Posts Tagged ‘twilight’

Spring Equilux 2013

March 17, 2013 1 comment

Today, Sunday 17 March 2013, it is the Spring Equilux throughout the UK (and possibly elsewhere too*) meaning that there are almost exactly 12 hours between sunrise and sunset.



This date differs from the Spring, or Vernal, Equinox (1102 GMT on Wednesday 20 March 2013) for a variety of reasons, which I explain in a previous post but here is a list of sunrise / sunset times for a variety of towns and cities throughout the UK:

Town / City Sunrise Sunset
Aberdeen 0617 1817
Glasgow 0627 1825
Belfast 0633 1831
Newcastle 0615 1815
Manchester 0618 1817
Birmingham 0617 1816
Cardiff 0622 1821
London 0610 1809

As you can see the time between sunrise and sunset is not exactly 12 hours everywhere but this is the day of the year when that is closest to being true everywhere*. Yesterday the sun rose a couple of minutes later and set a couple of minutes earlier, and tomorrow the sun will rise a couple of minutes earlier and set a couple of minutes later, as the days lengthen.

Also, the reason that sunrise and sunset do not occur at the same time everywhere* is due mainly to the longitude of the town; the further east a town is the earlier it sees the sun in the morning, and the earlier it loses it again at night.

So happy Equilux everyone*!

* interestingly, the equilux does not occur on the same same day for everyone, it depends on your latitude. The closer you are to the equator the earlier the date of your equilux. For example the equilux in most US cities occurred yesterday, 16 March, and in cities near the equator there is never a day with exactly twelve hours between sunrise and sunset! Take Quito, the capital city of Ecuador (latitude 0 degrees 14 minutes south) for instance. The length of day there only ever varies between 12 hours and 6 minutes long and 12 hours and 8 minutes long!

Autumn Equinox 2012

September 22, 2012 Leave a comment

Today, 22 September 2012, marks the moment of the Autumn Equinox. At 1449 UT (1549 BST) the Sun will cross from the northern hemisphere sky to the southern, and we’ll begin the slow approach to the Winter Solstice on 21 December.

The equinoxes (one in spring and one in autumn) are the two instances every year when the Sun makes that crossing from north to south and vice versa, and they’re commonly thought to be the days when day and night are equal length, but they’re really not, for reasons I’ve outline before:

  1. astronomers measure the timings of equinoxes, sunrises and sunsets based on the middle point of the Sun’s disk in the sky, so when you read a sunrise time it means the time that the centre of the Sun’s disk rises above the horizon. For a few minutes before that time the top of the Sun’s disk will already have risen, giving “daylight”.
  2. Even before this happens the sky is lit up by the Sun below the horizon, and we experience twilight. Most people would think that the sky is bright enough to call it “daytime” long before the Sun pops above the horizon, during the phase of civil twilight.

So today, even though day and night are said to be equal on the equinox, the “daytime” (i.e the start of civil twilight) started about 0630BST in Glasgow (where I am) and will end this evening around 2000BST, giving me 13.5 hours of “daylight”. (Londoners will have from about  0615 until 1930BST, or approx. 13.25 hours of “daylight”).

The day this year where I have exactly 12 hours of “daylight” (i.e. between the morning start and the evening end of civil twilight) is 11 October and this day is called the equilux. (In London the equilux falls on 12 October).

The Return of Darkness

August 21, 2012 2 comments

With summer coming to an end in the British Isles we start the return to the dark skies of autumn and winter. Depending on where you are in the country you will have been without truly dark skies for many weeks, maybe even months, as summer evening twilight lasts throughout the night during the summer.

This all-night-long twilight is almost gone throughout the UK, indeed anywhere on the mainland UK can see astronomically dark skies around 1am at the moment. Only the furthest north outpost of the British Isles still doesn’t have that opportunity.

On the island of Unst, the furthest north of the Shetland islands, lies the UK’s furthest-north town, Skaw, at 60°49’N and 00°47’W. This tiny village will see astronomical darkness return at 0043 on 24 August, lasting only 46 minutes until at 0129 the sun’s light begins to creep into the sky again.

Midsummer sunset (at 2241) from Unst. Image Credit Brydon Thomason, Shetland Nature Ltd.

The last time that astronomical darkness was seen at Skaw was on 18 April, over four months ago! Indeed this settlement is so far north that between around 13 and 29 June each year they never get out of civil twilight, meaning that the sky’s bright all night long!

Compare this with the furthest south town in the British Isles, Saint Clement in Jersey, in the Channel Islands. Astronomical darkness returned to Saint Clement on 4 July this year, having been absent since 8 June; only four weeks without true darkness!

Such is the effect of differences in latitude that these two settlements, separated by 1299 km, have such hugely different seasonal swings between summer and winter.



101 Darkless Nights

June 6, 2011 1 comment

As an astronomer in the depths of a Scottish summer I have a long time to wait until darkness falls again. Between 5th May and 8th August this year – for 101 nights – the sky never becomes truly dark.

In order to get a truly dark sky you need to wait for the end of astronomical twilight, when the Sun sinks lower than 18 degrees below the horizon. However in particularly northern (or southern) latitudes there is a stretch of time during the summer when the Sun never gets this low, and the closer to the poles you are the longer this run of darkless nights can be.

So astronomers often go in to hibernation during these summer months, unless they want to observe or image those astronomical objects that don’t require true darkness: the moon, say, or the planets.

The Sun will next dip 18 degrees below the horizon (as seen from central Scotland) on 9th August around 1am, and then only for half an hour. As the days go on though it dips lower and for longer each night, and the hours of stargazing stretch until around the winter solstice we will have close to 12.5 hours of absolute darkness.

Plenty of time then to stock up before another long darkless summer.

Twilight vs. Light Pollution

May 4, 2011 4 comments

A few weeks ago I was waiting for it to get dark so I could go out into my garden and use my telescope. I decided to wait until the end of astronomical twilight (when there is no light from the Sun left in the sky) so that the sky was “properly dark”, but of course given that I live in a city (Glasgow) the light pollution from street lights means that it never actually gets “properly dark”.

So I decided to figure out exactly how long I should wait after sunset before going out to observe, or put another way, at what point does light pollution take over from twilight as the dominant source of light in the sky?

Sunset behind Glasgow Science Centre

I carried out this little experiment on 28 April 2011, in my back garden on the Southside of Glasgow, under the following conditions:

Sunset Time: 2050 BST
Civil Twilight Ends (Sun 6 degrees below horizon): 2134 BST
Nautical Twilight Ends (Sun 12 degrees below horizon): 2232 BST
Astronomical Twilight Ends (Sun 18 degrees below horizon): 2358 BST
Longitude: 55.866 N
Latitude: 4.257 W
Sky conditions: 100% clear

[Incidentally, sitting out in my garden for four hours as it darkened was an absolute delight: I saw, as well as the emerging stars, many bats, some ducks, two foxes, and two passes of the International Space Station!]

Using a Sky Quality Meter I took readings of the sky brightness at the zenith every two minutes. The SQM-L makes measurements of the sky in magnitudes per square arcsecond, i.e. brightness per unit area in the sky. As a general rule, in city centres you would expect readings of 16-17, while in dark places you can get readings of 21-22. The higher the number the darker the sky. The darkest reading possible under a starry sky is around 22, as at that point the starlight itself becomes the limiting factor.

From experience I know that in my garden the darkest reading possible is around 18.5, so I decided to continue taking readings until I got fifteen in a row that were above 18.4, i.e. for half an hour the sky had not been significantly darkening. I got my first reading of above 18.4 at 2244 and the sky did not appreciably darken over the next 30 minutes.

At 2244 the sun was 13 degrees 07 minutes (13.117 degrees) below the horizon.

I graphed the results to see how they looked, and placed them alongside the projected results if I were under a dark sky free of light pollution i.e. so that the results could get as low as 22.0 at the end of astronomical twilight at 2358 BST

SQM-L Readings vs Time

Result: After sunset, and throughout civil twilight (Sun between 0 and 6 degrees below the horizon) and nautical twilight (Sun between 6 and 12 degrees) the dominant source of light in the sky is sunlight.

However shortly after the end of nautical twilight light pollution became the dominant source of light in the sky, when the Sun was a little more than 13 degrees below the horizon.

This means that, while observing from my garden in Glasgow, I shouldn’t wait much later than the end of nautical twilight to go out observing, since beyond that point the sky will not significantly darken.

CAVEAT: It should be noted that this information is really only relevant for my specific circumstances, i.e. the light pollution in your sky may be better of worse than mine, and mean that the point at which it begins to dominate twilight is different for you.

Twelve hours of “daylight”

March 1, 2011 4 comments

Today, throughout the UK, civil twilight began almost exactly twelve hours before it will end this evening meaning that we have, for the first time this year, twelve hours of “daylight”. Summer is on its way!

Sunset on Sark, 11 September 2010

Sunset on Sark, 11 September 2010

City Civil Twilight Dawn Civil Twilight Dusk
Glasgow 0632 1828
Manchester 0622 1821
London 0614 1814

Of course some of this “daylight” is what we call twilight, but if you’re outside between these times you will certainly think that the sky is bright, and that the day has begun.

Technically the equinox (“equal night”) doesn’t occur this year until 20 March, with the equilux (“equal light”) occurring a few days before that (it varies around the world but in most of the UK the equilux occurs on 18 March 2011). For a detailed explanation of equinox, equilux and twilight times see my blog post from last March.

Double Summertime and amateur astronomy

February 24, 2011 9 comments

The BBC news website today has a feature on Double Summertime (DST), the proposal to set all UK clocks forward by one hour throughout the year, so that we might all benefit from longer evenings. The argument is that this will boost tourism, reduce road traffic accidents, and give us more time to enjoy outdoor activities in the evening.

UK amateur astronomers would lose 25% of their dark evening observing hours under Double Summertime

The main argument normally put up against Double Summertime is that there will be an increase in road traffic accidents due to darker mornings. This is technically true, although it is more than offset by the reduction in RTAs as a result of the brighter evenings, and therefore overall it’s safer, saving an estimated 80-100 lives per year. (See section 4.6, pp. 49-50 in the report Road Safety Beyond 2010 for the estimates in detail).

The extra hour of daylight each evening could be worth £3.5 billion through increased tourism, as well as creating around 8000 new jobs.

And finally, the reduced use of lights at night might save an estimated 2% of our daily electricity use, or 1.2 million tonnes of carbon.

So what’s not to like?

Well, not everyone would welcome brighter evenings. It is definitely a minority interest when set against the pro-safety, pro-business, pro-environment arguments above, but the UK amateur astronomy community would be more than a little put out by the change, losing an hour of stargazing each night. Of course, that hour won’t be lost, they’ll simply have to stay up later to observe, but the fact is many won’t. Staying up until midnight on a weeknight when you have got work the next day is very different from staying up till 1am. In addition public star parties will have to start later, therefore attracting fewer people throughout the year. Small concerns maybe, but it’s worth recognising that not everyone in the country would welcome brighter evenings.

City Annual # of hours of darkness*
before midnight under present system
Annual # of hours of darkness*
before midnight under DST
% decrease
Glasgow 977 731 25%
London 1110 830 25%

* darkness = after the end of astronomical twilight

As you can see from the table above, amateur astronomers around the country would lose 25% of their dark evening observing hours throughout the year. Of course these “missing” hours could be made up by staying up an hour later, but that’s not always practical.

Just at the point where astronomy is starting to dramatically increase in popularity, with a surge in telescope sales due to projects like the International Year of Astronomy 2009 and BBC Stargazing Live, a switch to DST would put a serious dent in that enthusiasm. The table below is similar to the one above except that it shows the number of hours of darkness before 10pm, the time that an enthusiastic newcomer might stay up doing simple observing, or the latest that a public star party might run. As you can see the % decrease is even more dramatic here, with reductions of more than 1/3.

City Annual # of hours of darkness*
before 10pm under present system
Annual # of hours of darkness*
before 10pm under DST
% decrease
Glasgow 515 328 36%
London 584 379 35%

I’m not necessarily arguing against DST, given how many lives it could save, how much money it would bring in through tourism (although the change could seriously hamper an area that is developing its astronomy tourism), and how much it would benefit the environment, but it’s always good to get an alternative viewpoint.

UPDATE (28 Feb 2011): The British Astronomical Association has issued the following statement:

“The UK has a thriving amateur astronomy community who regularly observe the night sky in the evenings.

Many of these amateurs share their knowledge at organised public outreach events across the whole country every year. These evening events are attended by people of all ages and backgrounds. For many youngsters it will be their first experience of science outside the classroom and for some it will be a pleasurable and formative experience that may encourage them into a science-based education and career, something the government is keen to promote.

By delaying the time at which it becomes dark enough to observe the night sky, the proposals in the Daylight Saving Bill will have a detrimental effect on these outreach activities and reduce their benefit to society.

We encourage those deciding on the proposed Daylight Saving Bill to take this into account.”

Members may wish to write to their MP to make their own representations.

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