Ok, in response to your questions. A graphics designer creates graphics which are images but could also be animations depending on the skills of the graphics designer. A web programmer creates the interactive parts of the web application. For instance when you click a planet a popup might appear giving more information about that planet. Even revolving planets could be animated gifs and might be simply that unless you want the user to have control over the speed of the animation and then it will become more complex. So an animation where aspects of the animation can be controlled by the user would involve both graphics design and programming.
The graphics design covers any visual element and often includes @the shape and colour of any clickable buttons.
As far as I can tell there are no server side elements so that most, if not all, development concerns what happens in the browser.
In order to prepare to have this work done by a contractor you should consider breaking the project into milestones and then check the work is satisfactory to you at each stage. (You don't want to be stuck with a negligent contractor!) For instance:
Creation of single image backdrop of the night sky image on which to overlay planets.
Creation of planet animations
Programmer to create information popups for planets
Programmer to create animation controls for planets
That's just an example. Depending on what you need you may divide it up differently. Whatever you decide you do need to be able to track the progress of your project against your requirements.