Written by Anna K. Buk

As we ease into December, Christmas is almost upon us once again. It's time to do our Christmas shopping . . . or maybe ask Santa for his help. But with around 172 million homes to deliver to around the world, with 3 - 4 children per household, how does he do it all in one night? In this Edition, we uncover the physics of all Santa's Christmas secrets.

**What does the speed of his sled need to be?**

With a 31 hour window to deliver gifts to all the children of the world (accounting for different time zones), and 172 million homes - let's say evenly spaced at 1.25 km from each other - Santa must be cranking his sled to full speed to deliver all the gifts. The calculation for velocity is as simple as dividing the distance he has to cover, by the time he must deliver within. Doing this calculation gives us an incredible speed of 6.93 million kilometers per hour. Let's compare this speed to the fastest plane humanity has ever created, which is by the Lockheed SR-71 Blackbird, clocking in at 3,500 kilometers per hour. This means that Santa's sled is 1980 times faster than the fastest flying manned plane! Travelling at 2.31% of the speed of light is bound to have some interesting perks!

**How many visits does Santa make per second?**

We know that he must visit 172 million homes, in a 31 hour window. Converting hours to seconds and dividing homes per our new time window, we get an astounding 1540 home visits per second!

**Perks of travelling as fast as he does?**

Santa's sled travels at 2.31% of the speed of light, which definitely implies relativistic consequences. As Einstein taught us from his Theory of Relativity, time dilation is the phenomenon where the faster you travel, the slower you experience time. Because Santa travels so fast, for him time actually slows down, which allows him more time to deliver presents. Another consequence related to Einstein's theory is that the faster your speed, the less you weigh, which explains how Santa is able to carry the mass of all those presents on his sled. This also means Santa's own mass would be reduced, which would better allow him to sneak into chimneys for quick deliveries. But only if he keeps up with the speed of his sled! Running this fast for 31 hours would tire Santa as well as his reindeer, which is why we leave them milk, cookies, and carrots to energize their long journey!

**How does Santa know where to make his stops?**

It's actually not Santa, but his reindeer who are in charge of making the stops. Because Santa is human, he is only able to see between 380 to 700 nanometers, which is what we call the visible light spectrum. Reindeer on the other hand, are able to see ultraviolet light, which emits light of wavelengths ranging from 10 to 400 nanometers. The sun naturally emits UV rays, so Santa delivers at night so that the reindeer can see better. The lights on our Christmas trees actually emit UV rays, which help guide the reindeer from home to home so that Santa can deliver our gifts!

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