the south-east (i.e. towards the city center of Vienna) at an elevation of $\sim$45$^{\circ}$. Frames of 300s integration time were obtained. All spectroscopic observations were carried out during the evening hours. Some of our night sky spectra were obtained as dedicated observations, others were extracted as a by-product of stellar spectroscopy, in which case they were subtracted as background. The 2D raw spectra were dark-subtracted and flat-fielded with dome flats (using light bulbs mounted on the telescope). To extract the 1D spectra, all pixel rows that were illuminated by the narrowest slit were summed up along the slit direction. The wavelength calibration was done using the sky emission lines themselves by adopting a second-order polynomial to map pixel positions to wavelength values.

We expect that night sky spectra will change worldwide during the next few decades due to increasing numbers of light emitting diodes (LEDs) used for street and facade lighting. It is desirable to monitor the resulting changes in the average night sky spectra in order to see whether indeed the relative flux at short wavelengths ($\lambda$ $<$ 500nm) will thereby increase (which would be unfortunate for many reasons).

Data analysis
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In the following all given daily mean 