International Dark Skies Week 13-19 April 2015

This week is International Dark Skies Week, 13-19 April 2015.

Created in 2003 by high-school student Jennifer Barlow, International Dark Sky Week has grown to become a worldwide event and a key component of Global Astronomy Month.

“I want people to be able to see the wonder of the night sky without the effects of light pollution. The universe is our view into our past and our vision into the future . . . I want to help preserve its wonder.” – Jennifer Barlow

This year’s International Dark Skies Week coincides with the International Year of Light, which makes it particularly appropriate.

Why not head outside at night and explore dark skies? You could head to any International Dark Sky Places. In the UK we have six:

• Galloway Forest Dark Sky Park
• Exmoor Dark Sky Reserve
• Sark Dark Sky Island
• Brecon Beacons Dark Sky Reserve
• Northumberland Dark Sky Park
• Coll Dark Sky Island

Any of these sites will provide excellent views (weather permitting!) of a real dark sky.

If none of these are convenient why not visit a Dark Sky Discovery Site near you. These are sites that have been identified by local communities as being convenient for stargazing, although they don’t have the light pollution control measures that the International Dark Sky Places do.

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Shedding Light: a Survey of Local Authority Approaches to Lighting in England

Last Friday 25 April 2014 the Campaign to Protect Rural England (CPRE) released a report (pdf) entitled “Shedding Light: a Survey of Local Authority Approaches to Lighting in England”, that addresses the growing problem of light pollution in rural areas.

Light pollution from towns and cities spreads a long way, lighting the sky up and spoiling the view of the stars even in areas with few or no lights of their own. However that’s only the tip of the iceberg. According to CPRE:

Street lighting in England costs councils approximately £616m per year and can account for up to 30% of their carbon emissions so tackling light pollution will have a triple benefit – cutting costs and carbon too.

Although around one third of councils surveyed were switching lights off between midnight and 5am, when they aren’t needed, and around one half of councils dimming lights at similar times, there is still much more to be done.

Image from the CPRE report: Light pollution of Eastbourne, from Warren Hill

According to Emma Marrington, CPRE Dark Skies Campaigner:

‘The results of our survey show that many local authorities are taking steps in the right direction to manage lighting more effectively. But much more can be done to encourage all authorities to take this issue more seriously.

‘We urge councils to do more to control lighting in their areas, and ensure that the right lighting is used only where and when it is needed. We often hear concerns that changing street lighting can impact on public safety but our research revealed no evidence to support this. We’re not advocating changes where they’re not appropriate, but why shine bright lights on residential streets, quiet roads and open countryside throughout the night when it’s not needed?

‘Genuine dark starry nights are becoming harder and harder to find which is why councils should take action to control it now. Light pollution blurs the distinction between town and country, ruins the countryside’s tranquil character and denies us the experience of a truly starry sky.’

CPRE ends the report by making nine recommendations:

1. Light pollution policy All local authorities should have a policy to control light pollution in their Local Plan, in line with the National Planning Policy Framework and the associated National Planning Practice Guidance on light pollution. This should include identifying existing dark areas that need protecting.
2. Street lighting policy Local authorities should consider  preparing a Street Lighting Policy, which could include Environmental Lighting Zones to ensure that the  appropriate lighting levels are  used in each zone, with very strict requirements applying in identified dark areas.
3. Part-night lighting schemes We encourage local authorities to investigate how part-night lighting schemes (e.g. switching off between midnight and 5am) or dimming could work in their areas, including examining the cost, energy and carbon savings. This should be done in full consultation with the local community.
4. LANTERNS research project All local authorities who are switching off or dimming street lighting should monitor crime and accident statistics and consider taking part in the Institution of Lighting Professionals/LANTERNS research project which aims to quantify any effects of changes to street lighting on road traffic accidents and crime.
5. LED lighting Local authorities should give careful consideration to the type of Light Emitting Diode (LED) lighting they use and consider the potential impacts that higher temperature blue-rich lighting has on ecology and human health.
6. Targets for replacing lights Local authorities with responsibility for street lighting could set targets for replacing all their street and road lights with less light polluting types, such as full cut off flat glass lamps.
7. Testing new street lighting New street lighting should be  tested ‘in situ’ before a lighting scheme is rolled out across a wider area to ensure that it is the minimum required for the task and does not cause a nuisance to residents.
8. Preserving dark skies Local authorities should have a strong presumption against new  lighting in existing dark areas,  unless essential as part of a new development or for public safety reasons that have been clearly demonstrated.
9. Highways Agency guidance The Highways Agency should review the lighting section of the Design Manual for Roads and Bridges, which is used to design motorway and trunk road lighting, to ensure it remains relevant for local authorities.

The whole report makes for interesting reading, but a few things stand out:

• only 65% of councils in England have a policy on lighting
• 87% of these councils said it was a continuation of an existing policy; only 13% had adopted a new policy as a result of the 2012 National Planning Policy Framework (NPPF).
• The top three reasons that councils were switching off  lights at night are:

1. 95%: Energy Saving
2. 91%: Cost Saving
3. 43%: Reduced Light Pollution

• The top three reasons that councils were switching off  lights at night are:

1. 97%: Energy Saving
2. 78%: Cost Saving
3. 54%: Reduced Light Pollution

• 11 councils said that dimming schemes had gone largely unnoticed by the community
• 91% of councils that are switching off lights are continuing to work with local police to monitor local crime statistics

Dark Sky Islands

There are only two International Dark Sky Islands in the world, and both of them are in the British Isles: Sark in the Channel Islands; and Coll in the Inner Hebrides.

Sark

Coll

They were designated by the IDA (the International Dark-sky Association) under their International Dark Sky Communities programme, Sark in 2011 and Coll in 2013.

These beautiful short films show what you’ll see on a clear night:

The Starry Skies of Sark from Sue Daly on Vimeo.

Isle of Coll – IDA Dark Sky Community from Ewan Miles on Vimeo.

I’ve visited both islands several times, and they’re beautiful places, and not just at night when the stars come out. They’re very different: Sark is lush, with hedgerows and country lanes, and at 49°25’N latitude its climate is very continental. Coll on the other hand is almost entirely treeless, it’s rugged, boasts long sandy beaches, and lies at 56°38’N. Contrary to common impressions of the weather on the west of Scotland, Coll is one of the sunniest parts of Scotland and so has, like Sark, a high number of clear dark nights.

And it’s on dark nights when these islands are at their most stunning. Now that summer’s on its way though the dark nights will shorten and eventually disappear altogether until autumn, so you’ve plenty of time to plan your visit. Sark has a longer dark sky season, running from August till mid-May, as opposed to Coll’s which runs from September till mid-April, but the nights are longer on Coll than on Sark during the darkest winter months, the best time for stargazing.

Make sure that if you’re going to Sark or Coll for stargazing that you avoid the bright moon. Ideally you would be there during a new moon or thin crescent; at the very least you should avoid the week on either side of the full moon. To maximise your chances of seeing the wonderful dark skies make sure you stay for several nights!

Northumberland and Coll: The Newest International Dark Sky Places

The International Dark-Sky Association (IDA) announced today it has designated two new International Dark Sky Places in the UK, including one representing the largest land area of protected night skies in all of Europe. This brings to six the total number of IDA International Dark Sky Places in the UK, second only to the United States.

IDA is proud to recognise Northumberland  Dark Sky Park and Coll Dark Sky Island for their exceptional efforts in helping preserve and promote dark night skies over Britain.  I have worked with both of these areas as a dark skies consultant, advising them on the process of achieving dark sky status. To date this puts the number of dark sky places that I have been heavily involved in to five; more than anyone else in the world, I think!

The reasons for these areas seeking dark sky status are many and varied. Off-season winter astronomy tourism is one main driver, while for councils the economic and environmental benefits of night-sky-friendly zero-waste lighting are paramount. Northumberland County Council have recently announced an investment of £24million to refit all public street lights in the county to energy efficient LED lights, fittings which pay back the initial investment within 6-8 years through reduced operating costs, and which have a significantly reduced carbon footprint, due to their efficiency and the fact that no light is wasted – it all shines down to the ground where it’s meant to be, rather than into the sky.

Stargazers at Cawfields, Northumberland Dark Sky Park

Northumberland International Dark Sky Park

A UK National Park and adjacent forestry plantation encompassing nearly 580 square miles (1500 km²) of public lands in northern England, Northumberland National Park and Kielder Water & Forest Park are the first IDA-recognized International Dark Sky Park consisting of two independent parkland units.

Once at the frontier of Roman Britain where Hadrian’s Wall repelled Pictish invaders, Northumberland International Dark Sky Park now serves as a bulwark against the incursion of harmful light pollution into one of the darkest locations in England.

With today’s IDA announcement, National Parks UK and Forestry Commission England adds dark skies to their portfolio of protected natural resources including the largest manmade woodland and reservoir in northern Europe. Kielder Forest provides Britain with 200 million board feet (475,000 m³) of timber annually.

The dark night sky attracts an increasing number of visitors to the region. Kielder Observatory, the UK’s largest and most active public observatory, widely promotes local astronomy events and activities. “Dark skies and astronomy have become a passion in the area,” according to Heidi Mottram, Chair of the Kielder Water and Forest Park Development Trust and Chief Executive of Northumbrian Water.

As both Northumberland National Park and Kielder Water & Forest Park began to vie independently for IDA recognition, it quickly became evident that two heads were better than one.  “It made perfect sense to work together to protect one of our greatest assets and make it available to more people,” Mottram said.

Park officials hope that protecting dark skies through the promotion of responsible outdoor lighting will increase the allure of Northumberland as a tourism destination.

“Becoming a Dark Sky Park will reinforce the status of Northumberland as an unspoiled destination offering a true sense of tranquility and wildness – a tonic in this day and age,” said Tony Gates, Chief Executive of Northumberland National Park.

Coll International Dark Sky Island

A sparse population and geographic isolation make the night skies over the Isle of Coll among the darkest in Europe. The island adopted a quality outdoor lighting management plan to ensure Coll remains dark for many future generations of residents and visitors.

Coll lies about six miles (10 km) west of coastal Argyll and hosts just over 200 residents. It attracts dozens of bird species according to the Royal Society for the Protection of Birds, which owns an extensive reserve at the west end of the island and hosts one of Coll’s recognized night sky viewing sites on its land. Nature tourism in part draws thousands of visitors to the island each year.

“Achieving dark skies status will be great for the island in many ways,” Julie Oliphant, hotelier at the Coll Hotel, explained. “Not only will it ensure that any future development on the island is done in a way that protects Coll’s natural and unspoiled beauty, but it will also help promote winter tourism.”

Fred Hall of the Argyll and Bute Council echoed the sentiment. “The Isle of Coll is a unique island in many ways, not least of which is its beautiful countryside and sea views but also the lack of light pollution,” he said. “I can think of no better island in the inner Hebrides to gain the Dark Skies accolade.”

Northumberland  is IDA’s thirteenth International Dark Sky Park, while the Isle of Coll becomes the world’s fifth International Dark Sky Community. They join four existing International Dark Sky Places in Britain: Galloway Forest Park in Scotland, Isle of Sark in the Channel Islands, Exmoor National Park in England, and Brecon Beacons National Park in Wales.

If you’re interested in gaining dark sky status for your area, then get in touch!

Comet ISON Finally A Naked-Eye Object

Yesterday Comet ISON (C/2012 S1) brightened suddenly, meaning that it’s now visible to the naked eye – just.

Comet ISON, imaged by Nick Howes, Ernesto Guido and Martino Nicolini

According to astronomer John Bortle Comet ISON brightened from magnitude +8.5 on Monday and +7.3 on Wednesday,  to +5.4 on Thursday, meaning that it is now visible to stargazers without the aid of binoculars or telescopes – although those devices will help!

You’ll have to be determined to see it, as it rises in the east just before dawn, so an early start is required. At the moment it’s not spectacular, although some astronomers are still holding out hope that it might become the Comet of the Century as was predicted earlier this year. We’ll need to wait and see whether it continues to dramatically brighten. If it does then it may be visible high in the night sky in December.

How Best to See Comet ISON

Here are three simple steps you can take to maximise you chances of seeing this comet.

1. Find somewhere dark with a clear eastern horizon

Although it is now a naked-eye object, any light pollution in the sky will make it next to impossible to see, so head to your local dark sky site. If you don’t know how to find one then have a look at this light pollution map of the UK to give you an idea. In general you want to make sure that any nearby town or city is behind you, so head to the east of any populated are. You’ll need a flat horizon too – east coast is ideal – as hills and trees will block your view. At the moment the comet is still low in the sky when twilight brightens the sky making it impossible to see.

2. Keep an eye on the weather forecast

There’s no point heading out if it’s cloudy towards the east, but just because it’s raining when you go to bed doesn’t mean that it will still be raining at 6am. Check local weather forecasts for predicted cloud cover before dawn.

3. Find Mercury and Spica

The planet Mercury rises around 6am, and at that point the star Spica, in the constellation of Virgo, will be only a few degrees above the horizon. Comet ISON lies above and to the right of, approx. 10° higher than Mercury in the sky. Here’s a simple finder chart for approx. 6am.

Comet ISON marked with a red cross, approx. ESE in the pre-dawn sky mid November. Image via Stellarium

International Dark Sky Places

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.

The Church of the Good Shepherd, Aoraki Mackenzie IDSR Image by Fraser Gunn

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

 

The Starlight Declaration

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

Saint Helena Dark Sky Island

This blog post is a copy of one which I wrote for the Guardian Science blog, posted there on 13 April 2012.

The small South Atlantic island of Saint Helena is about as remote as any place on Earth gets. It lies 2000km from Africa and 3000km from South America, and I’m heading there for eight days this month to carry out a dark sky survey.

Saint Helena, 1815

This survey will allow me to determine the quality of the night sky above Saint Helena – the darkness of the sky, but also the clarity of the stars – in anticipation of the island becoming an International Dark Sky Place, a designation awarded by the International Dark-sky Association (IDA).

Light pollution is a common problem for astronomers living near cities; a familiar orange glow drowning out the light from all but the brightest stars in the night sky. With the spread of suburbia there are increasingly fewer places where stargazers can enjoy an unspoiled dark sky, but the further you travel from urban areas the more stars you will see, and Saint Helena as about as far as it’s possible to be from the next town.

Under such dark skies the Milky Way can be seen stretching from horizon to horizon in an arc overhead, and the heavens are studded with thousands of stars and many nebulae, including the dramatic Magellanic clouds not visible from far northern latitudes. Indeed its location at 16° south of the equator means that virtually every constellation is on display at some time throughout the year.

Small and Large Magellanic Clouds, Akira Fujii/Davidmalin.com

Saint Helena’s Astronomy Heritage

Saint Helena has long been used by astronomers as a site for making important observations. Edmund Halley – he of comet fame – visited the island in 1677 to catalogue the southern stars and observe a Transit of Mercury. The following century, in 1761 Neville Maskelyne, later to become Astronomer Royal, came to observe a much rarer Transit of Venus. (Incidentally, a Transit of Venus occurs this year on 5/6 June, only the fifth to occur since 1761, and the last for over 100 years).

The Dark Sky Survey

During the survey I’ll be using a Sky Quality Metre (SQM) to assess the brightness overhead. This device measures sky brightness in units of magnitudes per square arcsecond (magnitudes are a measure of brightness, the lower the number the brighter the sky; square arcseconds are a measure of area, where one arcsecond is 1/3600 of a degree).

In my back garden in the suburbs of Glasgow the SQM reads around 18 magnitudes per square arcsecond; in the centre of Glasgow it might read 16. The darkest readings come from remote places like Galloway Forest Dark Sky Park where 21.7 isn’t uncommon. In the very darkest places the limit of the device comes from the brightness of the stars overhead, and so you can’t expect readings much darker than 22.0 even in sites free of light pollution.

As well as these SQM readings I’ll be estimating the naked-eye limiting magnitude (NELM) of the night sky above Saint Helena. This basically involves looking for the faintest star I can see and reading its magnitude from a star atlas. In a city the NELM might be 3 or 4; in Galloway Forest Dark Sky Park it might reach 6.5 or even 7, where the only limit to what you can see is your eyesight.

Dark Sky Tourism

So why go to all this trouble? Well, an extensive dark sky survey is just one of the criteria expected of an International Dark Sky Place. Once this survey work is carried out, along with a lighting audit and adoption of new lighting codes on the island, the IDA might confer this status on Saint Helena. And the drive for all this work? Tourism. At the moment Saint Helena’s tourism is based almost exclusively on Napoleon’s exile there between 1815 and 1821. The Island also has several hundred species of flora and fauna which only found on this remote Island and is steeped in history from the Age of Discovery when it was a crucial staging post for sailing ships. The island attracts around 1000 visitors per year.

The main difficulty for the prospective visitor is travel to the island. The only way of getting there right now is on the RMS Saint Helena, on a six-day ocean voyage from Cape Town, something that may deter all but the most determined traveller. Come 2015 however, the island will have its own air strip, making it more accessible and tourism visits more regular.

The Saint Helena Tourism Association hopes to attract visitors with the prospect of the stunningly dark skies above the island. The concept of dark sky tourism has been growing over the past few years. There are currently 16 International Dark Sky Places recognised by the IDA, including three in the UK: Galloway Forest Dark Sky Park, Sark Dark Sky Island, and Exmoor Dark Sky Reserve. These sites are seeing an increase in visitor numbers in the dark winter season as keen stargazers, inspired perhaps by Prof Cox, flee the bright city lights for darker skies.

Naked Eye Limiting Magnitude: Assessing Sky Brightness

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.

Ursa Minor

Bright Star Name (Bayer Designation)	Magnitude
Polaris (α)	1.95
Kocab (β)	2.05
Phercab (γ)	3.00
Yildun (δ)	4.35
Urodelus (ε)	4.20
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 Above Map	Star Name	Visual Magnitude	Bortle Class
1	α UMi	1.95	9
2	β UMi	2.05	9
3	γ UMi	3.00	9
4	ε UMi	4.20	8
5	5 UMi	4.25	8
6	ζ UMi	4.25	8
7	δ UMi	4.35	8
8	4 UMi	4.85	7
9	η UMi	4.96	7
10	θ UMi	5.00	7
11	11 UMi	5.02	6
12	19 UMi	5.45	6
13	24 UMi	5.75	5
14	λ UMi	6.30	4
15	20 UMi	6.35	4
16	3 UMi	6.40	4
17	π1 UMi	6.55	3
18	HIP74818	6.65	3
19	14 UMi	7.35	2

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: A Flow Chart

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.