Sunday, June 9, 2013

Measure the Night Sky with your Phone

People with an interest in light pollution can now use their smartphones to measure the brightness of the night sky. There are new apps out for both iPhone and Androids that are easy, accurate and inexpensive.
The iPhone app is called Dark Sky Meter. It works very much like the much more expensive Sky Quality Meter and its results compare very favorably to it. It is available in both free (also called Lite) and Pro versions. The Pro version currently costs just $3.99.

It uses the iPhone's camera to first take a dark frame (done by placing your finger over it) and then measures the night sky. The app knows date, time, location, moon phase, weather and the phone's tilt so it can generally tell what data is good or not.

Here's a sample of what the readout looks like with the Pro version. 
That's a measurement made from my yard. 20.25 is the night sky brightness as measured in magnitudes per square arc second. While that may be arcane to some, it is a useful number to know. I wont go into the details here, but higher numbers are darker. There's also a description to go with it. Also reported is the naked eye limiting magnitude, which happened to 6.18 at the zenith for me.

The Pro version also lets you see your own data:
That's a nice feature as it lets you track what is going on from your observing site and, if you want, to send your numbers off to programs like GLOBE at Night. Yes, I need to make more measurements. The user also has the option of uploading the data to the app's database. You can view all the data on their interactive map, here. It is good stuff, but more people need to be contributing. The International Dark-Sky Association has worked directly with the app developers and is hoping that crowdsourcing will lead to many useful measurements.

The app is currently most accurate using the iPhone 4S or 5. It works for the 4, but the camera is not as good, so the quality of the data suffers.

Android users should not feel left out as there is an app for them too. The free app Loss of the Night allows Android users to measure night sky brightness by observing stars. The data from that program is also used to help track and study light pollution. As you may have guessed, I do not have an Android phone. So, while I have not personally tried it out I hear very good things about the app and how it works.

It has never been easier to measure the impact of light pollution on the night sky. So why not download one of the apps and help contribute to science?

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