Monitoring Light pollution

Why is this important?

Knowledge of light pollution is a crucial topic for astronomers, because optical observations need clear and dark sky conditions. But not only sky observations, also the ecological environment is disturbed - a large number of insects die, biodiversity gets lost, birds loose orientation in light polluted areas and perish due to debility. Our aim is to conserve the night sky and therefore we still have to learn more about the development of light pollution over time. What is the actual situation in Austria? And what are the most important impacts?

How we quantify light pollution

We measure zenithal sky brightness in mag/arcsec² with Unihedron's Sky Quality Meter. Our measurements are being carried out at the following locations
IFA: Institute for Astrophysics of the University of Vienna, 1180 Wien
FOA: Leopold-Figl-Observatory for Astrophysics, Mitterschöpfl, Lower Austria
BA1: Potsdam-Babelsberg, Leibniz-Institute for Astrophysics.
GRZ: Graz-Lustbühel, Observatory of the University of Graz, Austria
STO: Stockholm, Department of Astronomy, AlbaNova University Center, Stockholm University, Sweden
Our raw data have a time resolution of about 7 seconds, but the data can also be displayed in a smoothed form with 1 minute time resolution (option "noise reduced data"). The data from Graz-Lustbühel have a time-resolution of 90 seconds.

NEW: High level data products

Based on our measurements and those provided through our Upper Austrian partner network (running 23 SQM stations as of March 2018), we have derived high level data products that are available at

Select location and date (see above for the location codes)

SQM-LE calibrated data plot

We operate all our SQM as they get delivered by the manufacturer. All plots and values found on this site are already corrected, no further calibration is necessary.
Value for Calibration (already included): -0.110 mag/arcsec²
Calibration change in dataset: no

statistics for sky data after/before nautical twilight for the night from 24.05.2020 to 25.05.2020 at location IFA


sun eq. alt
mean 18.16 0.0059 0.0185 -10.5 0.97
std. dev. 1.11   0.87
best 19.52 0.0017 0.0053 -12.5 0.00
worst 15.97 0.0442 0.1389 -8.5 4.18


*) Sky luminance in cd/m² can be converted from mag/arcsec² by a correlation found at Unihedron's website.

**) The illuminance in lux can be found from luminance in cd/m² following Lambert's cosine law. Although the surface of the hemisphere is 2*Pi, we have to take in account that the luminous intensity decreases with the angle of incidence, whereas the luminance in cd/m² is isotropic due to geometrical aspects. Therefore we see that the illuminace in lux can be calculated by multiplying the cd/m² value by a factor of exactly Pi.

***) This is the equivalent altitude of the Sun below the horizon in degrees which would, under natural conditions, correspond to the measured sky luminance value.
The underlying table of values for solar illumination on horizontal surface at various solar altitudes above the horizon can be found on the website of Paul Schlyter, Stockholm.

****) CSF: see below

Naked eye limiting magnitudes (NELM) and Clear Sky Factor (CSF)

The following diagram shows the Naked Eye Limiting Magnitude (red dots) and the Clear Sky Factor (blue crosses). NELM is a calculated value which represents the faintest star barely visible by human eye. CSF is a number which represents the cloudiness. Basically it is noticed that the light fluctuates more when clouds are overhead, therefore CSF is the sum of the absolute differences between 10 readings taken 1 minutes apart. That is a pretty good indicator of a clear sky (0 = clear, above 1 = cloudy).


CSF calculations are inspired by Unihedron's "Clear Sky Detection Experiment".
Calculations of naked eye limiting magnitude are based on the formulas published by Bradley E. Schaefer, 1990. The result given in foot-candles is converted to lux by multiplying with a factor of 10.76. In the last step the conversion from lux to mag is done by knowledge of the illuminance in lux of an A0V star. You can download a Mathematica Notebook file (version 6).

Calculations of sun and moon for the given date

geographical coordinates of the
SQM-LE location @ IFA
* Civilian Twilight21:17:0858993.8869004:27:5558994.18605
* Nautical Twilight22:08:3858993.9226603:36:1558994.15017
* Astronomical Twilight23:18:0758993.9709102:26:1358994.10154
Moon data for 25.05.2020 00:00:00 ME(S)Z
 moon set
moon set
moon rise
moon rise
***24.05.2020 21:32:0058993.8972225.05.2020 05:58:0058994.24861


*) Calculations of Twilight are based on Almanac for Computers, 1990 published by Nautical Almanac Office, United States Naval Observatory, Washington, DC 20392

**) Calculations of moon's phase and distance are based on the CPAN Astro::MoonPhase module 0.60 which refers to the algorithms found in Practical Astronomy With Your Calculator by Peter Duffett-Smith, Second Edition, Cambridge University Press, 1981 Translated to PHP by Stephen A. Zarkos, 2007
The given diameter and distance refer to Earth's center

***) Calculations of moon rise and set found in Astronomy on the Personal Computer, 1994 by Oliver Montenbruck and Thomas Pfleger, Springer Verlag 1994, 3rd edition ISBN 3-540-63521-1
PHP port written by Matt "dxprog" Hackmann
bugfixed by Johannes Puschnig, 2011

Helpful online calculations can be found at the Astronomical Applications Department
of the U.S. Naval Observatory

For more information contact Johannes Puschnig or Web