PARI uses a Sky Quality Meter (SQM) to find out. It measures the darkness of the sky on our campus.
A dark sky at night allows quality astronomical observations to be made, but has a variety of other benefits as well. Wildlife, health, energy consumption, and safety, are all enhanced by a naturally dark site. You can learn more about the positive impacts of a dark sky from the International Dark Sky Association at www.darksky.org.
The values shown on the graph are in magnitude per arcsecond squared. On this scale, higher values indicate darker skies. Measurements are taken at 15 minute intervals and are shown in red.
The Moon is shown as a white circle, or crescent, depending on its phase. The height of the Moon indicates how high it is in the sky throughout the night. We expect brighter skies when the Moon is full, or high in the sky, and darker skies when it is new, or lower in the sky. The Sun is shown as a yellow circle, with its height showing its position in the sky.
The International Dark Sky Association designates three tiers to sites that it recognizes as dark sky sites. Bronze is for magnitudes greater than 20, silver for greater than 21, and gold for greater than 21.75. These magnitudes are marked on the graph with bronze, silver, and gold lines.
The left column shows the date and time that each pieces of data was collected.
The right column shows a value that represents how dark it was at that time.
PARI uses a Unihedron SQM-LE.
You can learn more about it at unihedron.com