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.

Monday, December 4, 2017

Light trespass

In previous "view from your app" images we have only had a few examples of light trespass: when light illuminates something it is not meant to illuminate.

My colleague Catalin-Daniel Galatanu sent me an excellent example of light trespass from Iași, Romania. In the photo below, you can see a residential building that is brightly illuminated by the spill light from church facade lighting:

Light trespass by Catalin-Daniel Galatanu is licensed under
a Creative Commons Attribution 4.0 International License.

Catalin used his images to calculate the luminance of the facades, and got the following result:

Light trespass luminance by Catalin-Daniel Galatanu is licensed under
a Creative Commons Attribution 4.0 International License.

The lit areas have a luminance of 2-4 cd/m2. To calculate the illuminance on the windows it would be necessary to know the albedo of the building, but with a rough guess of 0.7 that would imply around 15 lux. I think it would be pretty unpleasant to have such a bright light shining into either your bedroom or living room! Catalin points out that in addition to the light in the room, the people living here have lost not only the view of the stars from their window, but they wouldn't be able to enjoy their view over the city at night, due to the unpleasant glare.

It's important to stress that there's absolutely no reason that it has to be this way. With more carefully directed lighting, the church could be illuminated with limited or no spill light. Furthermore, it's not necessary to shine so much light on a facade for it to have the impression of being illuminated. I hope to soon be able to share a great example of how churches can be more effectively lit (paper in review).

Monday, October 16, 2017

Types of light pollution

Here's a handy image for explaining some of the different aspects of light pollution.
This work is licensed under a Creative Commons
Attribution-NonCommercial 4.0 International License

The image on top right is from Andrej Mohar, the other 5 are from me.

Wednesday, July 19, 2017

When church lighting goes wrong

A blog post in tweets

Friday, July 14, 2017

Great observation from Portugal

I wanted to share a great observation made a few months ago by a Loss of the Night project participant from Portugal:

As you can see, there is a very clear separation (white line) between the stars that were visible (yellow, orange, and brown) and those that were not (black). You can also see how thanks to the observer examining so many stars, you can have a lot of confidence in the result, and can even measure how consistent the observer's result is. When you first start using the app, you might not have results as consistent as this, but with a bit of practice it becomes easier and easier to cover a lot of stars quickly.

This also demonstrates why the app is a better method for estimating limiting magnitude in bright places than star chart based methods like Globe at Night. In this case, the naked eye limiting magnitude was around 3.9 ± 0.1. The star charts of Globe at Night only allow you to choose between integer limiting magnitudes, which in this case would be "about 4". In addition, with Globe at Night we can't be sure how careful a participant is, and we found that compared to skyglow models, the standard deviation of Globe at Night observations is about 1.2 magnitudes.

While the app can provide more accurate data, I want to stress that it's not a replacement for Globe at Night! The app doesn't include stars with limiting magnitudes above about 5.2, so in areas with little light pollution, Globe at Night is a better method. In addition, the Globe at Night time series goes back over 10 years, and there is therefore a lot of value in continuing to contribute to it. In my opinion, it's the best system we have for tracking global changes in skyglow. So please consider contributing to both projects!

If you make an observation with the app, you can easily see a similar plot for your own results. Just head to My Sky at Night, zoom in to the area where you made your observation, and click on it to bring up this chart.

Monday, March 27, 2017

Unnecessary light on a field

My colleague Andrej Mohar recently shared this image with me:

This work by Andrej Mohar is licensed under a
Creative Commons Attribution-NonCommercial 4.0 International License.

The photo shows the light spread from single 20W 4000K LED that was recently installed. The lamp itself is designed to have no direct upward emissions (which is good), but much of the light is shining into an area which doesn't need to be illuminated (which is very bad), and even the area that is intended to be illuminated happens to be a region of very low traffic. It is so out of character for the region it is installed in, that at least two people in the nearby village have already officially complained about it.

In these sorts of cases, it doesn't matter how efficiently the lamp converts electricity into visible light. The light itself is unnecessary, so it is an inefficient use of electrical resources.

Thursday, March 23, 2017

Milky Way and Skyglow from the Fürstein

Martin Würzer recently sent me this amazing panorama showing the skyglow over Switzerland:

Fürstein 360 Panorama v2 by Martin Würzer is available
under a CC BY-NC-SA license

He took the photo on New Year's Day 2017 around 8 pm, from the peak of the Fürstein. The light from Milan is visible at the far left. It's an amazing photo, and you can see it in full resolution by clicking on the name of the photo in the caption above.

Martin also sent me this image, which shows where at least some of that waste light is coming from:

Illuminated facade by Martin Würzer is available under a
Creative Commons Attribution 4.0 International License..

That photo was taken shortly before 3 am! There is some good news associated with this photo, however. By having a polite discussion with the architect and the firm that owns the building, Martin convinced them to remove the light that illuminates the facade, and to also improve the other lamps so that they produce less skyglow. Great job Martin!

For Swiss and other German speaking readers, Martin provided two useful references: