# Predictions of star positions

1. Apr 16, 2012

### bignevermo

Can the star positions be predicted with any degree of accuracy? If so are there any variables that woukld dramaticaly challenge said predictions? I say they can be but i am being challenged on it and i dont have any sources to back it up. Anyone? Thanks i am just ! :shy: newbie and dummie here..so take it easy huh?

Last edited: Apr 16, 2012
2. Apr 16, 2012

### SHISHKABOB

What do you mean by star positions? Like, in the sky so that you know where a star will be so you can see it with your telescope? Because yeah, stars can be predicted. The positions of stars in the sky change over the course of a day because of the rotation of the Earth, so there are two numbers we use to locate a star in the sky. Declination and right ascension.

Imagine, if you will, a big sphere around the Earth, this is called the celestial sphere. Everything in the sky is found on the celestial sphere, it's exactly like a planetarium, except it goes all the way around the planet. The equator lines up with the celestial equator and the same goes with the celestial north pole and south pole.

Basically, declination is the same thing as latitude and right ascension is similar to longitude. Declination is easy, because the Earth only rotates in one direction. A position in right ascension changes as the day goes by. They are measured in hours, seconds and minutes. 24 hours around the whole thing. So what you do is you take your current local time, compare it to the right ascension, and then subtract some number of hours from the right ascension value, and there's the current spot. Then you line up your telescope and find the star.

It's a lot more complicated than I made it sound, but that's the gist of the idea.

Over the course of tens of thousands of years, the axis of the earth's rotation moves a bit due to a thing called precession. Basically, the north star will change over time. I think it takes 27000 years to do a whole rotation.

Then of course there is the movement through the galaxy, but that takes tens of millions of years.

3. Apr 16, 2012

### Staff: Mentor

We can track stars as they move through space and calculate future and past positions/paths with varying degrees of accuracy depending on the accuracy of the measurements and calculations.

4. Apr 16, 2012

### bignevermo

thanks my "nemesis" said that there cannot be accurate predictions because there may be gravitational pulls on the stars so the predictions wont hold up. I stated that the predictions can be in the future by thousands of years into the future and he said: "Only to a very limited extent, and only based on what is known. The star may be headed toward a gravitational force that is yet unknown and the prediction would be false"

i have not been able to respond because the thread on that phorum is closed but with some good data i will open a new thread.

5. Apr 16, 2012

### Staff: Mentor

The problem with his argument is that he assumes that we don't see the source of this gravitational force and don't notice it's effect on other objects. This is...unlikely. And remember, it's all about accuracy, not a black and white case.

6. Apr 16, 2012

### DaveC426913

Agree on both counts.

It's easy for your opponent to claim there's lots of mysterious gravitational forces out there. He'd have to demonstrate that that precludes determining star positions with some degree of accuracy.

And it is a question of accuracy. How accurate does your opponent demand? You're both right if you each choose your own criteria for accuracy.

7. Apr 17, 2012

### Chalnoth

It's always easy to predict the paths of stars on the same time scales as we have recorded observations of said stars. It's not so easy to extrapolate their motions far into the future.