I draw a circle. I duplicate it and move it away. I rotate the second circle 90 degrees with the rotation tool, then right arrow. Then I select the outline edge of the first circle. I select the Follow Me tool. I click on the second circle interior (not edge). Boom. The second circle is now a sphere. I delete the first circle.
Now, how do I find the centre of this sphere?
Good question!
You might be better off finding the centre of the circle before you make the sphere.
On SketchUp desktop you Right Click the circle edge and choose âfind centreâ this creates a guide point
Hopefully that option is there on web too
yes it is. itâll create a guide point at the center
another way would be to show hidden geometry. Poles would be revealed, allowing to draw a line from one pole to the other. the center of the sphere will be at the middle of this line
Another option:
Place the origin of the sphere at the center before or after Follow Me. Here Iâm doing it before Follow Me. I also added a guidepoint as Adam described but it isnât really required if the sphere componentâs origin is at the center.
Please complete your forum profile with SketchUp version, current opertaing system version, and graphics card model.
Ah yes, hidden geometry will be very useful, thank you.
Iâm drawing a futuristic city in Sketchup because Blender is a mess. But Sketchup free is limited. Should I pay to upgrade? Can I put stars and planets in the sky etc using Sketchup?
Wow, thanks for the video tutorial!
Depends on how far away you want these celestial bodies to be from the Earth-based geometry. SketchUp has limitations in overall model dimensions. The larger the overall model (or equivalently the further any geometry is from the world coordinate system origin), the worse is how that geometry is rendered, and the overall model becomes âjitteryâ. Thus, creating stars and planets as geometry - even if only a few miles above the ground plane - might not work very well.
On the other hand, you might be able to use a sky sphere background image. I am not familiar with SketchUpâs features in that area; other experts can comment. 
I just downloaded the free trial of Sketchup Studio: AU$1200 a year! Hopefully this will allow me to create an image mesh to replace the sky.
3D tools tend not to use spheres for the sky. When you do that you have one texture for every direction, and so the image either has to be massive, or it ends up looking not so good.
They usually have a sky box, and thatâs something you can make in SketchUp. Try this model, and choose the Look Around tool, then look in various directions.
skybox.skp (2.3 MB)
Just an added idea, depending on what you need to do you may not need to mark the center of a sphere, if you make it a component which is good practice anyway. Using the Move tool you can toggle through the available grips (grey dots), one of them is objects center.
Thank you, but this cube has the same problems as my sky dome.
I have never made a component. I always make groups. Is there a difference?
A face can have clouds on one side while the other side seems to be not there.
Thatâs very interesting about JPG. How do I fly through objects instead of having to scroll the mouse wheel a million times when passing through it?
One way is to use the âOrbitâ tool to swing through faces sideways.
Or apply the âPosition Cameraâ tool while being in X-ray face style and with âHidden Geometryâ turned on.
This lets you click (and hold down L-mouse button!) on a starting vertex on the sky and drag the camera to the target inside.
(creating shortcut keys for âX-rayâ, âHidden Geometryâ and if needed also for toggling to âPerspectiveâ helps)
Or experiment with âWalkâ in perspective view. Pay attention to the status bar down left to see information about what keys help using it. [Alt] disables collision ditection (with faces)
I did do a variation where I have the outside faces be transparent. Also locked the sky box, but I donât think there is a way to make something be mouse-transparent, so mouse-wheeling gets slowed down. Shadows are tricky too, to make the sky box be seamless involves setting the shadows dark value too high to see shadows.
Clipping is also more of an issue. One way around that is to hide the sky box until you are ready to export some images.
skybox.skp (2.3 MB)
I would forget about making the sky dome to real astronomical dimensions or even near and just make it large enough that your model fits inside and that it looks convincing from the inside. People are able to perform sci-fi plays inside the walls of a theatre stage. Use more smoke and mirrors.
Both @colin and @Anssi are right.
My suggestions are for general solutions.
But in your case with an unrealistic size of a sky dome (not for what SketchUp was developed for!) I would follow their advice in bringing the dome size back to just covering your entire model.
Think of visiting a planetarium:
Yes. If you collect some geometry (e.g., edges and faces) into a component, then if you make copies of a component and edit any one of the copies all the copies will automatically stay in synchronization - they all receive the same edits instantly. If you collect some geometry into a group and make copies of the group, editing any of the copies only affects that one copy of the group. Sometimes that is desired, and sometimes the auto-updating behavior of multiple components is desired.
If you never expect to have more than one copy of the geometry, the difference between a group vs. component behavior might seem immaterial. However, there are cases when you only want one final copy of the geometry but could benefit from temporarily having a second copy - the so-called âDaveâ method of working around SketchUpâs inability to directly model end points that are very close together (e.g., small edges such as frequently occur when modeling small features such as screw threads or when boring holes through curved surfaces formed of small faces). In the Dave method, the geometry is made into a component and then a copy of the component is placed somewhere convenient in the model space. The copy is scaled up by 100x or 1000x, and then edited. At that scale, the âsmallâ features become 100 or 1000 times larger, which is generally large enough to prevent SketchUp from merging very-close endpoints. The original instance of the component, at its natural size, experiences the same edits instantly as you edit the large copy. When done editing, the large component can simply be deleted (or left in an unobtrusive location if you anticipate making edits in the future).
