Archive for Equinox

Finding Easter

Posted in History, The Universe and Stuff with tags , , , , , , on April 3, 2026 by telescoper

As an Astronomist I am often asked “How do they calculate the date of Easter?”, so here goes.

The simple answer is that Easter Sunday is on the first Sunday after the first full Moon on or after the Vernal equinox. The Vernal Equinox took place this year on March 20th and the first full moon after that was on April 2nd.

I say “simple” answer above because it isn’t quite how the date of Easter is reckoned for purposes of the liturgical calendar.

For a start, the ecclesiastical calculation of the date for Easter – the computus – assumes that the Vernal Equinox is always on March 21st, while in reality these days it is more frequently 20th March, like this year.

On top of that there’s the issue of what reference time and date to use. The equinox is a precisely timed astronomical event but it occurs at different times and possibly on different days in different time zones. Likewise the full Moon. In the ecclesiastical calculation the “full moon” does not currently correspond directly to any astronomical event, but is instead the 14th day of a lunar month, as determined from tables (see below). It may differ from the date of the actual full moon by up to two days.

There have been years (1974, for example) where the official date of Easter does not coincide with the date determined by the simple rule given above. The actual rule is a complicated business involving Golden Numbers and Metonic cycles and whatnot.

Here is an excerpt from the Book of Common Prayer that shows Anglicans how to determine the date of Easter for any year up to 2199:

The calculations are based on the approximately 19-year metonic cycle, which is why the above table will not work indefinitely

For this year we find that (2026+1=2027) ÷19=106 with a remainder of 13 (106 × 19 being 2014). The Golden Number for this year is therefore 13, or XIII in the Table. This gives the date of the Paschal Full Moon, which occured this year on 2nd April, which is indeed the day in the centre column next to XIII in the left-hand column in the table. The Sunday Letter is determined by the remainder of (2026+506+6)÷7, which is 4, so this year’s Sunday Letter is D. The date of Easter Sunday is given by the entry in the centre column next to the first occurrence of D in the right-hand column after the Golden Number XIII appears in the left-hand column, i.e. April 5th. I hope this clarifies the situation.

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The Vernal Equinox 2026

Posted in Biographical, Maynooth, The Universe and Stuff with tags , , , on March 20, 2026 by telescoper
Loughcrew Cairn: for a few days on and around the Vernal Equinox the rays of the rising Sun penetrate the passage and illuminate the back stone.

Just a quick note to mention that the Vernal Equinox (Spring Equinox) in the Northern hemisphere happens this afternoon, Friday 20th March 2026, at 14.46 UTC (which is 2.46 pm local time here in Ireland. Many people in the Northern hemisphere regard the Vernal Equinox as the first day of Spring, atlhough round these parts that is taken to be 1st February (Imbolg). Of course in the Southern hemisphere, this is the Autumnal Equinox.

The date of the Vernal Equinox is often given as 21st March, but in fact it has only been on 21st March twice this century so far (2003 and 2007); it was on 20th March in 2008, has been on 20th March every spring from then until now, and will be until 2044 (when it will be on March 19th). I’ll be retired by then.

Anyway, people sometimes ask me how one can define the `equinox’ so precisely when surely it just refers to a day on which day and night are of equal length, implying that it’s a day not a specific time? The answer is that the equinox is defined by a specific event, the event in question being when the plane defined by Earth’s equator passes through the centre of the Sun’s disk (or, if you prefer, when the centre of the Sun passes through the plane defined by Earth’s equator). Day and night are not necessarily exactly equal on the equinox, but they’re the closest they get. From now until the Autumnal Equinox, days in the Northern hemisphere will be longer than nights, and the days will continue get longer until the Summer Solstice on June 21st before beginning to shorten again.

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The Vernal Equinox 2025

Posted in Biographical, Maynooth, The Universe and Stuff with tags , , on March 20, 2025 by telescoper
Loughcrew Cairn: for a few days on and around the vernal equinox the rays of the rising Sun penetrate the passage and illuminate the back stone.

Just a quick note to mention that the Vernal Equinox (Spring Equinox) in the Northern hemisphere happens this morning, Thursday 20th March 2025, at 9.01 UTC (which is 9.01am local time here in Ireland, i.e. in about half an hour). Many people in the Northern hemisphere regard the Vernal Equinox as the first day of spring; of course in the Southern hemisphere, this is the Autumnal Equinox.

The date of the Vernal Equinox is often given as 21st March, but in fact it has only been on 21st March twice this century so far (2003 and 2007); it was on 20th March in 2008, has been on 20th March every spring from then until now, and will be until 2044 (when it will be on March 19th).

Anyway, people sometimes ask me how one can define the `equinox’ so precisely when surely it just refers to a day on which day and night are of equal length, implying that it’s a day not a specific time? The answer is that the equinox is defined by a specific event, the event in question being when the plane defined by Earth’s equator passes through the centre of the Sun’s disk (or, if you prefer, when the centre of the Sun passes through the plane defined by Earth’s equator). Day and night are not necessarily exactly equal on the equinox, but they’re the closest they get. From now until the Autumnal Equinox, days in the Northern hemisphere will be longer than nights, and the days will continue get longer until the Summer Solstice before beginning to shorten again.

P.S. This time last year I was in Barcelona. Time passes.

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Yesterday was nearly Easter

Posted in History, The Universe and Stuff with tags , , , on March 29, 2021 by telescoper

As as Astronomist I am often asked “How do they calculate the date of Easter?”, to which my answer is usually “Look it up on Wikipedia!“.

The simple answer is that Easter Sunday is on the first Sunday after the first full Moon on or after the Vernal equinox. The Vernal Equinox took place this year on March 20th and the more observant among you will have noticed that yesterday was (a) Sunday and (b) a Full Moon. Yesterday was not Easter Sunday because the rule says Easter is on the first Sunday after the first full Moon on or after the Vernal equinox, which does not include a Full Moon on the first Sunday on or after the vernal equinox. Accordingly Easter 2021 is next Sunday 4th April. If the Full Moon had happened on Saturday, yesterday would have been Easter Sunday.

That is just as well really because next weekend is when the holidays and sporting events have been arranged.

I say “simple” answer above because it isn’t quite how the date of Easter is reckoned for purposes of the liturgical calendar.

For a start the ecclesiastical calculation of the date for Easter – the computus – assumes that the Vernal Equinox is always on March 21st, while in reality it can be a day or two either side of that. This year it was on March 20th.

On top of that there’s the issue of what reference time and date to use. The equinox is a precisely timed astronomical event but it occurs at different times and possibly on different days in different time zones. Likewise the full Moon. In the ecclesiastical calculation the “full moon” does not currently correspond directly to any astronomical event, but is instead the 14th day of a lunar month, as determined from tables (see below). It may differ from the date of the actual full moon by up to two days.

There have been years (1974, for example) where the official date of Easter does not coincide with the date determined by the simple rule given above. The actual rule is a complicated business involving Golden Numbers and Metonic cycles and whatnot.

I’m grateful to Graham Pointer on Twitter for sending this excerpt from the Book of Common Prayer that sheweth how to determine the date of Easter for any year up to 2199:

I don’t care what happens after that as I’ll be retired by then. If you apply this method to 2021 you will find it is an 8C. Next year will be a 9B. Further calculations are left as an exercise to the reader.

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The Michael Green Experience

Posted in Biographical, The Universe and Stuff with tags , , , on October 30, 2009 by telescoper

It’s been a couple of weeks since the University of Cambridge announced that the successor to Stephen Hawking as Lucasian Professor  of Mathematics would be Michael Green, who is best known for his work on string theory. Heartiest congratulations to him for reaching a position of such eminence.

I was trying to think of a suitable way of marking the occasion of his election to this prestigious post when I suddenly remembered that we were actually on a TV programme together years ago. The show in question was called Unravelling the Universe and was first broadcast in December 1991 as part of a science documentary series called Equinox.

I eventually found my ancient VHS copy of the broadcast master tape of this show and persuaded Ed and Stephen, two of the excellent elves that work in the School of Physics & Astronomy here at Cardiff University, to transfer it to a digital format and put a bit on Youtube for all to see. Many thanks to them for their help.

Other people involved in the programme included Rocky Kolb, Chris Isham and Paul Davies but the short (2-and-a-half minute) clip below features just Michael Green (who basically put the show together) and myself (who was just there to make up the numbers), plus wonderful narration by the late great Peter Jones.

Michael Green hasn’t changed a bit in 18 years. In fact, I saw him last year and am sure he was even wearing the same sweater.

I, on the other hand….Oh dear.

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