# Precession

Hi,
in the last post about Polaris I mentioned precession as the effect that causes change of North and South stars. I did not really explain what is happening there so I decided to make a short post only on that topic.

There are two other good examples of objects that precess alongside Earth. Take gyroscope for example:

On the left you can see gyroscope. It consists of spinning wheel (orange/yellow) and a pin on which it stands. When you speed the gyroscope up it will be able to stand as you can see on the animation. This is basically what Earth looks like and what it does in matter of 26,000 years.

There is a thing called torque in physics. If you hold a pole on one side and something is hanging on the other side it will be very hard to lift it up. You will have to use both of your hands and one will push down and the other up to counter the torque that the objects has, that is a force causing it to rotate around one of your hands.

If you spin the thing though it will suddenly be easy to lift the thing, why? Because of angular momentum and conservation of angular momentum. If you spin on your chair and stretch your arms and legs you will slow down but if you do the opposite you will speed up. No force is acting on you only conservation of angular momentum takes place. Angular momentum is calculated as the amount of mass spread in distance from axis of rotation. If it is further away it does not need to be so fast as then the mass is closer to the axis.

When you speed the object up the angular momentum will stay conserved so you do not need to provide the torque anymore, in other words it is going to be easy to lift the thing up. If you try to change the angle in which it spins it will feel weird and it wont like it.

Even than gravity is still pushing down. If you add the vectors of the forces together, you will find out that the object will rotate –> precess. The slower it is the bigger the angle in which it precesses. If the force would be applied only on one spot all the time then it would not precess. Take a round sheet of cardboard and rotate it on tip of pencil. If you blow on the spot close to you the thing will tilt left from your point of view. Gravity though will try to tip the thing over always on the side that is lower and thus it will continously change and rotate – precess. If Earth had no tilt there would be no precession.

Dragallur

# Why is the Moon so strange?

Hi,
here it comes, here it goes. Today I will write about the strange shape of Moon which is actually pretty normal and you probably saw it some day, of course I mean that the strange thing is that it is “turned” by 90° degrees. I saw it yesterday/today during flight from Rio de Janeiro to London. It was very difficult for me to imagine the position of Moon, Earth and Sun to create this. But today I have downloaded Stellarium which seems to be pretty awesome and now it is clear to me so I will try to explain it.

Fine, the problem is the rotational axis of Earth. It is tilted by over 20°. This means that when you are standing on equator you are moving by a huge distance up and down like on rail coaster during day and night (this changes your point of view on the Moon so the lighted part seems to change). When you are far away from equator you are not moving that much. Parallel are not so long, the difference is less and less and when you get on pole it is not moving at ALL! By this move I mean rotating of the part of lighted Moon through its journey since from our point of view Moon seems to be lighted from different sides throughout the day.

Well it was kind of hard for me to find the right words but you can always ask in the comments.
Dragallur

Source of 1st picture: https://10minuteastronomy.wordpress.com/category/sky-motions/