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Autumn Equinox 2013

September 22, 2013 Leave a comment

Today, 22 September 2012, marks the moment of the Autumn Equinox. At 2044 UT (2144 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).

Categories: Time and Date Tags: , , , , ,

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).

Spring Equinox 2012

March 20, 2012 1 comment

Today is the first day of spring in the northern hemisphere! At 0514UT (GMT) this morning, 20 March 2012, the Earth’s axis of rotation went momentarily side-on to the Sun. For the past six months the Earth’s northern hemisphere has be angled away from the Sun, and now we’re angled towards it. Lot’s more info about equinoxes and equiluxes in a previous post.

As most people take delight in the lengthening daylight hours spare a thought for the amateur astronomers who, in a few weeks time, will be packing away their scopes for the summer, eagerly waiting for the darkening nights later in the year.

Happy Spring Equilux 2012

March 17, 2012 4 comments

Today, Saturday 17 March 2012,  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 1825
Belfast 0633 1832
Newcastle 0615 1815
Manchester 0618 1817
Birmingham 0617 1816
Cardiff 0621 1821
London 0610 1810

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, 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!

Easter Astronomy

April 22, 2011 4 comments

Being neither Christian nor Pagan I’m not inclined to pay much heed to Easter (except of course for the days off work and the chocolate eggs) but it does present an opportunity to talk about a little astronomy. Did you know, for example, that the date of Easter changes each year, and that it depends on the phase of the Moon?

The Venerable Bede

The Venerable Bede, a Northumbrian Monk, wrote, in his book “The Reckoning of Time” (725CE):

“The Sunday following the full Moon which falls on or after the [Spring] equinox will give the lawful Easter”

So that all seems fairly straight forward: wait till the Spring Equinox; wait till the next full Moon; wait till the next Sunday; and voila, Easter.

However complications arise because:

(a) the ecclesiastical date of the Spring Equinox does not always match the astronomical date; and

(b) the ecclesiastical date of the Full Moon does not always match the astronomical date.

Why is this?

Put simply the ecclesiastical Spring Equinox always falls on 21 March, whereas the astronomical Spring Equinox, that is the point in the year when the Sun passes from the southern to the northern celestial sphere, can occur on 19, 20 or 21 March (and occurs most often on 20 March), and so the starting date for the computation of Easter is often out.

Also the ecclesiastical date for the Full Moon takes no real account of the actual phase of the Moon, instead it refers to the 14th day of the lunar month in which the New Moon occurs between 8 March and 5 April, which can differ from the date of the Full Moon by up to three days. This is referred to as the Paschal Lunar Month, the word “Paschal” deriving from the Hebrew word for Passover, a Jewish festival commemorating the Exodus.

The table below shows how these two dates vary over the next decade, and how that effects the date of Easter.

 Year (A)
Date of Ecclesiastical Spring Equinox
(B)
Date of Astronomical Spring Equinox *
(C)
Date of Paschal Full Moon on or after A **
(D)
Date of Astronomical Full Moon on or after B
Date of Sunday on or after C (Easter) Date of Sunday on or after D
(Astronomical Easter)
 2011  21 March  20 March  18 April  18 April  24 April  24 April
 2012  21 March  20 March  8 April  6 April  8 April  8 April
 2013  21 March  20 March  30 March  27 March  31 March  31 March
 2014  21 March  20 March  15 April  15 April  20 April  20 April
 2015  21 March  20 March  4 April  4 April  5 April  5 April
 2016  21 March  20 March  24 March  23 March  27 March  27 March
 2017  21 March  20 March  12 April  11 April  16 April  16 April
 2018  21 March  20 March  1 April  31 March  1 April  1 April
 2019  21 March  20 March  19 April  21 March  21 April  24 March
 2020  21 March  20 March  9 April  8 April  12 April  12 April

* Despite these dates being all the same, the date of the astronomical Vernal Equinox can vary between 19, 20 and 21 March, but occurs most often on 20 March. Over the next 100 years it will fall on 19 March only 4 times, and on 21 March only 20 times.

** The dates of the Lunar Months (there are 13 Lunar Months in one Solar Year) repeat on a 19 year cycle, called the Metonic Cycle. 2011 in the 17th year in that cycle; in 2014 we’re back to year 1. The dates for the Paschal New Moon can be found in calculation tables.

So you can see that, despite the fact that the Ecclesiastical Spring Equinox date doesn’t match the Astronomical Spring Equinox date on any year over the next decade (and only matches it in 20 out of the next 100 years), plus the fact that the date of the Paschal Full Moon often doesn’t match the date of the astronomical Full Moon, the dates of Easter calculated using the Ecclesiastical and the Astronomical methods only differ on one year in the next decade – 2019CE, when those who are so inclined will celebrate Easter on 21 April, but when, according to Bede, the Sunday after the full Moon after the equinox actually occurs on 24 March!

Maybe I’ve got too much time on my hands…

[Please note that throughout this post I have calculated and referred to the dates of Gregorian Easter, that celebrated by the western Christian churches. Eastern Orthodox Christians calculate Easter using the Julian calendar, which is currently 13 days ahead of the Gregorian calendar, i.e. on 21 March (the Spring Equinox) the Gregorian calendar reads 3 April.]

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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