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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.

sunrise

Sunrise

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.

 

 

Winter Solstice 2011

December 21, 2011 3 comments

The northern hemisphere winter solstice occurs on 22 December 2011, at 0530 GMT At this point the Earth’s north pole will be tipped away from the Sun. As seen from Earth, the Sun will stop its slow daily decent south in our sky – over the past six months the Sun’s mid-day height above the horizon has been decreasing steadily – and once again turn north, getting higher in the sky at noon each day, until it gets to its highest point in midsummer 2012.

The actual day of the winter solstice – in this case 22 December 2011 – is commonly known as midwinter, the shortest day, and is the day when the Sun spends least time above the horizon. The further north of the equator you are, the more profound the effect. Indeed if you live within the arctic circle the Sun won’t actually rise today.

I’m not that far north, but by most standards I’m pretty far north, in Orkney delivering a midwinter astronomy festival. Orkney sits between 58°41′and 59°24′ North, and on midwinters day the Sun rises around 0905 and sets around 1515, and only spends 6h10m above the horizon. The winter nights in Orkney are long and dark.

But the morning after midwinter, the days will be lengthening. For many cultures then, midwinter symbolised the rebirth of the year, and ancient peoples often built monuments to celebrate the returning of the light.

And people in neolithic Orkney built some of the most incredible midwinter monuments that still exist. I’ll be spending this afternoon inside the 4700 year old chambered cairn at Maeshowe, built so that the passageway – which one has to crawl through to get into the inner chamber – points directly towards sunset on the shortest day.

Maes Howe sections

Given clear skies, the last rays of midwinter sunlight stream into the burial chamber for a few moments before the sun sets.

Maeshowe midwinter sun, Charles Tait

The Orkney poet George Mackay Brown said of midwinter at Maeshowe:

The most exciting thing in Orkney, perhaps in Scotland, is going to happen this afternoon at sunset, in few other places even in Orkney can you see the wide hemisphere of sky in all its plenitude.

The winter sun just hangs over the ridge of the Coolags. Its setting will seal the shortest day of the year, the winter solstice. At this season the sun is a pale wick between two gulfs of darkness. Surely there could be no darker place in the be-wintered world than the interior of Maeshowe.

One of the light rays is caught in this stone web of death. Through the long corridor it has found its way; it splashes the far wall of the chamber. The illumination lasts a few minutes, then is quenched

Winter after winter I never cease to wonder at the way primitive man arranged, in hewn stone, such powerful symbolism.

Happy Spring Equilux

March 18, 2011 5 comments

Today, Friday 18 March 2011,  it is the Spring Equilux throughout the UK (and possibly elsewhere too*) meaning that there are almost exactly 12 hours between sunrise and sunset.

sunrise

Sunrise

This date differs from the Spring, or Vernal, Equinox (2321 GMT on Sunday 20 March 2011) 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 0626 1826
Belfast 0632 1833
Newcastle 0615 1815
Manchester 0617 1818
Birmingham 0616 1817
Cardiff 0621 1822
London 0609 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 does 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, 17 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!

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.

Equinox, Equilux, and Twilight Times

March 20, 2010 21 comments

On or around 21 March each year it is the Spring Equinox in the Northern Hemisphere, the day when eggs can be stood on their ends, when the Sun crosses the celestial equator, and when night and day are equal length the world over.

But this last part isn’t entirely true, for a variety of reasons.

Equinox

Equinox is latin for aequus (equal) and nox (night) meaning, roughly, “equal night”. The moment of the equinox is defined as the point at which the centre of the Sun’s disk crosses an imaginary line in the sky called the celestial equator, the projection of the Earth’s equator out into space.

The Sun, courtesy of NASA

The Sun (and the Moon and all the planets) move along a line in the sky called the ecliptic, the projection of the disk of the solar system out into space. These two lines, the equator and the ecliptic, cirlce the sky, and because the Earth’s axis is tilted at 23.5 degrees the angle between the equator and the ecliptic is 23.5 degrees, and the two circles meet at only two points, called equinoctial points.

Over the course of the year the Sun, as seen from Earth, appears to make one complete circuit around the ecliptic, as the Earth in fact orbits the Sun. And so on two days each year the Sun’s path crosses the equator. This means a number of things:

1. that an observer at the equator will see the Sun directly overhead at mid-day on the equinoxes
2. that the Sun will rise due east and set due west on the equinoxes (on all other days the Sun will rise either north or south of east, and set north or south of west)
3. the length of day and night are nearly equal

On this last point, they are not exactly equal, for two reasons:

1. the Sun appears as a disk in the sky with a radius of around 16 arcminutes, and so the top of the Sun appears to rise while the centre of the disk is still below the horizon, and the instant of the equinox is measured with respect to the Sun’s centre, and
2. the Sun’s light is bent, or refracted, in the Earth’s atmosphere, so that rays from the Sun can light you up even before the Sun rises, and keep you lit after it sets, with the degree of refaction being around 34 arcminutes

These two factors combine to mean that the Sun will appear to have “risen” when the centre of the disk is still 50 arcminutes (16 + 34) below the horizon, making the amount of daylight longer than the expected 12 hours. How much longer depends on where on Earth you are, but in the UK the length of the day is approx. 12 hours 10 minutes, rather than exactly 12 hours.

Equilux

Because of this effect, the days on which the length of day and night are exactly equal, called the equilux, occur a few days before the spring equinox and a few days after the autumn equinox. This date will vary depending on where on Earth you are, and indeed equiluxes do not occur at all close to the equator, whereas the equinox is a fixed instant in time.

Twilight

But the story doesn’t end there. Even on the days of equilux, the sky will have been bright for some time before the first rays of the Sun hit you, and will remain bright for some time after the last rays disappear from view. This time of day is called twilight, starting at dawn and ending at dusk.

There are, in fact, three kinds of twilight.

Civil twilight is what most people mean when they talk about twilight. It starts in the morning when the centre of the Sun’s disk is 6 degrees below the horizon, and ends at sunrise. In the evening civil twilight stars at sunset and continues until the centre of the Sun’s disk is 6 degrees below the horizon. During civil twilights the sky is still bright enough that, in general, artificial illumination will not be needed when doing things outside. In reality though, most councils switch lights on a fixed time after sunset (say 30 minutes) and turn them off a fixed time before sunrise, rather than relying on the Sun’s angular distance below the horizon.

Nautical twilight is when the Sun’s disk is between 6 and 12 degrees below the horizon. During nautical twilight it is still possible to distinguish the sky from the distant horizon when at sea, thus allowing sailors to take measurements of bright stars against the horizon. Most of us would consider this “dark”, but it is still technicaly twilight.

Astronomical twilight is when the Sun’s disk is between 12 and 18 degrees below the horizon. During astromical twilight you’ll no longer be able to tell the sky from the distant horizon when at sea, but crucially for astronomers, there is still light in the sky. Not much – indeed most of us would say it’s properly night time at this point – but the faintest objects in the sky, such as nebulae and very dim stars, will only be visible after astronomical twilight ends. Also, if you intend to measure how dark the sky is using a Sky Quality Metre, say, it is important to wait till after astronomical twilight.

So how much daytime does this twilight add? Only during civil twilight can the sky really be considered “light”, and so we can say that the day begins (at the point we would call dawn) at the start of morning civil twilight, and ends, (at the point we would call dusk) at the end of evening civil twilight. This means that on the Spring Equinox, the “day” will be around 13 hours and 15 minutes long in the UK (depending on where you are).

The date at which the “day” (including dawn and dusk) is 12 hours long in the UK (and therefore when we had equal amounts of daytime and nighttime) occurs around 01 or 02 March, over two weeks before the so-called equinox!

To find out your local sunrise / sunset / twilight times visit www.timeanddate.com

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