Today marks the 175th anniversary of the birth of John Muir, the Scottish-born American naturalist, writer, and advocate for the preservation of wild land.
The protection of our wildernesses landscapes (defined as anywhere you cannot see the intrusion of human activity) is more important than ever, with the spread of suburbia and the urbanisation of more than 50% of the world’s population.
But one measure of what makes a wilderness has to-date been largely ignored: that of the darkness of the night sky. After all, if you can see the sky glowing orange at night then you are seeing the intrusion of human activity, and you can’t consider the land you’re in a true wilderness.
John Muir’s legacy as the founding father of the conservation movement lives on today, in part in the organisation The John Muir Trust.
The JMT estimates that the amount of Scotland’s landscape that is wilderness is rapidly diminishing, dropping from 31% of Scotland to 28% between 2008 and 2009, but I think if you added in the spread of man-made light pollution the situation would be decidedly worse.
I am fortunate to have been awarded the JMT’s Bill Wallace award to help fund a trip later this year (once the skies get dark again after the bright summer nights) to map light pollution in one of Scotland’s most wild landscapes, between the JMT properties of Quinag and Sandwood Bay.
Hopefully this project – the first of its kind in this remote area – will shed some light on the problem of the loss of our wilderness nightscapes.
The global family of International Dark Sky Places – areas with stunning night skies and exemplary lighting controls to preserve those skies – has grown again recently, with the addition of some huge parks and reserves. There are currently (as of June 2012) 18 places around the world that satisfy the International Dark-sky Association‘s (IDA) requirements.
I’ve been lucky enough to visit 12 out of these 18 incredible places, including the two most recent additions to the IDA family, NamibRand Nature Reserve in Namibia, and Aoraki Mackenzie in New Zealand, both of which have been awarded International Dark Sky Reserve status this year.
The IDA has three different designations: International Dark Sky Park (IDSP), International Dark Sky Reserve (IDSR), and International Dark Sky Community (IDSC).
IDSPs are areas of public land that are near-empty wildernesses, and which have enacted strict controls of outside artificial lighting throughout the entire park. There are currently ten IDSPs.
IDSRs are large areas centred on a dark sky core, a significant area – an observatory, say – in need of protection against light pollution, and a 15km-minimum buffer zone around that core, encompassing surrounding communities. The communities in the buffer zone have lighting controls that help minimise light pollution in the core area. There are currently four IDSRs.
IDSCs are communities – cities, towns, villages, islands – that have enacted exemplary lighting controls to limit the spread of light pollution into their night skies. There are currently four IDSCs.
The following table has some information about the various International Dark Sky Places:
|Name||Location||Park Area||Designation||Year Designated|
|Aoraki Mackenzie||New Zealand||4300 km2||Reserve||2012|
|Big Bend National Park||Texas, USA||3242 km2||Park||2012|
|Borrego Springs||California, USA||110 km2||Community||2009|
|Cherry Springs State Park||Pennsylvania, USA||4.3 km2||Park||2008|
|Clayton Lake State Park||New Mexico, USA||1.9 km2||Park||2010|
|Exmoor National Park||England, UK||692 km2||Reserve||2011|
|Flagstaff||Arizona, USA||255 km2||Community||2000|
|Galloway Forest Park||Scotland, UK||780 km2||Park||2009|
|Geauga Observatory Park||Ohio, USA||4.5 km2||Park||2011|
|Goldendale Observatory State Park||Washington, USA||0.2 km2||Park||2010, provisional|
|The Headlands of Emmet County||Michigan, USA||2.2 km2||Park||2011|
|Homer Glen||Illinois, USA||58 km2||Community||2011|
|Hortobagy National Park||Hungary||800 km2||Park||2011|
|Mont Megantic||Quebec, Canada||5000 km2||Reserve||2008|
|NamibRand Nature Reserve||Namibia||1722 km2||Reserve||2012|
|Natural Bridges National Monument||Utah, USA||31 km2||Park||2006|
|Sark||Channel Islands, UK||5.4 km2||Community||2011|
|Zselic Landscape Protection Area||Hungary||90.4 km2||Park||2009|
I recently attended the Third International Starlight Conference held by the Starlight Initiative near Lake Tekapo, New Zealand. The conference brought together a huge range of specialists who seek to limit the excesses of light at night, and the venue sat in the recently-announced Aoraki / Mount Cook International Dark Sky Reserve (IDSR) in New Zealand’s stunning south island.
The beauty of the night sky from somewhere like Tekapo is astounding, and the IDSR status will help keep it that way, limiting the amount of lighting that can spill into the sky from the surrounding communities. Under such starry skies it’s easy to understand why we’d want to protect them, but for most of the population of the planet starlight is becoming increasingly more elusive.
To help emphasise the importance of a dark starry sky the conference looked to build upon a document written at the first Starlight Conference in La Palma, in 2007, the Starlight Declaration in Defence of the Night Sky and the Right to See the Stars.
The Starlight Declaration states:
a. An unpolluted night sky that allows the enjoyment and contemplation of the firmament should be considered an inalienable right equivalent to all other socio-cultural and environmental rights. Hence the progressive degradation of the night sky must be regarded as a fundamental loss.
b. Knowledge—armed with education—is a powerful vector that can heal the growing rift between today’s society and science and contribute to the advancement of mankind as a whole. The dissemination of astronomy and of the scientific and associated cultural values should be considered as basic contents to be included in educational activities.
d. Control of obtrusive light must be a basic element of nature conservation policies since they impact on several species, habitats, ecosystems, and landscapes.
c. Protection of the astronomical quality of areas suitable for the scientific observation of the Universe must be given priority in national and international scientific and environmental policies.
e. The intelligent use of artificial lighting that minimizes sky glow and avoids obtrusive visual impact on both humans and wildlife should be promoted. This strategy would involve a more efficient use of energy so as to meet the wider commitments made on climate change, and for the protection of the environment.
f. Tourism, among other players, can become a major instrument for a new alliance in defence of the quality of the nocturnal skyscape. Responsible tourism, in its many forms, can and should take on board the night sky as a resource to protect and value in all destinations.
Necessary measures should be implemented to involve all parties related to skyscape protection to raise public awareness—be it at local, regional, national, or international level—about the contents and objectives of the International Conference in Defense of the Quality of the Night Sky and the Right to Observe Stars, held in the Island of la Palma.
Dated 20 April 2007, La Palma, Canary Islands, Spain
Spring in the northern hemisphere is the best time to view the elusive, faint astronomical phenomenon known as Zodiacal Light.
This light – literally “light from the zodiac” – appears only just at the end of evening twilight or just before morning twilight, and is seen as a cone of very faint light stretching up from the horizon, narrowing as it does so, following an imaginary line in the sky known as the zodiac, or to give it its more astronomically correct name, the ecliptic.
The angle which this line makes with the horizon varies throughout the year, and the steeper the angle the more evident the zodiacal light will be. The steepest angle for observers in the northern hemisphere occurs in the evening sky in March and April, or the morning sky in October and November.
How best to see Zodiacal Light
You will need to be as far as possible from any sources of light pollution. In fact the Zodiacal Light is one of the benchmarks of the Bortle Dark Sky Scale, which says that it is only visible in skies with Bortle Class 5 or better, and even in suburban/rural transition sites it is not striking. Under rural skies it is “striking”, while in a truly dark sky site it is bright enough to cast a shadow. Under exceptionally dark skies it might appear as a band stretching from horizon to horizon.
Once you’ve found your dark sky site, you need to find somewhere with as clear a western horizon as possible, and wait until the end of evening astronomical twilight (assuming you’re viewing it in the spring – if it’s the autumn then you need to start observing before the start of morning astronomical twilight). You can find your twilight times using timeanddate.com or the excellent Velaclock app for smartphones. As a general rule, for observers in the UK, you need to wait until two hours after sunset before you skies get dark enough to see this elusive light. But wait too long and the bulk of the cone of light may have set, so sunset+2hours is really the perfect time.
What Zodiacal Light looks like
As mentioned above, Zodiacal Light is a faint grey cone of light stretching up from the horizon. The darker your observing site the larger this cone will appear, and the higher into the sky it will stretch. From the very darkest sites on Earth it can stretch overhead and down to the far horizon.
What’s Zodiacal Light made of?
Zodiacal Light is sunlight reflecting off particles of dust and rock orbiting the Sun. This dust is in a lens-shaped cloud with the Sun at the centre, and the cloud lies in the same plane as the planets in the solar system (which is why it’s visible along the ecliptic). The particles in the Zodiacal Light are around 0.15mm in diameter (some smaller, some a little bigger) and probably come from shattered comets and asteroids.
Photographing the Zodiacal Light
As tricky as it is to see with your naked eyes, it’s even harder to catch on camera. Harald Edens has a great page about how best to photograph it.
Having just tried to assess Naked Eye Limiting Magnitude from a dark site, I realised that my previous post on the subject merited some amendments.
Rather than using the whole constellation of Ursa Minor to carry out your NELM estimate, it’s much simpler to use just part of it, that part around the “body” of UMi, roughly bounded by and immediately surrounding β, γ, ζ, and η UMi. Here’s a more detailed star chart of that part of the sky, with all 34 stars brighter than magnitude 7.2 labelled.
And here’s a list of the magnitudes of each of these stars:
| Star Number (Name)
||Magnitude|| Star Number (Name)
|1 (β UMi)||2.05||18||6.55|
|2 (γ UMi)||3.00||19||6.60|
|3 (ζ UMi)||4.25||20||6.60|
|4 (5 UMi)||4.25||21||6.65|
|5 (4 UMi)||4.80||22||6.70|
|6 (η UMi)||4.95||23||6.80|
|7 (θ UMi)||5.00||24||6.85|
|8 (11 UMi)||5.00||25||6.85|
|9 (19 UMi)||5.45||26||6.85|
|15 (20 UMi)||6.35||32||7.10|
|17 (3 UMi)||6.40||34||7.20|
As you can see, it’s much easier to fine-tune your NELM estimate using this chart compared to the previous one, as there are not such big jumps between brightnesses from one star to the next.
Colours in this table correspond to the Bortle Scale colour key.
Crucially, one thing I omitted to note in the previous post was that this process should be carried out when your target stars are high above the horizon. The stars of Ursa Minor, when observed from the UK, vary in altitude between 40° and 70° roughly speaking, so ideally you’d wait until they were higher than 60° above the northern horizon.
|Month||Times when Kocab (β UMi) alt > 60°|
|mid Jan||0300 till start astronomical twilight (~0600)|
|mid Feb||0100 till start astronomical twilight (~0530)|
|mid Mar||2330 till start astronomical twilight (~0430)|
|mid Apr||2230 till start astronomical twilight (~0400)|
|mid May||end astronomical twilight till start astronomical twilight|
|mid Jun||no hours of darkness|
|mid Jul||no hours of darkness|
|mid Aug||never > 60° during hours of darkness|
|mid Sep||never > 60° during hours of darkness|
|mid Oct||never > 60° during hours of darkness|
|mid Nov||never > 60° during hours of darkness|
|mid Dec||0500 till start astronomical twilight (~0630)|
UPDATE: Here’s the chart with the magnitudes written directly beside each star.
There are a variety of ways of measuring your night sky quality, and one of the most effective ways is by looking for the faintest star you can find with your naked eye, and noting its brightness, or magnitude. This provides what is known as Naked Eye Limiting Magnitude, NELM.
Of course just randomly casting about the sky for faint stars can lead you on a merry chase, and so a very useful method is to use one specific constellation – one you can always see, no matter what time of year – and look only at stars within that one constellation. This narrows the field somewhat, and makes your task that much easier.
For observers in Europe and North America the constellation of Ursa Minor, the Little Bear, provides an excellent choice for estimating NELM.
The overall shape of Ursa Minor is made up of seven bright-ish stars, but around and amongst these are many more fainter stars.
|Bright Star Name
|Ahfa al Farkadain (ζ)||4.25|
|Anwar al Farkadain (η)||4.95|
Even some of these “brighter” stars might not be visible from city centres. For example, if you are observing from a site with Bortle Class 8 you would not see η-UMi, while those unhappy stargazers under a Bortle Class 9 sky would only be able to pick out the three brightest stars, α-, β-, and γ-UMi. Only at Bortle Class 7 and darker will you make out all seven of the main stars of Ursa Minor.
But what if you’re at a good dark sky site? Well, you’re going to need a longer list of magnitudes, and a more detailed map of Ursa Minor.
|Star Number on
|Star Name||Visual Magnitude||Bortle Class
The stars in the map and table above have been numbered (by me – these aren’t official designations) from 1 to 19, with 1 (Polaris) being the brightest, and 19 (14 UMi) being the dimmest. You will only be able to see all 19 numbered stars from exceptionally dark places, virtually free of light pollution, what Bortle called “typical truly dark sky sites”. From my garden in the outskirts of a major city I can see numbers 11 and 12, but not number 13, giving me an NELM of 5.45.
The Bortle Scale is a useful way of estimating your sky brightness, i.e. to what extent light pollution affects your view of the night sky. By going outside on a clear moonless night and recording what astronomical objects you can see you can assign a Bortle Class rating to your observing site.
I have used the Bortle Scale to assess night sky quality many times, and always felt the lack of a handy flow chart to lead me through it. So I made one. Enjoy. (You can also download the pdf version.)
PS The content of this chart assumes some prior knowledge of astronomy, but any of the terms used are easily google-able.
In case you haven’t heard the BBC are running another series of Stargazing Live starting on Monday 16 January for three nights. Each hour long programme will be presented by Professor Brian Cox and comedian Dara O’Briain, and will feature a wealth of information about what’s visible in the night sky.
This series will focus on light pollution, and the benefits of a dark sky.
On Wednesday 18 January, Dulverton in Somerset [in Exmoor Dark Sky Reserve] will attempt to become one of the first towns in the UK to have every single one of its lights turned off at the same time, as part of a Stargazing Live demonstration showcasing the beauty of a night sky free of the effects of light pollution.
There are 177 street lights in Dulverton making the night sky significantly brighter and making it much harder to see the stars. At roughly 8.15pm on Wednesday (or at the sound of a unique set of church bells), the Stargazing Live team want every single person in Dulverton to turn off every single light in the town, giving people in the area the unique chance to take in the wonders of the night sky free of the effects of light pollution.
To support this series, and encourage people to get out and look up, the BBC are sponsoring hundreds of events around the country, from planetarium shows to star parties, from lectures to observatory visits. You can find out what’s on near you on their events page.
To find out more about the shows visit their website, where you can view images, download their excellent star guide and activity pack, listen to some audio guides, watch “how to” videos, and take part in live web chats. You can also follow the series on Twitter using the hashtag #BBCstargazing.
Once again the Campaign to Protect Rural England and the British Astronomical Association’s Campaign for Dark Skies are running a UK-wide star count programme. This year’s event takes place between 20-27 January 2012. On any of these nights the skies will be dark enough to begin your star count by 7pm.
To make your own observations for Star Count 2012 find Orion in the sky and count how many stars you can see within the rectangular boundary formed by the four brightest stars in Orion. Those boundary stars are called Betelgeuse, Bellatrix, Rigel and Saiph.
You should count the three belt stars – Alnitak, Alnilam, and Mintaka – plus any other stars that are visible. The above star map shows around 40 stars within that boundary. If you can see that many stars then you’ll be in one of the darkest places in the UK. For most of us we’ll count far fewer stars than that. People in very bright urban areas may only see the three belt stars.
UPDATE: I should have mentioned that the CPRE will accept observations from anywhere in the UK, not just England.
Following on from my last blog post (“Do brighter street lights make you safer from crime?“), a Guardian “Comment is Free” editorial was published on 26 December 2011 under the title “In praise of leaving the lights on“. This article came after the widely reported words of Stella Creasy MP who called for a halt to street-light switch-offs until studies had been carried out into the effect on crime of reduced light at night.
In my previous post I cited several US studies that have shown that switching street lights off at night does not result in an increase in crime, and indeed in many cases brighter street lights resulted in an increase in crime.
I have subsequently read several British studies that also support this view, such as the 1991 Home Office Crime Prevention Unit Papers No. 28 and 29, entitled respectively “The Influence of Street Lighting on Crime and the Fear of Crime” (pdf) and “The Effect of Street Lighting on Crime and Fear: A Review” (pdf). These reports state that
no evidence could be found to support the hypothesis that improved street lighting reduces reported crime (Paper No. 28)
improvements to street lighting can help to reduce the public’s fear of crime, but that they make less of a difference to the prevailing level of crime than many people would expect (Paper No. 29)
However the Guardian editorial cites
a 2002 study by the Home Office [Home Office Research Study 251, HORS251] [which] found that “improved street lighting led to… an overall reduction in recorded crime of 20%” (pdf).
This seems to contradict the earlier Home Office studies. Might things have changed in the intervening 11 years? It appears not. The author of the 1991 Paper No. 29, Dr Malcolm Ramsay, a Senior Research Officer in the Crime Prevention Unit of the Home Office, wrote a 2004 paper in the British Journal of Criminology criticising the methods used in the HORS251 study.
The arguments in Dr Ramsay’s research paper are predominantly of a statistical nature. The introduction states that
the [HORS251] review at first sight appears to be an appropriate statistical synthesis of all studies on street lighting and crime across the world. However on close examination, the statistical claims and methods are unfounded.
According to Dr Ramsay, not only does the 2002 HORS251 report
use methods that ignore the large variation (known as “overdispersion”) in the data and implicitly assume that crimes are independent events, which is implausible in the extreme
but it also
is not comparing like with like, for the individual studies, in general. This is because brighter street lighting is applied to more crime ridden areas and the comparison areas are less crime-ridden and this will lead to an effect known as regression to the mean.
Further problems are apparent with HORS251. For example small studies are excluded for no good reason. Dr Ramsay’s paper concludes:
Crime reduction is frequently presented as a potent argument for increased lighting – here it has been shown that there is no scientific basis for this claim.
I for one welcome any new data on this issue, but in the light of the above criticisms of HORS251 it is unfortunate that the Guardian used it in a comment piece in praise of leaving the lights on.