the tens or hundreds of years and multiple planets can make the situation complicated to unravel with insufficient data.

10. Originally Posted by StupendousMan
Well, it's a standard problem for undergraduate astronomy students to figure out the magnitude of the Sun's motion around its center of mass with Jupiter. The answer turns out to be about 12 meters per second. Since Jupiter is by far the largest influence on the Sun's motion, you can make a rough estimate that at some times, the Sun will appear to be moving toward us at around 12 m/s, and at other times, moving away from us at around 12 m/s.

You could use the ordinary Doppler formula to compute how large a shift in the wavelength of sunlight this motion would cause. And then you could examine the properties of some standard devices for measuring wavelengths (or frequencies) of visible light, and see if those standard devices could detect these shifts.

Have fun!
My bold. No, it would not do so. The Earth, relatively close to the Sun compared to Jupiter's orbit, will be moved nearly in unison with the Sun by the gravitational action of Jupiter. Of course there will be differences because of the gradient over the radius of Earth's orbit, but I would expect the net radial velocity of the Sun relative to the Earth to be well under the gross amount of plus or minus 12 m/sec.

11. D'oh! Of course Hornblower is right.

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