Apparent and real trajectory?

[Diego]

While observing Sirius a few minutes ago I was looking up some info about it. This lead me to read up about Sirius B, even though I wouldnt be able to observe it with my 4.5" reflector. I read about it having an apparent and real trajectory around Sirius A. Does this imply that it appears to orbit around Sirius A in a certain way to us that would be... I dont know, false??? If so, how is that possible? Would it have anything to do with how motion and position are relative to the observer etc?

Although interesting, I dont really have a vast understanding of physics. Maybe this question doesnt even have to do with physics :p

Wikipedia has a diagram of what im talking about if you look up Sirius A.

Thanks for any help

[Merlin66]

The real motion of a double star will have both stars revolving around their common centre of gravity - like the earth revolving around the sun.

Let's assume this is basically a circular orbit.

If it were viewed from the side the apparent orbit would look like an ellipse, getting narrower as we view closer to the plane of the real orbit.

To find the real orbit from observations of the apparent orbit requires a bit of maths..

Hope this helps.

[Diego]

Its still a bit fuzzy, but im getting it a little better with what youve just said.

many thanks

In reference toy our analogy of it to the earth orbiting the sun, wouldnt a binary systen orbiting a common center of gravity be more like the earth and moon going around the sun thatn the earth alone going around the sun though?

[Merlin66]

No.

All orbiting bodies revolve around their common centre of gravity...for the earth this is firmly inside the Sun.......

[dmahon]

But not for Jupiter

[lunator]

Hi Diego

The way I find easiest to visualise it is to imagine two footballs orbiting around each other. If you look stright down on them it is easy to see the shape of the true orbit.

If you then drop down so you are looking at them from 45 degrees or even in the same plane - exactly side-on then you will see the orbit looking very different although the footballs are still following the same true orbit.

Hope that helps

Cheers

Ian

[JulianO]

There are some good animations on this wiki page

Multiple star - Wikipedia, the free encyclopedia

[Diego]

Thanks for the help guys

[jimmyjamjoejoe]

There are some good animations on this wiki page

Multiple star - Wikipedia, the free encyclopedia

Nice link, i like the picture E.

Interesting pic of a quadruple star system too...