Celestia Forums

I want to be able to show a demo to my class on the phases of the Moon, and I need to be able to show the position of Earth, Moon and Sun, as well as an image of the moon as it would look from Earth. I've successfully divided up the window into panes showing everything I want, but the problem is that my view of the Moon does not change phase. Basically, the camera needs to stay on an imaginary line between Earth and the Moon, always staying aimed at the Moon, but I can't seem to figure out how to do this.

Any advice?

thomasareed wrote:I want to be able to show a demo to my class on the phases of the Moon, and I need to be able to show the position of Earth, Moon and Sun, as well as an image of the moon as it would look from Earth. I've successfully divided up the window into panes showing everything I want, but the problem is that my view of the Moon does not change phase. Basically, the camera needs to stay on an imaginary line between Earth and the Moon, always staying aimed at the Moon, but I can't seem to figure out how to do this.

Any advice?

Try this:

Position the camera between the Earth and Moon. Select the Moon and press ":" to lock the camera on the Earth-Moon line. Press "T" to keep the Moon centered.

- Hank

hank wrote:Position the camera between the Earth and Moon. Select the Moon and press ":" to lock the camera on the Earth-Moon line. Press "T" to keep the Moon centered.

Well, I tried that, but it didn't work. Then, while tinkering, I noticed something... at the bottom right corner of the window, it said "Lock Moon -> Sol". So I selected the Earth, then pressed ':' again, and it changed to "Lock Moon -> Earth". Then I re-selected the Moon and hit 'T', and now all works well.

However, I'm baffled as to what exactly I did, and want to understand it. How does this lock control work, exactly? Why is it that it worked with Earth selected, and what exactly am I locked to?

thomasareed wrote:

hank wrote:Position the camera between the Earth and Moon. Select the Moon and press ":" to lock the camera on the Earth-Moon line. Press "T" to keep the Moon centered.

Well, I tried that, but it didn't work. Then, while tinkering, I noticed something... at the bottom right corner of the window, it said "Lock Moon -> Sol". So I selected the Earth, then pressed ':' again, and it changed to "Lock Moon -> Earth". Then I re-selected the Moon and hit 'T', and now all works well.

However, I'm baffled as to what exactly I did, and want to understand it. How does this lock control work, exactly? Why is it that it worked with Earth selected, and what exactly am I locked to?

In Celestia, the location of the camera is defined with respect to a coordinate system. The coordinate system is usually attached to an object, which defines the coordinate origin, so that the camera moves with the object. When the camera is in "lock" mode (activated by pressing ":" ), the coordinate system is attached to two objects, where one defines the coordinate origin and the other defines the axis orientation. Pressing ":' doesn't change the coordinate origin, but "locks" the axis orientation to the currently selected object. You needed to have the camera already in "follow" or "sych orbit" mode with Earth for my original instructions to work.

- Hank