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.

Leap Seconds

Today the International Telecommunications Union are voting on whether to abolish the leap second. This miniscule measure of time is added in to our time-keeping systems every so often to make sure they align more accurately with the time as measured by the spin of the Earth.

The original definition of the second was 1/86400 of a mean solar day which is related to the speed of the Earth’s spin about its axis. We might call this the “Earth second”. However the Earth’s spin in not regular. To begin with the Earth is slowing down by a couple of milliseconds per century due to tidal breaking. This breaking action is as a result of the drag of the Earth spinning beneath the tides created by the Moon. In effect the Moon is “stealing” energy from the Earth, increasing in its orbit about us while our spin slows.

In addition to this discrepancy the Earth is occasionally wobbled off course by major geological events, such as earthquakes. The 2004 Pacific earthquake which resulted in the Boxing Day Tsunami actually caused the Earth to speed up by over 2 milliseconds.

To avoid the problem of an irregular length of day – and therefore an irregular length of second – scientists adopted the much more regular SI second, which is the length of time it takes for 9,192,631,770 cycles of vibration of atomic caesium. This “atomic clock second” is accurate to one part in ten billion, and since 1972 this has been the international standard in timekeeping.

But time kept using the the SI second doesn’t match exactly with time kept based on the spin of the Earth, which after all is the time we experience every day. In order to make these two time signals match leap seconds are added every so often. Since 1972 25 leap seconds have been added. The last leap second was added at 23:59:59 on 31 December 2008, and the next one is due to be added at 23:59:59 on 30 June 2012. But leap seconds themselves are irregular, and are decided on by the ITU whenever the two time signals drift by more than 0.9 seconds.

The argument for abolishing these additional leap seconds is that it creates problems for modern computing and navigation systems that use the atomic clock second. Every time one of these irregular leap seconds is added the world’s hi-tech time keeping devices need to check and adjust by one second. It would be far simpler for us to use only “atomic clock seconds”.

However if we were to ditch the leap second then our civil time keeping would begin to drift with respect to “real” Earth time, so that in thousands of years time our clocks might read 8am just as the Sun is setting. This might seem like a minor concern right now – after all a millennium is a long time – but it’s something that astronomers and scientists do need to consider to avoid future problems. One alternative would be to introduce a “leap hour” to be introduced every few hundred years to keep the clock aligned with the real world.

BBC Stargazing Live Goes Dark: 16, 17, 18 January 2012

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.

Professor Brian Cox and Dara O'Briain will host BBC Stargazing Live 2012

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.

 

Star Count 2012

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.

Count all the stars within the rectangle of Orion's brightest four stars

This is the third such star count, previous ones having taken place in 2007 and 2011. Last year’s star count resulted in lots of observations, presented in this map.

Map of observations from Star Count 2011

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.

Orion the Hunter

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.

London Borough Of Ealing Declared International Dark-Sky Reserve: Spoof

Hilariously, I have been “quoted” in a fictitious article on the website The Spoof. This article boasts the headline: “London Borough Of Ealing Declared International Dark-Sky Reserve”!

The police have been inundated by calls from anxious Ealing residents, worried about strange white dots in the night sky

The piece continues:

‘London is an international centre of excellence for numerous endeavours,’ explained Mayor of London, Boris Johnson. ‘It was total madness that Londoners had to travel to the mountains of Chile or to Exmoor, wherever they are, to get a decent view of the night sky.’

So the author(s) of the piece know their stuff: the Atacama Desert in Chile is widely recognised as one of the very best places for stargazing on the planet, while Exmoor became an International Dark Sky Reserve in November 2011, following on from Galloway Forest Dark Sky Park in 2009, and Sark Dark Sky Island in 2010.

The article introduces me as:

UK astronomer Steve Owens, chair of the IDA‘s Dark Sky Places Development Committee

again true, but not nearly so widely known. They must really have done their homework on this story. They then put words in my mouth that are entirely reasonable:

‘To be declared an International Dark-Sky Reserve,’ he explained, ‘an area must possess an exceptional or distinguished quality of starry nights. Light pollution from conurbations is the most significant barrier to this aspiration, and areas in London possess quite exceptional challenges in that respect.’

Ha! Very true. Then the article continues:

As part of the bid for Reserve status for Ealing, street lights were disabled within a ten mile radius… In addition, local byelaws were passed to enforce the use of blackout curtains after dark within the Reserve area. Car headlights and other sources of light, necessary to facilitate travel, were required to be red in order to not affect night vision.

From now on my analysis of this article might read as somewhat po-faced, as I point out their errors, but it does at least allow me to talk a little bit about what it means to be a Dark Sky Place.

The IDA doesn’t require the extinguishing of all street lights, nor black out curtains, nor red car headlights. All nice ideas though, from an astronomical point of view. Just not very practical.

The article then quotes several fictitious Londoners:

‘Since the changes, I have been almost constantly working outside at night,’ said emergency paramedic and ambulance crewperson, Ursula Major. ‘I often point out the constellations to distract RTA and other casualties from their injuries.’ [actually, studies have shown no connection between reduced light at night and RTAs.]

‘Sometimes,’ admitted Leo Regulus, an unemployed young person from Boston Manor, ‘I stop when lootin’ from Ealin’ Broadway ta wunda at the splenda of the Miwkey Way. Last week,’ he continued, ‘I even went back ta Argos ta nick a Newtonian reflecta telescope, init.’ [similarly, there is no good evidence that reduced light at night increases crime.]

Astronomers from across the UK have visited Ealing to avail themselves of its crystal clear view of the heavens. ‘Many initially complained about mugging and the loss of equipment,’ admitted sergeant Izar Bootes from the Metropolitan Police, ‘but the ability to repurchase their kit, or better gear, at Leeland Road market on Saturday mornings has more than compensated for such inconveniences.’ [see above]

Even everyone’s favourite astronomer Prof Brian Cox gets a mention:

The quality of the sky over Ealing means that key astronomical features are clearly visible for the first time in over two centuries. The police have been inundated by calls from anxious residents, worried about the appearance of strange white dots in the night sky.

‘It looks like that Brian Cox bloke was telling the truth, after all,’ said one amazed Northfields resident.

Make sure you read the whole article, but remember: the London borough of Ealing isn’t really an International Dark-Sky Reserve!

 

Quadrantids 2012: Peak Activity Rate

This morning (Wednesday 04 January 2012) at  around 0530 the Quadrantids meteor shower reached its peak activity rate. According to the International Meteor Organisation the ZHR(max) was 78 +/- 7. As is usual with this meteor shower the peak was quite narrow, with activity starting to increase after 0000 on 04 January, and dropping off again by 1800 the same day.

Quadrantid Meteor Shower 2012, credit imo.net

As was predicted, observers who were out under clear skies between 0430 and 0630 would have got the best views, as within this two hour window the Moon had set and the Sun had yet to brighten the sky. The ZHR(max) of 78 was rather disappointingly low compared with the predicted maximum rate of ~120, but predicting these things isn’t an exact science. The IMO estimated that the rate usually falls somewhere between 60 and 200, so this year’s shower was certainly at the lower end of that, however that might be down to the fact that few people had clear skies (a common problem at this time of year) and so some meteors were missed.

A ZHR (max) occurring at 0530 on 04 January 2012, when the radiant was around 60° above the eastern horizon (in the UK), if seen from a cloudless dark sky site with no light pollution (i.e. a limiting magnitude of 6.5) would have meant that you’d have seen around 70 shooting stars an hour, still quite an impressive show

Perihelion 2012

At around 0100 GMT on 5 January 2012 the Earth will be at perihelion, its closest approach to the Sun this year.

If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.

The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis. Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).

The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.

This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.

The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (5 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.

Quadrantids Meteor Shower 2012

On the night of 03/04 January 2012 the first meteor shower of the year will take place, the Quadrantids. This shower ranks as one of the best performers of the year, assuming your skies aren’t clouded, as they so often are in winter. If the peak of this shower occurs under ideal conditions – i.e. perfectly clear skies, free from light pollution – then you can expect to see in excess of 100 meteors every hour. The peak for this shower is very brief though, so you’ll have to catch just the right conditions at just the right time to see a display this good. This year’s peak is estimated to occur just before dawn on 04 January 2012.

Not only do you have the weather to contend with, but this year the waxing gibbous Moon will be up for much of the night. However the Moon sets at around 0415, giving you a couple of hours before the sky starts to brighten before sunrise. Given that the peak of this shower will probably occur within this short window, things are looking pretty good for this year’s display.

Last year’s graph of meteor activity shows how sharp the peak is, so you probably won’t see many Quadrantids on the nights either side of the peak, but it’s worth a look if you have clear dark skies. ZHR for this year may be anywhere between 60 and 200.

Quadrantids Activity from 2011, credit imo.net

How best to view the Quadrantids 2012

1. Get somewhere as far from street lights and city glow as possible, preferably somewhere really dark, like your nearest national park or one of the UK’s dark sky places: Galloway Forest Park, Sark or Exmoor.
2. Go out at the right time, which for this year’s shower is between around 0400 and 0700 GMT.
3. You don’t need binoculars or a telescope, your eyes are best for viewing meteors.
4. Wrap up warm, as if you have clear skies (which you’ll be hoping for) it will be very cold in these early morning hours.
5. Bring a reclining deck chair so you don’t have to stand all night, and a blanket to wrap yourself in!
6. Although the radiant of the meteor shower (the point where the meteors will appear to stream from) is high in the E around 0400 you don’t need to worry about facing in any particular direction, just position yourself so that you can see as much sky as possible, and enjoy the view!

You can follow the progress of the meteor shower at meteorwatch.org, or on twitter via @VirtualAstro and the #meteorwatch hashtag.

If you want to make more serious observations of this shower you can submit them to either the International Meteor Organisation, the British Astronomical Association, or the Society for Popular Astronomy.

Street lights vs crime: redux

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)

and

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.

Thanks (again) to @darkskyscott of the International Dark-sky Association for links to these reports, and to the Campaign for Dark Skies for the information in their “Lighting and Crime” section.