Santa’s trips both down and up the chimney pose puzzles from a physicist’s point of view: if the typical household chimney is 30 feet tall, simply dropping down the chimney would be dangerous indeed.

" ‘Twas the night before Christmas and all through the house, not a creature was stirring, not even a mouse.”

So begins the much-loved poem by Clement Clarke Moore about Santa’s visit to a household where "the children were nestled all snug in their beds, while visions of sugar plums danced in their heads."

The poem has inspired generations of children and grown-ups with the wonders and the magic of Christmas. Much of the Santa Claus story invokes magic, but there’s physics involved, too. Take just one aspect of Santa’s visit: At each household he parks his sleigh, goes down the chimney and delivers presents under the Christmas tree for the good boys and girls of the house. According to the poem – our definitive source for Santa facts – the trip down the chimney is described as something of a free-fall – “Down the chimney St. Nicholas came with a bound.”

After dropping off the gifts and taking a quick snack of milk and cookies, he leaves: “Laying his finger aside of his nose giving a nod, up the chimney he rose.”

Santa’s trips both down and up the chimney pose puzzles from a physicist’s point of view. If the typical household chimney is 30 feet tall, simply dropping down the chimney would be dangerous indeed. More than 350 Yuletides ago, Galileo examined the motion of bodies falling under the constant acceleration of gravity. From his results it is easy to calculate that in free-fall, Santa’s descent would take about 1.4 seconds, and he’d hit the Earth at about 30 miles per hour. As Galileo showed, aside from air resistance, the drop time and impact speed do not depend at all on the mass of the body. So let us assume that Santa probably slides down a rope tied to his sleigh on the rooftop. Also, as we know, he is “chubby and plump,” with a “little round belly that shook when he laughed like a bowl full of jelly.”

No doubt, by bumping his tummy on the bricks during descent he would further brake his downward plunge, while rappelling down the rope like a mountain climber.

In a chimney, this would be messy, so it’s no surprise that “his clothes were all tarnished with ashes and soot.”

Going back up the chimney offers a much more challenging physics problem. Santa could just climb, hauling himself hand-over-hand up his rope. But for such a chubby fellow, that would be slow and strenuous – much too demanding in view of his incredibly busy schedule.

Even if Santa could leap like Michael Jordan in his prime, a quick jump up the chimney is not feasible.

With no room for a running approach, the jump would have to be from a standing start, and the world record for a vertical high jump is only 4 feet. In order to exceed that, and reach 30 feet, Santa would need a means to blast off like a human rocket.

Simple calculations show that, in order to exit in a second or so, he’d have to be subjected to uncomfortably strong forces; Santa would emerge not merely tattered but fractured.

Surely, “laying his finger aside of his nose” is a telling clue. Our conclusion is that in his cap he has a little electrical device that would have delighted Ben Franklin, who pioneered the understanding of electricity.

The device evidently must be triggered by his finger, probably by interrupting a faint light beam near his glowing cheek. That would generate a photoelectric signal in a way described by Einstein in 1905. The device could then send a radio signal to activate a winch on the sleigh, thereby winding up Santa’s tether and enabling him to rise effortlessly.

So up the chimney he goes – with a little help from physics – and into the night he flies in his reindeer-driven sleigh – with a lot of help from magic.

Merry Christmas to all, and to all a good night!

Dudley R. Herschback, Ph.D., is a professor of chemical physics at Harvard University and a Nobel laureate. He is an expert on molecular collisions and has devoted much of his time in recent years to enhancing public understanding of science. This article has been reprinted as a Christmas tradition since 1998.