What prevents us from seeing the stars?

Last night I was in Brussels, Belgium, and as the sky was clear I had the chance to look for stars at an open area just outside of the Brussels Central Station. When I first looked up, it seemed as if there weren't any stars at all, and I think this is the impression that most people would have. But when you look at the dark sky for a bit longer, and screen out some of the glaring light using your hands, it turns out you can see quite a few stars!

Here is my observation:

The limiting magnitude was around 3.5, which means that a few constellations (including Cassiopeia and the Big Dipper) were visible. But I think if I had asked passersby how many stars they thought they could see, their initial guess would be either zero or perhaps "one or two". The problem is not only that the sky is bright, but especially that glaring lights and illuminated buildings make it hard to look at the sky with dark adapted eyes. Here is a street view several blocks away from the station:
 

"Street view in Brussels" by Christopher Kyba is licensed
under a Creative Commons Attribution 4.0 International License.

If you have to look up directly into a streetlight, you're not going to be able to seem much more than the streetlight... Contrast that contemporary view with van Gogh's impression of a night scene in Arles.

Van Gogh's scene is notable in how there are no glaring lamps at all, and for the relative darkness of the building facades. The scene provides an inspiration of what I think we should aim for in urban lighting: warm feeling light, no glare, no street light directly shining on windows, good uniformity in the pedestrian areas, and above it all, bright stars shining visibly in the night sky.

---

If you look up again at my observation, you will see that for a number of stars I had to report that I wasn't sure if I could see them or not. These stars were mainly in the constellation Draco, which is quite extended, and therefore hard to orient towards when the sky is bright. I hope that in a future version of the app, we will be able to further optimize which stars the app asks you to examine. I recently requested funding from the EU for a big project, in which the Loss of the Night app plays a small part. If the proposal is successful, I would be able to produce a new version of the app sometime in 2019 or 2020. Please cross your fingers for me!

Timelapse video of conversion of lamps from HPS to LED in Innsbruck

Christoph Malin sent me this video quite a while ago, and I have been meaning to post it to the blog for ages. Check out his time lapse showing the same streets in Innsbruck, Austria lit with HPS and LED lights.

Christoph is a photographer and cinematographer. You can follow him on twitter.

Luojia-1, a new night imaging cubesat

This week I had a visit from Xi Li, a professor at Wuhan University in China. Among many other things, we discussed the Luojia-1 cubesat which was designed by Wuhan University and launched several months ago. The satellite is already providing exciting imagery from a number of locations around the world. For example, compare the image of Berlin from the DNB monthly composite (left) to the Luojia-1 image (right) below.

Two images of Berlin at different resolutions. Tegel airport is at top left,
Schoenefeld/Berlin Brandenburg International airport is at bottom right.

Luojia-1 takes images at a resolution of about 130 meters, a major improvement over the ~750 meter resolution of the DNB. The higher resolution makes it possible to see smaller features, such as the major road network of cities or individual bright large buildings. The downside of high resolution is that Luojia-1 can only take images of smaller areas, rather than imaging the entire world the way DNB does each night.

In the coming years, Luojia-1will create a map of light emissions for the entire country of China. It will also acquire images of a number of cities worldwide, and will be tasked to take images of some areas for humanitarian regions, such as to provide information about infrastructure damage in war zones or overview of areas experiencing wildfires.

The data acquired by Luojia-1 is made freely available a short time after it has been transmitted to Earth. The data is not yet radiometrically calibrated, but it is already georeferenced. (Note: in some of the photos from June, I noticed some relatively large errors in the georeferencing.In images from July, the error is typically only a few pixels).

The team from Wuhan University has already produced a web portal to allow everyone to discover and download the data.When you go to the site, a login screen will appear with the public login information already filled in. Click up at the top right to change the interface to English and log in.

From the next page, you can use a tool to draw a polygon to select your region of interest and check whether imagery is available. Keep in mind that the satellite has only been acquiring data for a few months, and because it is a cubesat the total bandwidth available for downloading data is limited.

I am looking forward to seeing the interesting results that come out of this new data source (as well as the data from another upcoming cubesat from the Adler Planetarium in Chicago)! If you have questions about the data or would like to request imagery for a specific location, contact Xi Li.

 

Update 2022/06/06: Nicholas, for some reason Google isn't letting me reply to your comment below, so I'm adding a line here. I'm not sure what your units are (e.g. nW/cm^2sr or W/m^2sr), and in any case I'm not quite sure what sort of typical values one should expect from Luojia one. You may wish to contact Xi Li.

Perspective of a facade from above

One of the systematic issues that nighttime imagery has to deal with is the fact the orientation of lights and their position relative to buildings and other objects affects whether they can be seen from above. Take for example, these two images of the area near Berlin's Zoologischer Garten station.

This work by Alejandro Sanchez de Miguel, Christopher Kyba, and Freie Universität Berlin
is licensed under a Creative Commons Attribution 4.0 International License.

When our aircraft was north of the area, it was able to see north facing facades that were illuminated. These illuminated facades are invisible, however, when viewed from the South. This means that when satellites take images of the Earth at night, the "answer" for how bright a city is depends to some extent on where the satellite was when it took the image.

Update (August 5, 2019)

My student Jacqueline Coesfeld wrote a paper that uses these images, and considers how this effect (and others) result in variations in how much light is seen by satellites from night to night. The paper is Open Access, so check it out!

Great streetlamps in a Spanish village

Last summer I spent a single night in the Spanish village of Sant Pau d'Ordal. My room had a balcony, and a colleague and I spent several hours in the evening sitting on the balcony, talking shop, and watching a distant thunderstorm. The experience was really enjoyable, thanks to the well directed lighting in the village. Here is the view of the street from the balcony:

This work by Christopher Kyba is licensed under a
Creative Commons Attribution 4.0 International License.

This work by Christopher Kyba is licensed under a
Creative Commons Attribution 4.0 International License.

If the village lamps had directed light upwards, then the balcony would have been flooded with glaring light, and the experience would have been much less enjoyable. Beyond the full cutoff nature of the lamps, I was also impressed with the extent to which they kept the light on the street and sidewalk, and avoided lighting the trees. Here's the view from street level in daytime, notice the flat full cutoff design:

This work by Christopher Kyba is licensed under a
Creative Commons Attribution 4.0 International License.

Great job Sant Pau d'Ordal, and thanks for a wonderful experience!

Instructional video for the Loss of the Night app

Here is a video tutorial for using the Loss of the Night app.

It was created by two students at Brandeis University, Anna Bulger and Mei Cornue-Hollander. The video was part of their coursework in Colleen Hitchcock's "Citizen Science: Bridging Science, Education and Advocacy" course at Brandeis. Thank you very much Mei and Anna!

If you prefer text to video, you can also find instructions for both Android and iOS here on the blog.

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.