A YOUNG lady, aged seven, wrote to me recently from her home near York to ask if I could tell her how the date of Easter is calculated.

She needed the information for a school project and pointed out that the other major Christian festival, Christmas, falls on the same date every year. I responded with an edited version of the following, adding a note for the teacher detailing the more complicated bits!

One astonishing fact is that Easter continues to be calculated on a system which used a hypothetical moon devised by an Athenian astronomer as long ago as 432 BC.

By using this artificial "moon", the date of Easter falls on the same day in every part of the Christian world and so it was adopted by Pope Gregory when he introduced his reformed calendar in 1582 (which was not adopted by the English until 1752).

If the phases of the real moon were used as a basis for these calculations, there would be variations of up to a lunar month between countries in the different hemispheres; the artificial "moon", however, has phases which alternate between 29 and 30 days.

This system established, world-wide, that Easter Day is the Sunday after the full moon which happens on or next after the 21st day of March, and if that full moon happens to fall on a Sunday, then Easter Day is the Sunday following.

Thus Faster Sunday can never arrive earlier than March 22 nor later than April 25. In fact, the rarest date for Easter Sunday is March 22. It last occurred on this date in 1828 and it will not fall again on this date until 2285 - unless the method of calculating Easter is altered.

For many years there has been a desire that Easter Sunday should fall on a fixed date. The Vatican council has agreed to work towards this idea and in 1979 it established talks between the Catholic church and the Orthodox churches but although an earlier vote among council members was overwhelmingly in favour of fixing the date of Easter, this has not yet happened.

To do so requires the agreement of all Christian churches and although the World Council of Churches agrees that a fixed Easter is favoured by most, world-wide agreement has yet to be reached.

England tried to "go it alone" in 1928 when our government passed the Easter Act. This authorised Easter to be celebrated annually on the first Sunday after the second Saturday in April - but neither the English parliament nor the Church of England acts on behalf of world-wide Christianity and this law could become effective only if an order in council brought it into effect - and if all the Christian churches agreed. To date, there is no such agreement and so everyone waits!

The fact that the date of Easter varies each year also means that several associated Christian festivals have variable dates, including Shrove Tuesday, Ash Wednesday, Lent and Pentecost.

While this ever-changing system of dates may not be too disruptive from a churchgoer's viewpoint, it does affect the tourist industry and its representatives have constantly pressed for Easter Sunday to become a fixed date.

So far as this country is concerned, there was a dispute over the date of Easter as long ago as AD 664.

Two branches of the Catholic church then existed in England, one following the Celtic tradition and the other supporting Rome. It meant that the Celtic church in the North of England celebrated Easter at a different time from Christians in the South.

King Oswy of Northumberland wanted everyone to celebrate Easter together and so the matter was discussed at the famous Synod of Whitby. After much argument, St Wilfrid of Ripon reminded the gathering that St Peter and his successors in Rome held the keys to the Kingdom of Heaven, and recommended adoption of the Roman system. This was agreed and, in spite of the Reformation, has continued in England since that date. But the Vatican does favour a fixed Easter - if all churches can agree.

Carroll connection

Following my notes about pele towers, a Knaresborough correspondent has provided some interesting facts about one of the towers which I featured. This was Elsdon Tower in Northumberland which is one of those known as a parson's pele, built to protect the local priest from invaders. It is believed to date from around 1400.

One of his snippets is that the spiral staircase in Elsdon Tower ascends anti-clockwise and this has made him ponder whether the original owner of Elsdon was left handed.

Bearing in mind that a man defending his home in such a tower, and on such a staircase, would require his right hand to be free to grasp his sword, then a staircase which can be defended only by a left-handed person is, perhaps, unique.

The second piece of information is that, in 1760, Eldson Tower was occupied by the local rector, the Rev Dodgson, who was great-great grandfather of Charles Dodgson, whose father, also called Charles, was rector of Croft-on-Tees, near Darlington.

Charles Dodgson junior is better known as Lewis Carroll, the author of Alice in Wonderland and he has many associations with this region. It is not widely known that Lewis Carroll began his literary career in Whitby. He was a regular visitor to the town and regularly walked along the beach, His first published work was Lady in the Ladle, a poem he wrote while staying in East Terrace near Whitby's Royal hotel. He also attended his brother's wedding in Sleights in 1871.

So far as Elsdon Tower is concerned, it continued to be the home of priests and vicars until 1962, after which it was restored to become a rather spectacular private house. On a stone panel in the front wall there is a coat of arms of the Umfraville family, with those of the famous Percys on the east wall and the Howards on the north wall.

I thank my correspondent for his information.

Garden drama

Yesterday evening, drama came to the bird population of our garden with the unexpected arrival of a male sparrowhawk. A wonderful variety of small birds were feeding from our peanuts and seeds - we'd just spotted a goldfinch, a pair of siskins, several great tits and blue tits, umpteen house sparrows, four greenfinches and a chaffinch - when, in a trice, they all vanished. As dusk was almost upon us, I thought they had all rushed off to roost.

Then I spotted the crouching figure of the sparrowhawk on the garden wall. Clearly, our regular guests at the bird feeding station had noticed him long before I did and had made themselves scarce, but the hawk began to show intense interest in one of our borders, particularly a substantial evergreen shrub.

I realised that, within that shrub, a host of little birds had already settled down for the night, a number of blue tits among them.

The sparrowhawk then began a sustained attack on the bush, trying unsuccessfully to dive into its depths, then dropping to the ground and even clambering up the base of the trunk. The surprising thing was that the little birds deep inside did not leave - I think they realised the density of the branches and leaves would protect them.

I was tempted to go outside and shoo away the sparrowhawk but realised that if I did so, I would also scare away those little birds - and if they left the security of the shrub, at least one of them would fall prey to the hawk. So I decided to let nature take its course.

The sparrowhawk remained in that area for about 15 minutes, by which time dusk had turned into night and he flew off without his supper. This morning, I sought further signs of his presence but found none and as I compile these notes, he has not made a return visit.

The little birds are back at their feeding station, the tits are examining our nest boxes - and we have a pair of pied wagtails who may be the pair who nested in our ivy last year. They are examining that same ivy again; it contains their old nest but are these new wagtails? Do wagtails mate for life and return to their former haunts? It seems we are due for another wagtail summer.