Posts Tagged ‘astronomical’

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.




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
Date of Astronomical Spring Equinox *
Date of Paschal Full Moon on or after A **
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.]

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