Measuring and Mapping Light Pollution

As mentioned in a previous post , mild air pollution is a significant nuisance to astronomers each beginner and professional.

When astronomers classify how a lot mild air pollution there may be at a specific location, they usually use the Bortle Scale, devised by John Bortle and first revealed within the widespread astronomy journal Sky and Telescope (Bortle 2001). Moderately than taking scientific measurements of the brightness  of  the night time sky, Bortle designed his scale for beginner astronomers to the use the visibility of the Milky Manner and different astronomical objects to evaluate how darkish the sky is and provides it a ranking between 1 and 9 on the size he’d designed.

The Bortle Scale

• The primary column offers the Bortle classification between 1 and 9
• The second column offers the magnitude of the faintest star that an observer with good eyesight can see as soon as their eyes have totally tailored to the darkish. The upper the magnitude the fainter the star. (See notes on the finish of this put up for extra particulars on the magnitude scale.)
• The Zodiacal mild is a really faint yellowish-white glow seen within the night time sky and seems to increase from the path through which the Solar has set or is about to rise. Daylight scattered by interplanetary mud causes this phenomenon.

On Bortle’s scale, Scale 1 are the darkest skies on Earth. Only a few individuals are lucky sufficient to stay in such a location. One of the best most individuals can hope for in Europe is to be inside three hours drive of a Bortle Scale three location. As urbanisation will increase and light-weight air pollution will get steadily worse increasingly individuals are dwelling in Bortle Scale eight and 9 areas. For instance, in England the world round Manchester Metropolis Centre is Bortle Scale 9.

To additional illustrate how the variety of astronomical objects seen declines as the quantity of sunshine air pollution will increase I’ve used the Stellarium planetarium software program for example how the world of sky across the constellation Orion would appear like for Bortle Scale 1, Bortle Scale 5 and Bortle Scale 9 (with limiting magnitude three) skies.

Simulation of how the world round Orion at a Bortle Scale 1 location would seem to a naked-eye observer. This simulation was run for mid-March, three hours after sundown, at a latitude of roughly 53 levels north. In actuality, the brighter stars don’t truly  seem as discs – simply brighter factors of sunshine.

Simulation of the world round Orion at a Bortle Scale 5 location. Discover that there are far fewer stars seen and the Milky Manner can not be seen.

Simulation of the world round Orion at a Bortle Scale 9 location. Discover that there are solely a really small variety of stars seen and it’s tough to make out Orion

Gentle Air pollution Mapping

Within the UK, as in most industrialised international locations, you’re by no means too far-off from brightly lit areas. The image under reveals a satellite tv for pc picture of the UK from area.

As you may see in England there may be nowhere that’s greater than 30-40 km from a closely lit city space. The place is healthier in Scotland, when you get away from the so known as Central Belt centred round  Glasgow and  Edinburgh.

Gentle Air pollution Mapping

There have been numerous works in current a long time which have produced mild air pollution maps primarily based upon satellite tv for pc information. A notable current work have been the maps produced by Falchi et al (2016). They took satellite tv for pc information over numerous nights for your entire Earth and from this information they created an upwards emission operate to foretell how a lot of this mild being emitted at floor degree could be scattered by the ambiance, giving the sky a faint man-made glow. They fitted their upwards emission operate to 24 000  ground-based measurements made by astronomers at totally different areas who measured the precise brightness of the night time sky. Falchi et al.’s maps have been colour-coded as follows.

Key to the Falchi et al maps

Even when there isn’t a Moon above the horizon and no man-made mild air pollution the night time sky isn’t completely black and glows extraordinarily faintly.  The most important contributor to this pure night time sky brightness is one thing known as airglow. This  is attributable to varied processes within the higher ambiance of Earth, such because the recombination of atoms which have been ionised by the Solar throughout the day, emission of sunshine attributable to cosmic rays hanging the higher ambiance, and chemiluminescence brought about primarily by oxygen and nitrogen atoms  reacting with free radicals.  The usual unit of luminance (which is how vibrant a lightweight supply seems) is candela per sq. metre. On account of airglow and different elements the night time sky instantly overhead (when there isn’t a Moon) has a pure luminance of round 174 microcandela per sq. metre (μcd/m²). If the Solar is extra lively, there might be extra airglow and the night time sky might be brighter.

Column headings

Column 1 offers the color coding, column 2 offers the quantity of synthetic luminance in microcandela per sq. metre and column three offers this as a proportion enhance over the background degree. Column Four offers mappings from the microcandela per sq. metre vary to the Bortle Scale worth taken from https://www.handprint.com/ASTRO/bortle.html

 

Description of first few rows within the desk

• All of the areas within the maps colored black have a synthetic luminance of lower than 1.74 microcandela per sq. metre. Which means man-made actions have added lower than 1% to the pure sky brightness.
• Areas colored darkish gray have a synthetic luminance of between 1.74 and three.48 microcandela per sq. metre. Which means man-made actions have added between 1% and a couple of% to pure sky brightness.
• Areas colored mild gray have a synthetic luminance of between three.48 and 6.96 microcandela per sq. metre. Which means man-made actions have added between 2% and Four% to pure sky brightness.

and so on

The maps from Falchi et al

Gentle Air pollution Map of Europe

As you may see most of Europe is pretty closely mild polluted. Apparently, the North West of Scotland is likely one of the areas in Europe least affected, as you may see in additional element under.

Gentle Air pollution map of the UK

 

This begs the query as to why there aren’t any astronomical observatories within the north west of Scotland. The reply is sort of easy, the north west of Scotland is likely one of the wettest areas within the UK and the skies have on common a excessive diploma of cloud cowl. This limits the time that observers can truly see the night time skies!

Gentle Air pollution map of the US

 

It’s attention-grabbing simply how mild polluted the jap half is the US in comparison with its western half. There are nonetheless sizeable areas of the western US the place there may be just about no mild air pollution.

Gentle Air pollution map of your entire world

For those who take a look at the image of the entire world we see that there are nonetheless substantial land areas the place there may be little or no mild air pollution. Except Africa, these are all areas the place there’s a low inhabitants density reminiscent of northern Canada, elements of South America, the Himalayas and the inside of Australia. Australia is a very attention-grabbing case. 88% of the nation’s land space has just about no mild air pollution, which is likely one of the highest figures for any nation on this planet.  Nevertheless, solely 2% of the nation’s  inhabitants truly stay in an space the place there isn’t a mild air pollution. It is because the overwhelming majority of Australians stay in city areas. Actually, the proportion of Australian’s dwelling underneath closely mild polluted skies is definitely increased than within the rather more densely populated European Union.

I hope you might have loved this put up. In a future put up I’ll discuss how citizen-science tasks can enhance mild air pollution mapping by getting members of the general public to submit their very own night time sky brightness observations.

Notes

Magnitude

When discussing the brightness of objects within the sky, astronomers use a scale known as magnitude, the place the decrease the magnitude the brighter the item.  The dimensions was initially invented by the traditional Greek astronomers who labeled all the celebrities seen to the bare eye into six magnitudes. The brightest stars got a magnitude of 1, and the faintest a magnitude of 6.

Values within the magnitude scale have been standardised by nineteenth century astronomers to make every lower in magnitude worth by 1 a rise in brightness of two.512. The vary of values was additionally prolonged as effectively, to cater for the brightest stars and most planets that are brighter than magnitude 1 and stars fainter than magnitude six.

Within the standardised scale for instance

• a vibrant star having magnitude 1 is 9 instances brighter than a star of magnitude Four. It is because 2.512 x 2.512 x 2.512 = 15.9.
• a star having magnitude 1 is 100 instances brighter than a star of magnitude 6. It is because 2.512 x 2.512 x 2.512 x 2.512 x 2.512 = 100

 

The brightest pure objects within the sky are (clearly) the Solar, which has a magnitude of -26.7, adopted by the Moon, which has a magnitude of -12.7 at a typical full Moon.  Third comes Venus, with a magnitude of round -Four.5 on the two factors in its orbit when it’s brightest.  The fourth brightest pure object, and the second brightest planet, is Jupiter, which has a magnitude of -2.7 at its brightest.   The brightest star within the sky is Sirius, with a magnitude of -1.46, making it 16 instances fainter than Venus. The magnitude of the faintest star that may be seen by somebody with good eyesight in a rural location, as soon as their eyes have totally tailored to the darkish, is often taken to be round 6.5 to 7.zero. Though Bortle claims that in excellent circumstances folks with good eyesight can see fainter objects.

 

 

References

Bortle, J. E. (2001) ‘Introducing the Bortle Darkish Sky Scale’, Sky & Telescope, 101(2), pp. 126.

Falchi, F, Cinzano, P, Duriscoe, D, Kyba C M C, Elvidge C D, Baugh, Ok, Portnov, B A, Rybnikova, N A, Furgoni, R (2016) ‘The brand new world atlas of synthetic night time sky brightness’, Science Advances, 2(6), pp. [Online]. Obtainable at: https://advances.sciencemag.org/content/advances/2/6/e1600377.full.pdf (Accessed: 15 March 2020).