Friday, August 8, 2014

Sky Brightness

"The sky above the port was the color of television, tuned to a dead channel."
- Neuromancer,
William Gibson, 1984.

At the Ladd Observatory we operate a weather station and a number of other rooftop instruments to monitor the environment. One of them is a sky brightness meter. On a regular basis we use the live data to judge the quality of the sky for observing. It is also used to document long term changes such as the increase in light pollution.

Sky brightness meter and camera on the roof
Sky brightness meter
and camera on the roof.
The meter is contained in a weather proof housing next to a wide field sky camera. The camera takes a low resolution image of nearly the entire sky every 10 seconds and these images can then be compared to the brightness readings. I can then verify what the sky looked like when a measurement was taken. When the sky is very cloudy it scatters light from the city and the readings are very bright. Haze or high humidity can also cause elevated readings.

The sensor is too sensitive to take a measurement during the daytime. It starts collecting data shortly after sunset when the sky begins to darken and stops during morning twilight just before sunrise. Last summer I calibrated the meter and we've now collected 300,000 data points in about one year. I thought this would be a good time to analyze what we have so far.

A lens is used to only detect light coming from a spot in the sky about 20 degrees in diameter directly above the Observatory. It reports the value in units called magnitude per square arcsecond or mpsas. For the convenience of local amateur astronomers I have converted this to the more common naked eye limiting magnitude or nelm. The conversion is only approximate for a number of reasons. For example, that nelm depends on the eyesight of the observer. But we'll use it here as a familiar way of thinking about brightness. Here is a graph of the data from a recent night.

Sky brightness graph

There is a sharp drop in brightness as the sky darkens just before 9 pm local time and a steep rise after 4:30 am when twilight begins. At the start of the night the full Moon was very bright but it was low near the horizon and outside the direct field of view of the sensor. Only moonlight scattered by the sky registers. The sky gradually darkens as we approach the time that the Moon sets at 3:15 am. During the early morning stars of about magnitude 4.5 would be visible overhead.

Plotting a histogram of all the data from the past year shows the values that we typically see. Readings between 2 and about 3.5 (or brighter readings not shown here) are from overcast nights with thick cloud cover. Between about 3.5 and 4 are hazy or very humid nights, but these are nights when some stars can be seen. When the meter registers between 4 and 5 the skies are dark and as good as it gets for observing at this location. At a place farther away from artificial lights you would see readings greater than 5 indicating a darker sky. We never see skies that dark in Providence.

Histogram of sky brightness for one year

The limiting visual magnitude is about 4.1 - 4.3 for most clear nights (or more accurately early mornings which is when we see the darkest skies.) There is Space Math (PDF) document from NASA that gives a formula for the number of stars that can be seen for a given nelm. It works out to about 600 - 800 stars for the conditions that we most frequently get here. That estimate is a bit optimistic, however. We are only measuring the brightness directly overhead and there is more glare from street lights lower in the sky. I would guess that only about 300-400 stars are visible before dawn and fewer still in the evening when most people observe.

I'll be writing more about the analysis of this data and what it means for the loss of our ability to view the wonders of the night sky. But for now I'm going outside to tune the sky to a dead channel...

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