# Does light emitted from the edge of the sun have a different wavelength than light em

#### mcgucken

When we look at the sun, does light emitted from the edges of the sun have a different wavelength than light emitted from the center of the disk?

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#### GOD__AM

I would imagine that there is a small difference. Since the sun rotates on its axis, one edge is accelerating towards us and the other away. So the biggest difference would be from edge to edge across its equator.

#### mcgucken

Has anything like this been measured? Is there any experimental data regarding this? Thanks!

#### Mariko

I would tend to think it does not because all light moves through space at the speed of light but there could be a bit more light dencer perhaps at the center of the sun disk than at the edge hmm ill do some more research interesting question.

I am thinking the inverse square law would apply to measuring...

#### Astronuc

Staff Emeritus
mcgucken said:
When we look at the sun, does light emitted from the edges of the sun have a different wavelength than light emitted from the center of the disk?
You would be referring to the Doppler effect. It would be a simple matter to take the speed of the emitting surface and calculate the Doppler effect. One could confirm by looking at the Doppler shift of emission lines from the edge of the sun rotating toward and the edge rotating away, to which GOD__AM alluded. I don't believe the effect is significant, IIRC.

Certainly the speed of light is constant, the frequency however is influenced by the speed (velocity) of the emission source.

#### Chronos

Gold Member
There is a small shift favoring longer wavelengths due to limb darkening. There is also a very slight doppler shift.

#### Dr.Brain

Fraunhoffer Lines first analysed by Fraunhoffer himself depicted different wavelengths coming from the sun.

BJ

#### Creator

mcgucken said:
When we look at the sun, does light emitted from the edges of the sun have a different wavelength than light emitted from the center of the disk?
Yes, and the difference in wavelength (doppler shift) between the eastern to the western solar limb (at the equator) is how we can determine the amount of rotation of the solar surface...

Creator

#### Labguy

Creator said:
Yes, and the difference in wavelength (doppler shift) between the eastern to the western solar limb (at the equator) is how we can determine the amount of rotation of the solar surface...

Creator
Being just gas of increasing density, I don't really know if we can call it (the photosphere) a surface.(?) Also, since the Sun has a differential rotation of about 25 days at the equator and ~29 days at high latitudes, does anyone know (legit question) if we have instruments sensitive enough to measure the very small difference (doppler) between the approaching and receding "edges"? 25 days in a long time for one rotation...

I had always thought that we observed the rotation rate by observing sunspots and convection granules at different latitudes.

#### Chronos

Gold Member
Labguy said:
...does anyone know (legit question) if we have instruments sensitive enough to measure the very small difference (doppler) between the approaching and receding "edges"? 25 days in a long time for one rotation...

I had always thought that we observed the rotation rate by observing sunspots and convection granules at different latitudes.
The Michelson Doppler Imager has a resolution of about 20m/s [compared to the solar equatorial revolution speed of about 2000m/s].
http://soi.stanford.edu/science/obs_prog.html
Astrometric dopplers can resolve line of sight velocities down to about 3 m/s [!].

#### laam

There are gravitational effects that draw the photons emmited from the outer edges toward the centre of the sun-observer axis in accordance with einsteins theory of general relativity. So the photon energy is decreased as energy is lost gravitationally, therefore further redshifting emmitted photons from the outer edges in addition to the aforementioned rotational doppler shift and limb broadening.

#### jns90

Labguy said:
Being just gas of increasing density, I don't really know if we can call it (the photosphere) a surface.(?) .
The layer of gas in the sun that emits visible light (the photosphere) is relatively thin, and from Earth it looks like a surface.

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