Dr Shane Bergin, a physicist at UCD, has been answering some questions posed by first class children at the Divine Word National School, Marley Grange, in Rathfarnham in Dublin.

Speaking on RTÉ's Today with Seán O'Rourke, Dr Bergin said the elves at UCD, also known as 'students' sharpened their pencils and primed their calculators to come up with the science behind Santa.

How many children will Santa Claus visit? How does he get to visit all of them in one night?

There are around two billion people in the world who will be celebrating Christmas, and we guess that about 20% of them will be expecting a gift from Santa Claus, so that would mean 400 million children are due to be visited on Christmas Eve. At an average of three children per house, Santa will have to get to about 130 million houses in one night.

How does Santa visit all of the children in one night?

He will have to move fast to cover that ground and visit 400 million children but Santa Claus is smart.  He knows that if he moves east to west he can take advantage of timezones. So instead of having 24 hours, he will have 31 hours to do his work. Santa Claus will travel at a speed that is about ten thousand times faster than a jumbo jet in order to cover that ground. 

How does he fit all of the presents in his sleigh?

Well, this is a tricky one. If you imagine there are 400 million children in the world, and they each get a present weighing 1kg, like a lego case for example, he would need to have some something that would be hundreds of times bigger than the size of an Olympic swimming pool. So we reckon, and this is where our limits of knowledge come in, we reckon that he uses nano technology in order to shrink presents to put them in to his sleigh.

Why is Rudolph's nose red?

Great question. Well, as we were saying Santa Claus is moving at speed, so if he is moving that fast the light that his sleigh is giving off is starting to bend. That is due to something called the "Doppler effect". (Gardaí use this to figure out how fast cars are going on the motorway, but we can also see strange effects from Santa's sleigh.) So, because the light at the end of Rudolph's nose at the very tip of the sleigh is going to be bending, and bending in colour, as it moves away from us it will appear much, much redder. Interestingly, if Rudolph was coming right at you, his nose would be bright blue.

Do grown-ups have to go to bed early on Christmas Eve?

Yes, everyone has to go to bed early on Christmas Eve, and more importantly, Santa can visit at anytime during the hours of darkness so everyone must stay in bed until the sun comes up.

How does Santa fit down the chimney?

This is a tough one. Well, Santa, who is travelling at huge speeds, might experience the effects of something called Relativity, this is the work of Einstein. So, Santa Claus might stretch as he moves very, very fast. This would elongate Santa Claus, so he would look like a piece of spaghetti and could easily fit down the chimney. Alternatively, if Santa got stuck in the chimney as the song goes, then the fire below him would go out because there would be no oxygen to feed it. In that case, the pressure in the chimney flue would increase and it would pull Santa through. (*If anyone wants to see this, google "egg squeezing through a tight tube"*).

What are the best foods to leave out for Santa Claus?

Fruit and vegetables are good, but we at UCD think carbohydrates would be better. They give out lots of energy but at a slower rate, so perhaps something like crackers. Santa Claus has to visit about 130 million homes, so if he doesn't get to eat what you have left out for him, it's nothing personal. He has a lot to get through in one night, and don't forget he will share this with his reindeer too, as they will need plenty of fuel to do all of that flying.

Can bad weather affect Santa Claus' journey?

No, is the short answer. Santa flies at very high altitudes in order to avoid bad weather. Yes, he has to come down through it in order to land, but remember that his sleigh has been doing this for over 1,000 years so he knows exactly what he is doing.