Thursday, January 12, 2017

Converting World Atlas floating point values into sky brightness predictions

Over the past several years, I was involved in the team that published the "The new world atlas of artificial night sky brightness" last summer. My main role in the project was in calibrating the map based on sky brightness observations. The two main sources of observations were the all-sky brightness surveys of the US National Parks Service Natural Sounds and Night Skies Division and observations with hand-held or vehicle mounted Sky Quality Meters (SQMs). In the end, we chose to do the main calibration with the SQM dataset, because thanks to the participation of citizen scientists, it covered locations on all 6 inhabited continents, including many more urban locations than were otherwise available. The NPS surveys, some additional telescopic data, and data from permanently mounted SQMs were then used to verify the result from the SQMs.

Now that the Atlas is published, many researchers are interested in using the map to understand their area. For most people, and also for many researchers, the colorized version of the map that we've made freely available is sufficient. For example, if you're looking for a good place to go stargazing, you'll do fine with the colored map. You can either view the Atlas with from within your web browser, or you can  download a set of tiles for Google Earth.

The rest of this post is technical, and will only interest a small subset of blog readers.

Some researchers will want to use the floating point dataset for further analyses. The data is currently not openly available, but can be requested via a form from this page. The form sends an email to Fabio Falchi, who will follow up with you regarding terms of use*. (Publicly funded researchers from the USA and Germany intending to use the data for non-commercial research purposes should be allowed access without fees. Other national research organizations and anyone interested in using the data or imagery for commercial purposes may be asked to license their use.)

The floating point data are stored in a single GeoTIFF file covering nearly the entire world's extent (arctic latitudes excluded). The map reports simulated artificial zenith luminance in mcd/m2. If you want to use the data to estimate how bright a real sky is, you need to add in the natural portion manually. Natural light at night is quite variable, mainly due to the position of the Milky Way and the amount of airglow that is present.

When we calibrated the World Atlas, Dan Duriscoe from the US NPS provided me with estimates of the natural sky brightness for the specific date and time the observations were taken at each of the tens of thousands of locations worldwide. These observations were taken from 2007-2015, with the largest number of observations taken in 2013 and 2012. Over this time period, the predictions for the natural sky brightness ranged from 21.01 to 22.11 mag/arcsec2. The most typical value was 21.65 mag/arcsec2 (or about 0.236 mcd/m2). The average value changed over the years (due to changing solar activity), from faintest values of 21.88 mag/arcsec2 in 2007 to brightest values of 21.58 mag/arcsec2 in 2014.

Once you choose a natural sky brightness**, you should add that value (in mcd/m2) to the artificial sky brightness from the Atlas. Let's call this value "X". To convert this value into a prediction of what an SQM would see, use this equation:

SQM_pred = -2.5 log10(X/ (10.8 x 107))

I'd like to thank to Salvador Bará for sending me a question that prompted me to write this blog post.

* For researchers using the data in publications, please note that the data DOI is separate from the World Atlas publication. If you write a manuscript using the data, you should separately site both the World Atlas publication and the data DOI.

** Note that in the model we also included an additional free parameter "S", which is a linear scaling factor for the natural sky component. We put it in to account for the fact that the SQM band does not match the V band. The best fit for this value was 1.15, meaning that the average natural sky brightness was increased from 21.65 mag/arcsec2 (0.236 mcd/m2) to 21.50 mag_SQM/arcsec2 (0.271 mcd/m2).

Monday, November 14, 2016

Four super activities for tonight's "Supermoon"

Tonight is the night of the "supermoon", meaning the moon happens to be close to it's "perigee" (the closest it gets to the Earth) at the same time as it is full. As it happens, tonight's supermoon is an especially good match between moment of fullness and perigee: the last time the full moon was this close to Earth was in 1948, and the next time it will be this close is in 2034.

When the moon is closer, it appears ever-so-slightly larger than normal. This makes some people roll their eyes at all the attention, but I think anything that gets people out to experience the night should be celebrated! With that in mind, here are four things you can do to celebrate tonight's moon (the first 3 are great for kids):

1) Watch the moonrise

When the moon is near the horizon, an optical illusion makes it look bigger than usual. This makes every moonrise special, but the experience is most exciting when the moon is big and full. One fun activity, especially for kids, is to bend over and look at the moon upside down through your legs. For many people, this destroys the illusion, so you can switch back and forth between having a big and small moon.

You can look up the moonrise time at your location here. For observers at high Northern latitudes where night is already falling, the moon will appear quite red, because the blue light is scattered by the atmosphere.

2) See how well you can see with only ~0.2 lux.

Once the moon has risen high in the sky a few hours later in the evening, go out into an area that's as free as possible from artificial light. A big (unlit) park or sports field will work in a city, and an open field works best in the country, but a back yard will do in a pinch. You'll probably find that you can see better than you can on a typical urban street, even though lit patches under streetlights are usually 100-500 times brighter than full moonlight. The reason for this is that the moon lights the landscape uniformly, and most importantly without glare. A recent paper argues that even older pedestrians need only 1 lux to avoid stumbling, and this experience shows to what extent we could reduce energy use and light pollution if we improved urban lighting to make it more uniform.

3) Test whether you can see color and read text under full moonlight

Before you head out the the park, grab a colorful magazine and take it with you. Many people can read text and see colors in full moonlight, but a lot of people incorrectly believe that the moon isn't bright enough. Who in your family can read the easiest? Can you tell what all colors are, or just some of them? Speaking of colors, use something to block out the moon and look at the sky around it. Many people experience the sky as shining blue near the moon.

4) Take moonlit landscape photos

If you have a camera with a programmable shutter speed, you can take absolutely wild photos by moonlight. All you need is a tripod or stable surface to rest your camera on (here's a moon landscape photography tutorial in case you want to get really serious). Once the moon is high up in the sky, take a photo of the landscape, and you'll end up with a fully lit scene, but with stars shining in the blue sky!

An example of one of my favorite moonlit photos is below (although this was far from full moon, and the exposure was kept short because the moon was in the photo, so the sky doesn't appear blue):

Moonrise over Nationalpark Müritz by Alejandro Sánchez de Miguel is licensed
under the Creative Commons Attribution 3.0 Unported License.

If it's cloudy or raining where you are, you could try for two other phenomena. One of the hardest things there is to photograph is a moonbow: a rainbow lit by moonlight:

Moonbow, Kula, Hawaii by Arne-kaiser is licensed
under the Creative Commons Attribution-Share Alike 4.0 International license.

Finally, if it's overcast where you are, I'm still waiting for someone to answer my challenge of taking a landscape photo on an overcast night in an area without significant light pollution. Maybe tonight will be the night!

Thursday, September 8, 2016

Second community App experiment!

It's high time for another community experiment using the Loss of the Night app!

Last year we examined how stars come out during twilight, which is also useful for understanding the different times at which stars come out for people. This time, we're going to try seeing how much variation there is in an individual observation due to the random set of stars selected by the app.

To take part, you should make an app observation with 8 stars (quit once you've reached 8 stars). Then, start the app again, and do a second 8 star observation. To make sure that we don't see changes due to city lights going off, both observations should be completed within a single 30 minute period.

Both times you run the app it will probably start with the same first and second stars, but the rest of the stars are likely to be different. Since you are the same observer looking at the same sky, your data will help us understand how much of the variation in observations is due to the design of the app itself, rather than differences in sky brightness.

You can do this on as many evenings as you want to from September 22 until about October 6. I'll present the results in an app newsletter email and here on the blog towards the end of year.

Friday, May 27, 2016

Skyglow surveys with an SQM and the Loss of the Night app

I was just looking through the Loss of the Night app data, and noticed that a project participant used the app and an SQM to do a skyglow survey of the island of Öland, Sweden:

Öland skyglow survey is licensed under a
Creative Commons Attribution 4.0 International License.
Link to the interactive map at

I reached out to Jörgen Tannerstedt, in order to get the story behind the map. Here's what he told me:

The story behind the measurements is that we last year started a project called "Dark sky Öland" in the local astronomical society on the Island of Öland, Grönhögens Astronomiska Förening (GAF). We are a rather small astronomical society, with fewer than 30 members, but we like our island and the darkness that we've got, and we want to protect it for the future and make others aware of it. There are also some plans/thoughts of applying to IDA and try to make some part of the island a dark sky park or reserve. The southern part of Öland is a world heritage site, and perhaps one could try to make that to a light protected area as well.

"Night over the lake" is copyright Jörgen Tannerstedt.
Used with permission.

As a first step in this project we bought a SQM-LU meter to start measuring the sky brightness all over the island. I guess I´m the most active member in our society, so I got the meter in my hands and have brought it with me most of the time when I´m out shooting. I take mostly astroscape images from the island, some of them can be seen here.
I recently got myself a telescope, and had it just set up before we lost the dark nights here up in the north. So now we just want the summer to end and get the darkness back :P August 10th is the first night with astronomical darkness again after the summer break for the island.
"Stargazing" is copyright Jörgen Tannerstedt.
Used with permission.

So far I have measured over a hundred different locations on the island, several locations multiple times and I will continue doing this during the autumn. There are still several good locations left, and I also want to measure in and close to villages to see how much influence they have, and how much light is spread to the nearby surrounding.

It´s rather recent that I discovered the Loss of the Night app, and it has made things so much easier for documenting the measurements. I really like the ease of use, and that the observations are automatically GPS tagged.

Our measurements will be used to evaluate how good and dark the sky is, and serve as a "before" value to see if it gets better or worse in the future. For example, the area near the bridge to the mainland is under heavy construction, and a lot of new houses are being built with road and street lights etc.

The measurements are also going to be used in a guidebook that another member, Lars Magnusson, is working on. In the guidebook, we will include a lot of good locations for astronomers that want to come to the island and experience our dark skies. We will also include information about the different locations, availability, public toilets, photos etc. Hopefully will we have a first version ready this autumn

When shooting from the southern cape of the island, the camera can easily pick up light from cities the other side of the sea, like Gdansk/Gdynia area in Poland. For example, in this picture the light pollution out to the right under the central parts of the Milky Way are from Gdansk/Gdynia, about 250 km away.

If anyone would like to come and visit the island we have the astronomical darkness back again august 10th, and then later in the beginning of september with the new moon, we have our yearly star party "Sagittarius". It's a rater small party with some spontaneous lectures during the daytime and most often a geological excursion and then stargazing all night :D It's always nice to meet others with the same interest. During summertime, there are a lot of tourists here, from Germany, the Netherlands, and Belgium. We've even got a hotel here that is named "Drei Jahreszeiten*". The winter is not that fun, and pretty windy here, so I understand they skipped that season :D But so far I haven't heard anything about any astrotourists.

* "Three seasons"

My hat goes off to Jörgen and the other members of GAF for documenting, sharing, and especially working to preserve your natural starry skies on the island of Öland! Hopefully your book will lead to a few extra astrotourists to fill up the Drei Jahreszeiten hotel!

While it's nowhere near as organized as what GAF is doing, I have taken advantage of my trips to the state of Mecklenburg-Vorpommern in Germany with some similar goals in mind. I also hope this area will someday be home to one or more recognized International Dark Sky Places, where the communities have recognized the value of the night sky as a natural resource, and agreed to work together to protect it. Here's my map:

Skyglow survey MVP is licensed under a
Creative Commons Attribution 4.0 International License.
Link to the interactive map at

If you have an SQM, this method of surveying is a wonderful way to contribute to environmental monitoring of Earth's night! The data are shared with everyone around the world, and they contribute to a permanent archive of the skyglow conditions at the site. If you are planning on founding an International Dark Sky Park or Reserve, this allows you to document the conditions at your site transparently, and future visitors will be able to verify your results and contribute to monitoring changes in the sky condition.

Please note that the best way to do an SQM measurement is to average the result of 4 measurements with your body oriented in the four different compass directions. This is the recommendation from the Loss of the Night Network, and we have found that it considerably reduces the size of the uncertainty compared to taking just a single measurement. Here is the full text of the recommendation from the LoNNe report:

Recommendation #1: When making observations with a handheld SQM-L, you should average the result of four observations, rotating your body after each observation to a different compass direction. If the SQM-L is being affected by stray light, this may minimize or reveal the effect. If the four observations are not self-consistent (maximum range about 0.2 magSQM/arcsec2), then it is probably not a good location, and the data should not be recorded. This technique has been suggested by Andreas Hänel in the past, and we advise all handheld SQM-L users to adopt it.

Wednesday, May 25, 2016

Good Morning Twitter!

Together with Tatjana Scheffler, I recently published a paper where we looked at what time German Twitter users wake up, and how it varies throughout the year. Here's a really short explanation.

What we did

Dr. Scheffler collected and saved (nearly) all the German language tweets for an entire year. She then selected all the tweets that included the German phrase for "good morning" (Guten Morgen). We then analyzed the data to find what time each morning the phrase really starts to take off, and called that the "onset of twitter activity" (i.e. what time people woke up).

What we found

The data can be best explained visually. Here is a plot of what time the sun rises (in Frankfurt) throughout the year. Winter is near the middle, and the "fall back" and "spring ahead" of Daylight Savings Time are shown as dashed lines. We don't change the clocks on this plot, to make it easier to see how wake times relate to the sun.

Now here's what wake times look like on weekdays:

Throughout the year, the typical* wake time on a weekday is around 5:50 am (in local time). You can see that there are a few weekdays that look very different from typical. Those are holidays, and the bunch in the middle are the days between Christmas and New Years.

Now here's what it looks like on Saturdays:

And here's Sundays:

You can see that during the late fall winter, and early spring, the wake up times on weekends are closely related to the time that the sun comes up. But then Daylight Savings Time comes and the relationships break down. Here's what it looks like with all the data together:

The gap between the blue compared to the black and red lines shows you how badly people are punishing themselves by using an alarm clock. About 80% of people in Europe use alarm clocks, which means that they aren't getting a full night's sleep. But what can we do about it?

Take a look at the difference between the blue and red lines. You can see that at the end of March, they're almost reaching each other. If they were to touch, it would mean that millions of people would manage to wake up fully rested without needing an alarm clock.

The plot shows that Daylight Savings Time lasts too long. If it started later in the spring and ended earlier in the summer (or if it was eliminated altogether), millions more people would get a good nights sleep. We would be healthier, feel better, be less likely to be involved in car crashes, and our entire society would be more productive. High school and university students would benefit in particular, because they need to sleep in the latest. Schools would have fewer disciplinary problems, students would fall asleep less often in class, and because they are alert, they would learn more.

If you'd like to read our full paper, it's freely available online. If you'd like to have a laugh and hear more about how terrible Daylight Savings Time is, here's John Oliver asking how it's still a thing:

* This might seem a bit early, and it is. It's related to the method we chose to select a single time. Long story short, some people wake up earlier than this, most people wake up later, but the time shown here is the most stable measure for start of twitter activity.

Wednesday, May 18, 2016

A tale of two lamps

This photo is taken from the bridge over the Warschauer Strasse S Bahn station in Berlin (daytime street view). Good lamps aren't visible from the side or above.

Lamp comparison by Christopher Kyba is licensed
under a Creative Commons Attribution 4.0 International License.

Unfortunately most lamps aren't good lamps, as you can see in the original:

Warschauer Strasse railway at night by Christopher Kyba is licensed
under a Creative Commons Attribution 4.0 International License.

It's pretty likely that the brightly lit area under the "good" lamps is overlit, because it's so much brighter than the other lit areas. So actually these lamps aren't really ideal either. But it demonstrates the point that there is no need to waste energy and destroy the night by shining light in directions that aren't at all useful.

Monday, May 16, 2016

Night lights and prosperity don't always go hand in hand

North Korea is famously dark compared to South Korea and China at night, and images like the one below are often used to demonstrate the consequences of its "unenlightened" policies.

Photo ISS043-E-247811 from the International Space Station ISS.
See more like this at Cities at Night.

I certainly wouldn't want to live in North Korea, but is it an absolute truth that bright lights indicate prosperity, and lack of bright lights poverty and backwardness? The border between Germany, Belgium, and The Netherlands would suggest otherwise:

The border of The Netherlands, Belgium, and Germany at night.
Image from an International Astronomical Union press release.

The area of Germany shown in the photo is part of the Ruhrgebiet, home to 8.5 million people and one of the most industrialized areas in Europe. Nevertheless, the comparison of Germany to Belgium and The Netherlands is nearly as visually striking as that between North and South Korea. This is at least partly due to German lighting policy: Germany rarely lights its Autobahns (highways), and cities and towns are conservatively lit, often intentionally not following the European (DIN) norms for street lighting.

Last year we published a paper that examined the differences in lighting between cities and towns in the USA and Germany. American towns of 10,000 emit on average three times more light per capita than German towns, and cities of 100,000 emit more than five times more light per capita.

Total light emission from cities in Germany and the USA compared to community population.
Figure 5 from "High-Resolution Imagery of Earth at Night: New Sources, Opportunities and Challenges".

Germany uses far less light than its neighbors and the USA. Despite this, it is a prosperous country that is highly visited by tourists. It has low crime rates (the burglary rate is only 1/3 of that in the brightly lit Netherlands and just over half that of Belgium) and low rates of death due to traffic (about 1/3 less per 1 billion vehicle kilometers than USA or Belgium).

So if bright lights aren't needed to attract tourists, reduce crime, or make driving safer, then why do so many cities have such bright lights? Now that's a $100 billion question.

Note: Thanks to Alejandro Sanchez de Miguel for sharing the two ISS images with me.

Update: If you liked this post and want to learn more, check out this blog's "view from your app" photo series, that often highlights how good lighting is about more than how bright lights are. Some other blog highlights are about what a single floodlight can do to a natural area, the promise and peril of LED lighting, and citizens push back on LED lighting.