##### Why do Christmas pyramids turn?

It is impossible to imagine living rooms and lounges at Christmas without Christmas pyramids now. For many they are the epitome of the Christmas and Advent period and it is only really Christmas once their blades gently turn above the candleholders in the candlelight.

However, children in particular have been asking in amazement for generations: Why do the figures on the Christmas pyramid turn so silently and seemingly as if by magic?

The solution to the puzzle lies in the physics: Strictly speaking the pyramids are a simple thermal engine. The candles in the candleholders heat up the air above them. As they are heated up the air molecules move faster and need more space. The molecules collide with each other and bounce off each other. This leads to not as many molecules fitting into the same space as before they were heated up. As a result the air above the candles expands and its density decreases. Due to the lower density the heated up air is lighter than the cold air and rises. Up there the heated air molecules hit the slanting blades of the pyramids and make them move by discharging part of their energy.

The Christmas pyramid turns fastest when the blades are at an angle of 45 degrees. If they are steeper the molecules flow past them without transferring their energy to them. If the blades are flatter the molecules tend to push them upwards rather than setting them off in a rotating motion.

Seen purely physically a Christmas pyramid is still way behind the possibilities of modern physics as far as speed is concerned. Thin windmills made from metal instead of wood with twisted instead of straight wheels would enable a higher speed.

However, luckily with the Christmas pyramids the important thing is not speed but creating a peaceful mood through candlelight and gentle movement.

What should you do if the pyramid is not turning properly?

Besides the thermals a Christmas pyramid also turns as a result of the fine tuning between height of the candles, the distance between the candles and blades, the aforementioned alignment of the blades and the resulting friction of components when turning. Perfect craftsmanship is demonstrated in these factors being perfectly tuned to one another and the pyramid rotating smoothly and regularly.

If the pyramid as a whole is so contorted and twisted that the friction is too high and the energy from the candles is not enough to set it off in a rotating motion it can often only be repaired by the manufacturer, if at all.

If the pyramid turns easily and smoothly when manually pushed then the lack of momentum is often due to the position of the blades. You can often easily remedy this yourself by putting all the blades at the ideal 45 degree angle. If the speed of a basically well running pyramid is to be controlled a realignment of the blades is also a good method for this.

If the pyramid is not running smoothly the reason for this may also be the pyramid bearing. Here the tip of the pyramid axis lies on top of a plate and ensures rotation with as little friction as possible. Sometimes the tip of the pyramid eats a little bit too far into the plate due to long use and at some point it becomes stiff and brings the pyramid to a halt. If this is the case it is recommended that you replace the pyramid bearing.
Particular caution is called for when operating a stiff pyramid. If the pyramid stops turning unnoticed the blades can quickly get too hot and ignite.

Also take care with where you place the pyramid. If it is too high in the room, possibly close to the ceiling, the heat accumulated there from other heat sources, for example the heating, can negatively affect the thermals.
If the pyramid is in a draught, for example in a hallway, the heat from the candles cannot rise in an ideal way and set the pyramid turning.

For many people the Christmas pyramid is a permanent fixture of their Christmas decorations and brightens up the living room every year. It creates a fascinating play of light in the room through its gentle movement and conjures up a unique atmosphere. We hope you enjoy this spectacle and getting in the mood for Christmas.