As discussed in a previous post , light pollution is a major nuisance to astronomers both amateur and professional.
When astronomers classify how much light pollution there is at a particular location, they often use the Bortle Scale, devised by John Bortle and first published in the popular astronomy magazine Sky and Telescope (Bortle 2001). Rather than taking scientific measurements of the brightness of the night sky, Bortle designed his scale for amateur astronomers to the use the visibility of the Milky Way and other astronomical objects to assess how dark the sky is and give it a rating between 1 and 9 on the scale he’d designed.
The Bortle Scale
- The first column gives the Bortle classification between 1 and 9
- The second column gives the magnitude of the faintest star that an observer with good eyesight can see once their eyes have fully adapted to the dark. The higher the magnitude the fainter the star. (See notes at the end of this post for more details on the magnitude scale.)
- The Zodiacal light is a very faint yellowish-white glow visible in the night sky and appears to extend from the direction in which the Sun has set or is about to rise. Sunlight scattered by interplanetary dust causes this phenomenon.
On Bortle’s scale, Scale 1 are the darkest skies on Earth. Very few people are fortunate enough to live in such a location. The best most people can hope for in Europe is to be within 3 hours drive of a Bortle Scale 3 location. As urbanisation increases and light pollution gets gradually worse more and more people are living in Bortle Scale 8 and 9 locations. For example, in England the area around Manchester City Centre is Bortle Scale 9.
To further illustrate how the number of astronomical objects visible declines as the amount of light pollution increases I have used the Stellarium planetarium software to illustrate how the area of sky around the constellation Orion would look like for Bortle Scale 1, Bortle Scale 5 and Bortle Scale 9 (with limiting magnitude 3) skies.
Simulation of how the area around Orion at a Bortle Scale 1 location would appear to a naked-eye observer. This simulation was run for mid-March, three hours after sunset, at a latitude of roughly 53 degrees north. In reality, the brighter stars do not actually appear as discs – just brighter points of light.
Simulation of the area around Orion at a Bortle Scale 5 location. Notice that there are far fewer stars visible and the Milky Way can no longer be seen.
Simulation of the area around Orion at a Bortle Scale 9 location. Notice that there are only a very small number of stars visible and it is difficult to make out Orion
Light Pollution Mapping
In the UK, as in most industrialised countries, you are never too far away from brightly lit areas. The picture below shows a satellite image of the UK from space.
As you can see in England there is nowhere that is more than 30-40 km from a heavily lit urban area. The position is better in Scotland, once you get away from the so called Central Belt centred around Glasgow and Edinburgh.
Light Pollution Mapping
There have been a number of works in recent decades which have produced light pollution maps based upon satellite data. A notable recent work were the maps produced by Falchi et al (2016). They took satellite data over a number of nights for the entire Earth and from this data they created an upwards emission function to predict how much of this light being emitted at ground level would be scattered by the atmosphere, giving the sky a faint man-made glow. They fitted their upwards emission function to 24 000 ground-based measurements made by astronomers at different locations who measured the actual brightness of the night sky. Falchi et al.’s maps were colour-coded as follows.
Key to the Falchi et al maps
Even when there is no Moon above the horizon and no man-made light pollution the night sky is never totally black and glows extremely faintly. The largest contributor to this natural night sky brightness is something called airglow. This is caused by various processes in the upper atmosphere of Earth, such as the recombination of atoms which were ionised by the Sun during the day, emission of light caused by cosmic rays striking the upper atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen atoms reacting with free radicals. The standard unit of luminance (which is how bright a light source appears) is candela per square metre. As a result of airglow and other factors the night sky directly overhead (when there is no Moon) has a natural luminance of around 174 microcandela per square metre (μcd/m2). If the Sun is more active, there will be more airglow and the night sky will be brighter.
Column 1 gives the colour coding, column 2 gives the amount of artificial luminance in microcandela per square metre and column 3 gives this as a percentage increase over the background level. Column 4 gives mappings from the microcandela per square metre range to the Bortle Scale value taken from https://www.handprint.com/ASTRO/bortle.html
Description of first few rows in the table
- All the regions in the maps coloured black have an artificial luminance of less than 1.74 microcandela per square metre. This means that man-made activities have added less than 1% to the natural sky brightness.
- Regions coloured dark grey have an artificial luminance of between 1.74 and 3.48 microcandela per square metre. This means that man-made activities have added between 1% and 2% to natural sky brightness.
- Regions coloured light grey have an artificial luminance of between 3.48 and 6.96 microcandela per square metre. This means that man-made activities have added between 2% and 4% to natural sky brightness.
The maps from Falchi et al
Light Pollution Map of Europe
As you can see most of Europe is fairly heavily light polluted. Interestingly, the North West of Scotland is one of the areas in Europe least affected, as you can see in more detail below.
Light Pollution map of the UK
This begs the question as to why there aren’t any astronomical observatories in the north west of Scotland. The answer is quite simple, the north west of Scotland is one of the wettest areas in the UK and the skies have on average a high degree of cloud cover. This limits the time that observers can actually see the night skies!
Light Pollution map of the US
It is interesting just how light polluted the eastern half is the US compared to its western half. There are still sizeable areas of the western US where there is virtually no light pollution.
Light Pollution map of the entire world
If you look at the picture of the whole world we see that there are still substantial land areas where there is very little light pollution. With the exception of Africa, these are all areas where there is a low population density such as northern Canada, parts of South America, the Himalayas and the interior of Australia. Australia is a particularly interesting case. 88% of the country’s land area has virtually no light pollution, which is one of the highest figures for any country in the world. However, only 2% of the country’s population actually live in an area where there is no light pollution. This is because the vast majority of Australians live in urban areas. In fact, the percentage of Australian’s living under heavily light polluted skies is actually higher than in the much more densely populated European Union.
I hope you have enjoyed this post. In a future post I will talk about how citizen-science projects can improve light pollution mapping by getting members of the public to submit their own night sky brightness observations.
When discussing the brightness of objects in the sky, astronomers use a scale called magnitude, where the lower the magnitude the brighter the object. The scale was originally invented by the ancient Greek astronomers who classified all the stars visible to the naked eye into six magnitudes. The brightest stars were given a magnitude of 1, and the faintest a magnitude of 6.
Values in the magnitude scale were standardised by nineteenth century astronomers to make each decrease in magnitude value by 1 an increase in brightness of 2.512. The range of values was also extended as well, to cater for the brightest stars and most planets which are brighter than magnitude 1 and stars fainter than magnitude six.
In the standardised scale for example
- a bright star having magnitude 1 is 9 times brighter than a star of magnitude 4. This is because 2.512 x 2.512 x 2.512 = 15.9.
- a star having magnitude 1 is 100 times brighter than a star of magnitude 6. This is because 2.512 x 2.512 x 2.512 x 2.512 x 2.512 = 100
The brightest natural objects in the sky are (obviously) the Sun, which has a magnitude of -26.7, followed by the Moon, which has a magnitude of -12.7 at a typical full Moon. Third comes Venus, with a magnitude of around -4.5 at the two points in its orbit when it is brightest. The fourth brightest natural object, and the second brightest planet, is Jupiter, which has a magnitude of -2.7 at its brightest. The brightest star in the sky is Sirius, with a magnitude of -1.46, making it 16 times fainter than Venus. The magnitude of the faintest star that can be seen by someone with good eyesight in a rural location, once their eyes have fully adapted to the dark, is normally taken to be around 6.5 to 7.0. Although Bortle claims that in perfect conditions people with good eyesight can see fainter objects.
Bortle, J. E. (2001) ‘Introducing the Bortle Dark Sky Scale’, Sky & Telescope, 101(2), pp. 126.
Falchi, F, Cinzano, P, Duriscoe, D, Kyba C M C, Elvidge C D, Baugh, K, Portnov, B A, Rybnikova, N A, Furgoni, R (2016) ‘The new world atlas of artificial night sky brightness’, Science Advances, 2(6), pp. [Online]. Available at: https://advances.sciencemag.org/content/advances/2/6/e1600377.full.pdf (Accessed: 15 March 2020).