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Perseids Meteor Shower 2016
August sees the return of the most reliable meteor shower of the year; the Perseids, with rates of shooting stars possibly increasing to over 100 per hour under perfect conditions.
Read my previous blog post Meteor Showers: The What, How, Where, When, Why for general advice on how best to observe meteor showers.
A shooting star – otherwise known as a meteor – is a tiny piece of space dust that burns up in our atmosphere, forming a bright, brief streak of light in the sky. Many people have never seen a shooting star, and think they’re rare events, but given dark skies you can expect to see a few every hour on a clear night. From cities, under light polluted skies, you can’t see most of the faint ones, and so only the rarer bright ones are visible.
However at regular times each year the Earth moves through thick clouds of space dust – left behind by comets – and we get a dramatically increased rate of meteors. On the night of 12/13 August we’ll pass through the densest part of a dust cloud left behind by Comet 109P/Swift-Tuttle, and will see the rate of meteors increase by a factor of 20!
You can begin watching for Perseid meteors now, and the shower will last until late-August, but the peak of the shower occurs overnight on 12/13 August 2016, which means that the nights on either side of this will be best for meteorwatching.
Location of the Perseids Radiant at 0001 on 13 August
The best time of night to watch the meteor shower is from around 2200 onwards on 12 August, once the radiant, the point from where the meteors appear to originate, rises above the horizon. However the moon will be in the sky until after midnight, and will interfere slightly for observers in dark sites. The later you observe the higher the radiant will be, and the more meteors you’ll see.
The number of meteors that you will observe every hour depends on a number of factors:
•the density of the dust cloud that the Earth is moving through, that is causing the shower in the first place;
•the height above the horizon of the radiant of the shower, the point from which the meteors appear to radiate;
•the fraction of your sky that is obscured by cloud;
•the darkness of your sky, measured using naked-eye limiting magnitude, a measure of the faintest object you can see.
This year the Perseid meteor shower has an expected zenith hourly rate (ZHR) of around 150. This is the number of meteors that you can expect to see if the radiant is directly overhead (the point in the sky called the zenith), and you are observing under a cloudless sky with no trace of light pollution.
However conditions are rarely that perfect. In the UK, for example, the radiant of the shower will not be at the zenith; it will be around 30° above the horizon at 2200, 40° high at midnight, and 50° high at 0200.
Assuming a clear night, the other factor is the limiting magnitude of the sky, a measure of the faintest object you can see. Man-made light pollution will be an issue for most people. From suburbia the limiting magnitude of the sky is ~4.5 (around 500 stars visible), so you will only be able to see meteors that are at least this bright; the fainter ones wouldn’t be visible through the orange glow. In a big city centre your limiting magnitude might be ~3 (only around 50 stars visible); in a very dark site like Galloway Forest Dark Sky Park the limiting magnitude is ~6.5 (many thousands of stars visible), limited only by the sensitivity of your eye. So in most cases it’s best to try and get somewhere nice and dark, away from man-made light pollution.
The calculation that you need to make in order to determine your actual hourly rate is:
Actual Hourly Rate = (ZHR x sin(h))/((1/(1-k)) x 2^(6.5-m)) where
h = the height of the radiant above the horizon
k = fraction of the sky covered in cloud
m = limiting magnitude
Let’s plug the numbers in for the Persieds 2016.
ZHR = 150 at the peak, say.
h = 30° at 2200, 40° at 0000, 50° at 0200, 65° at 0400
k = 0 (let’s hope!)
m = 6.5 (if you’re observing under skies free from light pollution)
So your actual hourly rate under clear dark skies is
(150 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 75 meteors per hour at 2200
(150 x sin(40))/((1/(1-0) x 2^(6.5-6.5) = 96 meteors per hour at 0000
(150 x sin(50))/((1/(1-0) x 2^(6.5-6.5) = 115 meteors per hour at 0200
(150 x sin(65))/((1/(1-0) x 2^(6.5-6.5) = 135 meteors per hour at 0400
Remember though that these numbers apply only to the peak of the Perseid occuring at these times. If the peak happens at 0400 on 13 August (and we’re not sure exactly when it’ll occur) then you might see 135 meteors per hour, but “only” 75 per hour if the peak occurs at 2200 on 12 August.
Remember that these rates are for perfectly dark skies. If you live in suburbia then divide these numbers by 4 or 5; if you live in a bright city divide these numbers by 10. Take home message: get somewhere dark!
It is worthwhile having a look on the days leading up to the peak, when the numbers of meteors will be gradually increasing towards this rate.
Live in or near Glasgow? Come and join me for the Perseids #Meteorwatch at Whitelee Wind Farm!
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:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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:
- 95%: Energy Saving
- 91%: Cost Saving
- 43%: Reduced Light Pollution
- The top three reasons that councils were switching off lights at night are:
- 97%: Energy Saving
- 78%: Cost Saving
- 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
International Dark Sky Week 20-26 April
Sunday marked the start of International Dark Sky Week 20-26 April 2014, a global initiative to get people out of towns and cities and seeing a night sky as it’s meant to be seen, unspoiled by light pollution.
City Lights To Dark Skies from Mark Gee on Vimeo.
Most of us live in urban environments these days, with the ever-present orange glow lighting the night sky. From my garden in Glasgow I can see only a few hundred start on a clear night. But if I traveled south by car for an hour down to Galloway Forest Dark Sky Park, or north by the same distance to Loch Lomond National Park, I’d count thousands of stars.
And then if I made more of an effort to get to the very darkest parts of our country (the heart of Galloway Forest, or Northumberland Dark Sky Park, or up to Coll Dark Sky Island) the number of stars would be overwhelming, too many to count.
At this time of year in the UK (especially in Scotland) you’ve got to wait until quite late to see the sky free of twilight (2230 in Exmoor Dark Sky Reserve for example, 2330 in Coll, and after midnight up in the Orkney Islands), but it’s worth making the effort if it’s clear.
If you do head out this week here are a few things to watch for:
The Lyrids meteor shower will reach its peak on 21/22 April but you might catch some early Lyrids in the days beforehand, and some after the peak; the darker your skies the more you’ll see.
Cygnus the Swan, and the other stars of the Summer triangle will be rising high in the east after midnight. In the right wing of the swan is the star Kepler-186, with the new-found twin Earth, known as Kepler-186f. The star is far, far too faint to see, even with a very powerful telescope, but you can still look in that direction and give a little wave.
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!
Light Pollution and Birds: Early Bird Survey
The negative effect of light pollution on wildlife has long been known, specifically – but not exclusively – its effect on bats, bugs, and sea turtles. Now the British Trust for Ornithology (BTO) are running an Early Bird Survey, asking people in the UK to monitor the pre-dawn feeding times of garden birds to see what – if any – effect light pollution is having.
To take part you need to get up before dawn* on 9** January 2014 (tomorrow, as I write this), watch your garden bird feeders, and record the times that the first ten species arrive to feed. You can download the full instructions here (pdf), and submit your observations here.
* dawn occurs at different times around the UK, so you should find your sunrise time and get up half an hour earlier than that, during civil twilight.
** observations on 10, 11, and 12 January are welcome too.
As the BTO website says:
Winter is not an easy time for birds. They need extra energy to keep warm, especially during long winter nights. To cope with this, they lay down extra fat reserves, though small birds quite often only lay down enough for a single night. Longer nights not only affect the amount of energy a bird uses, they also reduce the amount of time that birds can feed in. Birds, therefore, have to make the most of the daylight hours to replenish their energy reserves before it gets dark.
The 2004 BTO Shortest Day Survey, run in association with BBC Radio 4, investigated the patterns behind birds arriving at garden bird feeders first thing on a winter’s morning. Building on observations from the Shortest Day Survey, the Early Bird Survey will investigate what effect, if any, light and heat pollution have on the feeding patterns of birds during a cold winter’s morning.
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!
Light Pollution and the Loss of Wilderness
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.
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 |
Zodiacal Light
Spring in the northern hemisphere is the best time to view the elusive, faint astronomical phenomenon known as Zodiacal Light.
Daniel López, IAC Zodiacal Light on the left and (false colour) Milky Way on the right
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.
Naked Eye Limiting Magnitude: Redux
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.
UMi detail down to mag 7.2
And here’s a list of the magnitudes of each of these stars:
| Star Number (Name) |
Magnitude | Star Number (Name) |
Magnitude |
| 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 |
| 10 | 5.55 | 27 | 6.85 |
| 11 | 5.70 | 28 | 6.85 |
| 12 | 6.00 | 29 | 6.90 |
| 13 | 6.25 | 30 | 6.95 |
| 14 | 6.30 | 31 | 7.00 |
| 15 (20 UMi) | 6.35 | 32 | 7.10 |
| 16 | 6.35 | 33 | 7.20 |
| 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.
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