View Full Version : question about Spectroscopy of the sun

2012-Jan-05, 07:08 AM
Hi, first post. Found this site through the Skeptic's Guide to the Universe.

The Observation: The sun changes colors from a whitish yellow overhead to an orangish yellow near the horizon. As I understand it, the process is attributed to Rayleigh scattering, where the shorter blue light scatters more than the longer red light in a prism-like fashion, and this effect is more pronounced as the light passes through a longer gauntlet of atmosphere near the horizon.

But the illusion of a color shift persisted, and I began to wonder: What would I see in a spectroscope? The above description asserts a rainbow that is bright in the red, fading to dim in the blue, with all the absorption lines in the exact same place.

So: Is this what we see? I've tried to make a homemade spectroscope with a prism and a box, but it is a crude instrument. I've heard that astronomy spectroscopes are very sensitive, so I'm wondering if anyone has tried this little confirmation.

The Question: Is there any measurable shift in the spectral lines between observing the sun overhead and near the horizon?

2012-Jan-05, 04:58 PM
Indeed, the solar absorption lines stay at (essentially*) the same wavelength when observed at various altitudes. However, absorption lines imprinted by the Earth's atmosphere (which are very numerous in the infrared) do naturally get stronger as we look through a longer column of air, and this distinction has been used in making solar spectral atlases to correct for the Earth's atmospheric contribution (as here (http://diglib.nso.edu/Wallace_2011_solar_flux_atlas.html)). The atomic and molecular processes giving these absorption lines are different from the wavelength-dependent scattering that gives the overall color.

(*There is a small change in the Doppler shift of the Sun due to different directions that the Earth's rotation is carrying us, zero near solar noon and largest at sunrise/sunset, with maximum amplitude +/-0.3 km/second. People working on detection of exoplanets have to deal with this or it would swamp the signals they are measuring, which may be 100x smaller).

2012-Jan-07, 10:13 AM
Here is a nice image of the solar spectrum