Depending on the position of the sun, especially at different heights of the sun above the horizon, the solar spectrum changes due to varying degrees of absorption of radiation by the layers of air passing through.

Here we show a series of spectra taken on a rather humid, cloudless summer day. This meant that there was a relatively large amount of water vapor in the atmosphere, which was particularly noticeable in the solar spectrum when the sun was low.

Spectrum when the sun is high in the sky

When the sun is high in the sky, the effects of the atmosphere on the light passing through it are relatively small. The Fraunhofer lines that can be observed therefore consist mainly of absorptio lines that have already formed in the solar atmosphere, such as the metal lines caused by iron (Fe), magnesium (Mg) or sodium (Na).

All spectra shown here were recorded with a spectrometer from our optics kit in a Czerny-Turner arrangement. This is unprocessed raw data from the sensor. As the spectral sensitivity of the detector has not yet been corrected, the shape of the sensor signal does not correspond to that of an approximately ideal black body at a temperature of 5900 K (approximate temperature of the sun's surface). This means that neither the ultraviolet component at wavelengths below 400 nm nor the infrared component above 800 nm is detected, as the CCD sensor used is not sensitive in this range.

Spectrum when the sun is low in the sky

When the sun is lower in the sky, however, the light travels greater distances through the earth's atmosphere, which means that absorption effects caused by components of the atmosphere come to the fore.

The following series of images shows the changes in the spectrum as the sun gets lower and lower above the horizon. The last image was taken shortly before sunset.

¹ The data of the Fraunhofer lines shown are taken from the (german) Wikipedia page on Fraunhofer lines. Here you can also find further information on the Fraunhofer lines.