How they counted the years in the BC era; what noise does a giraffe make? How to solve sudokus - including the killers

Before AD, what did people of the BC era call their years?

The dominance of one calendar for world events is quite recent and many other calendars remain in use: the Ethiopian calendar, for ­instance, has 13 months. The references AD and BC are sometimes ­replaced by CE and BCE: Common Era and Before the Common Era.

The Roman calendar was counted Ab urbe condita ("from the foundation of the city"), in 753 BC; and it continued in use until the Anno Domini calendar was introduced in AD 525. The monk who calculated AD from AUC forgot that the Emperor Augustus ruled for four years as Octavian before he changed his name, and this error remains in the system. Also, as he counted in Roman, not Arabic, numerals, he did not include the years 0 BC and AD 0.

The Muslim calendar runs from the Hijra, Muhammad's flight from Mecca to Medina in AD 622. Like the Christian calendar, it displaced earlier calendars such as the Zoroastrian one in Persia, which dates from about 1200 BC. The Muslim calendar is a lunar one, but Iranians still celebrate Nowruz, the new year in the solar ­Zoroastrian calendar, at the spring equinox each March.

The Chinese calendar dates back to about 2700 BC and the Hindu calendar to about 3100 BC. The Jewish calendar has an even earlier starting point, 5,770 years ago, calculated as the date of the creation as described in scripture.

Roger Crosskey, London W10

Official records of the Roman ­empire and its successors used two systems in parallel. One, used in legal documents, dated from the accession of the current emperor, and started again with each new emperor (a system still used with each new ­monarch in English law). The other, used in historical works, was AUC, Ab urbe condita. In "the year of the consulship of Probius Junior" (1278 AUC) Dionysius Exiguus, a member of the Roman curia, invented the AD system by recording that it was 525 years "since the incarnation of our Lord Jesus Christ".

He probably arrived at that date by looking up the recorded dates of incidents mentioned in the Christian gospels. Matthew 2:1 has Jesus born during the reign of Herod, who died in 749 AUC after a long illness. Luke 2:2-6 has Jesus born at the time of the census of Judea instituted by Quirinius, which took place in 759 AUC. Dionysius seems to have decided on a compromise, putting the birth of Jesus between the two ascertained dates, at 753 AUC.

Much later, in AD 731 (1484 AUC), the custom of dating events AD, using Dionysius's date, was originated by the historian the Venerable Bede.

Donald Rooum, London E1

I can do passable dogs, pigs, lions, snakes etc for the children, but what noise does a giraffe make?

When my daughter was two I ­recall seeing her turning the pages of the children's book Dear Zoo, making the relevant animal noises – a roar for the lion, a hiss for the snake and so on. When it came to the giraffe there was a pause for thought, then she shouted "Tall! Tall!"

Royston Robertson, Broadstairs, Kent

I'm not sure giraffes make a noise, but in Kenya on holiday I was told giraffes employ a foster-mother ­system whereby one or two adults look after the young of the whole group so that the rest of the adults can forage for leaves. So if you're making animal noises to keep the ­children amused because you're minding them, then you're probably imitating a ­giraffe already.

Louise Harley-Smeur, Munich, Germany

A giraffe is the only animal with no vocal chords, so makes no noise at all. Source of this information – a quiz at the Dunelm Ridge, 29 December 2009.

Mike Rayner, Durham

Who cares? Make any noise you like and tell them that's the noise a giraffe makes – who's going to know any different?

Alan Paterson, London N8

Why is it that I can solve the Guardian's easy and hard sudokus, but never manage to complete the medium ones?

I put it to the test myself, playing sudoku 1,429 and would have judged it about medium difficulty as stated. On this statistically significant sample of one, I judge that the assertion that medium puzzles are harder than hard puzzles to be incorrect.

Allan Postgate, Bromsgrove, Worcs

Anyone can solve any sudoku ­(including killers) any time by ­using a pencil with a rubber on the end. You just put in the possibilities and rub out as you eliminate them. Simple!

Jane Jones, Hoole, Chester

Any answers?

My computer is getting rather slow. It's five years old. What's that in human years?

S Kirby, Esbjerg, Denmark

What happens to the food that TV chefs cook? Does the camera crew eat it, or is it given to the homeless?

Melanie Murray, Salford

Where does the curry in "curry favour" originate?

John Robinson, Liverpool

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