Thursday, January 25, 2018

What's the difference between Globe at Night and the Loss of the Night app?

I am frequently asked about the differences between Globe at Night and the Loss of the Night app. The main differences are in the ease of making an acquisition, and the accuracy of the data.

Globe at Night

Globe at Night works by asking you to compare how your sky looks compared to a series of star charts:

GLOBE at Night star charts for Orion (from the webapp).

After you've looked at the sky, you go to a computer and report what you saw (you can also report your observations immediately on a mobile phone). Depending on how bright the sky is where you live, you will need to give yourself 2-5 minutes before you make a decision. If you are in a natural area tens from any artificial lights, then you may have to wait up to 30 minutes for full dark adaptation (and in that case, you shouldn't use your cell phone!). In general, for someone who lives in the city, you should be able to accomplish the observation in a few minutes. You can do observations anywhere, from the center of an extremely bright city to a wilderness area, and you could write down your observations on paper and report them later.

Since you select the sky brightness based on maps with integer steps, the accuracy of your observation is limited. This means that individual observations are not particularly good measurements of sky brightness,and Globe at Night isn't a good method to see how skyglow is changing in your backyard. However, when we consider a lot of data together, the combined observations are very powerful, and that means Globe at Night data is best for tracking changes at the global, national, state, or potentially city level (if the city is extremely active, with hundreds of observations per year).

Loss of the Night app

The Loss of the Night app was designed to compliment Globe at Night, and to allow people to make more accurate observations (more info). We do this by asking participants whether individual stars are visible. When a participant tags enough stars, we can get a very accurate measurement of the sky brightness for that particular night:

Highly accurate and self-consistent observation from a participant in Portugal.
The more stars you observe, the more accurate your observation can be. The minimum number of stars that we request is 8, which takes an average user about 6 minutes. (The first time you use the app, it will probably take a bit longer. The more practice you have using the app, the faster you will get.) In addition to an estimate of how bright the sky is, we also get information about whether the observations were self-consistent or not. This is useful for researchers, because it provides direct information about the data quality. The more practice you get using the app, the more accurate your observations will become.

We take some precautions to avoid spoiling your night vision while using the app (e.g. we use an all red/black night mode). But because of the backlight from screens, and the need to look at the phone before staring the app, the app is only designed for areas that have quite a bit of light pollution. If you use it at a cabin on the lake, you'll probably be able to see all the stars the app will suggest, and that's not as valuable as a Globe at Night observation would be.

Because the Loss of the Night app observations are more precise, it should be possible to test the effects of fairly large local changes (e.g. a complete change to LED lighting, a new shopping center right next to your location). However, there are variations in how many stars you can see from night to night due to changes in the atmosphere (for example smoke and humidity make it harder to see stars). So to make sure that changes are really due to a change in lights, it would be most helpful to have observations from several nights per year.

 Which app is best for me, or for my group?

If you don't have any background in astronomy and are only interested in donating a few minutes of your time to light pollution research, then Globe at Night is probably for you. If you particularly enjoy spending time looking at the stars, if you have a background in amateur astronomy, or if you are interested in making a precise observation, then you should give the Loss of the Night app a try.

If you want to involve a larger group (like an elementary school class), or if you are planning a citywide campaign, I would suggest that you default to using Globe at Night, unless you've got a really dedicated or specialized group of individuals (e.g. amateur astronomy club).

If you live in an area where you can regularly see the Milky Way, you should definitely stick with Globe at Night, because the Loss of the Night app is intended for places with quite a bit of light pollution (cities and suburbs).

How can I access my data?

I'm glad you asked me that! Thanks to funding from the European MYGEOSS project, we have developed "My Sky at Night" to help you access and visualize your data, and to look at trends (more info here).

Anything else?

Thank you so much for taking part in observing the changes to our shared night sky! If you'd like more information or want to help fight light pollution, please visit the International Dark-Sky Association.