Hipparchus was the first to develop a star catalog of the night sky using his naked eye. This catalog contained about 850 stars that he discriminated by their brightness into six categories or magnitudes (1).
Nowadays that would have been an impossible task to repeat from a human settlement. The artificial lights that illuminate our cities are being poorly addressed and they emit light into the sky waning the darkness of night.
The transparency of the sky is altered by this new and subtle ambient pollutant, Light Pollution, which is defined as: excessive, misdirected, or obtrusive artificial (usually outdoor) light (2). The statement of defense the night sky and the right to starlight (StarLight Declaration) warns of the progressive degradation of the night sky, which should be considered equally to other problems related to the environment.
Astronomical observatories located in cities, which usually are of great importance in the academic training of future scientists, are seriously affected by artificial light, which is main responsible for dimming the light of stars.
The Central American Astronomical Observatory of Suyapa (OACS), located at the campus of the National Autonomous University of Honduras (UNAH), is immersed in the city of Tegucigalpa, so it is surrounded by lights from early afternoon (Figure 1).
Our work has consisted in estimating the sky background brightness (SBB), fitted with photometers Sky Quality Meter (SQM) that measure the SBB in magnitudes per square arc second (mag / arcsec2). These devices, which offer fairly reliable measurements, have allowed the construction of polar maps of the sky brightness.
The results (Figure 2) show that the OACS sits under a sky brightness reported magnitudes ranging from 16.78 mag / arcsec2 near the horizon, 18.09 mag / arcsec2 at 60° in height and 18.74 mag / arcsec2 at zenith.
For comparison, measurements were also made in the area of Monte Fresco (Figure 3), located outside the city, the map does show the improvement of sky brightness in remote areas. Heading into the city, measured values were around 18.47 mag / arcsec2 between azimuths of 210° and 270°, other measurements show a dark sky of 18.74 mag / arcsec2.
Additionally, a photographic study of the city, which can be consulted on the website http://luminicaunah.blogspot.com evidences a luminous halo that rises above the horizon reaching heights of up to 15° (Figure 4). This usually is oriented between the SW and NW, which produce the dimming of the stars as they set.
In order to know the evolution of SBB over time, we have installed a SQM-LU which performs monitoring during nights, the plots are available here. These figures reveal that brightness variability might be explained by the presence of clouds. Clear nights at OACS (Figure 5) have reported a SBB of 19.0 mag / arcsec2 classifying them as suburban skies suitable for astronomical observation.
1 – In astronomy, magnitude is the logarithmic measure of the brightness of an object, measured in an specific wavelength or passband. The magnitude system was introduced by Hipparcus, who introduced six categories with the stars he could observe using his naked eye. Note that the brighter the star, the smaller the magnitude. For instance, the Sun has an apparent magnitude of -26, the Full Moon of -13, objects with magnitude 6 are the typical faintest light that we could observe with the naked eye, and magnitude 31 is the faintest light that can be observed with the Hubble Space Telescope. More information: http://en.wikipedia.org/wiki/Magnitude_%28astronomy%29
Javier Mejuto (@JavierMejuto) is Head of Department of Archaeoastronomy and Cultural Astronomy Department in Universidad Nacional Autónoma de Honduras. He is a member of SEAC (European Society of Cultural Astronomy) and SIAC (Interamerican Society of Cultural Astronomy). Besides several Cultural Astronomy research projects he is currently involved in projects in History, Archaeology and Astronomy, mainly related to American indigenous people.
Ricardo Pastrana is a teacher of astronomy at the National Autonomous University of Honduras, developing teaching activities, entailment university-society and research. Taking place in the latter works on light pollution. He is also ambassador NASE (Network for Astronomy School Education) IAU program that has as main objective to educate the new generations of teachers and retrain the current on the teaching of astronomy.
David Espinoza is the coordinator of the Central American regional academic master’s degree in astronomy and astrophysics from the Universidad Nacional Autónoma de Honduras. He earned a bachelor’s degree in Physics at the UNAH and his master’s degree in physics and mathematics at the University of Granada in Spain.