DARK SKY
MEASUREMENTS
Getting to Know the Borrego Springs Night Sky
Why do we measure the night sky brightness?
Being a certified international Dark Sky community comes with responsibilities, among which is a strong commitment to protect the nighttime environment of our community.
The first step to be able to act and make the right decisions to protect our dark skies is to measure the night sky’s brightness on a regular basis at defined locations chosen for their representativity of the night sky in the area. This is a requirement to determine the success of our lighting regulations.
The Borrego Springs Dark Sky coalition has determined 5 locations around town where the sky brightness is manually measured and recorded regularly. Click the buttons below to discover the latest manual reading data.
In addition, we maintain a permanently mounted Sky Quality Meter (SQM) on Saint Vincent Drive. This device records readings every 10 seconds from sunset to sunrise. A graph is produced, that shows the time of night along the X axis and the measurement in astronomer units on the Y axis. Hovering your mouse on the measurement dot will provide indications about the conditions of the measurements, such as the lunar and solar altitude, azimuth and the lunar illumination. You can pick a particular date to produce a graph, and you will find the best reading for that date at the top of the page. Click the button below to discover the data recorded ay this location.
how do we mesure the sky darkness?
Measurements are taken using a device called a Sky Quality Meter (SQM).
The SQM is pointed to sky’s apex and thus detects the brightness of the light in a section of the sky. It provides a reading expressed in magnitudes per square arcsecond (mag/arcsec2) or mpsas, also called “astronomer units” by Dark Sky International. This uses a negative logarithmic scale, and lower numbers indicate brighter skies. Bright daylight will give readings below 5 mag/arcsec2 while the darkest skies on earth are 22 mag/arcsec2. Measurements around Borrego Springs are typically 20 – 21.2 mag/arcsec2.
Readings are affected by moon shine, clouds, and dust. Light pollution can only be quantified in the absence of residual sunlight, moon shine and clouds. That is why measurements are taken, if possible:
- during astronomical night, which is the period when the sun is 18 degrees or more below the horizon;
- when there’s no moon present in the sky;
- when the sky is clear, as much as possible.
More polluted skies are brighter at night and fewer stars are visible. Keeping a record of the measurements over time, as well as the conditions in which they were made, enables the Dark Sky Coalition to follow the quality of the Borrego Springs night sky and to take action as needed to protect it.
