Celestia Forums

is the size of stars viewing from earth is accurate?the moon seems smaller than the real one.

Please read the FAQ for an explanation that the moon is the correct size.
The stars should also appear the right size, within the limits of the calculated radius in Celestia, which shows a considerable error for red supergiants (too small) but is otherwise reasonably accurate.

Grant

Hmmm.
It occurs to me that, even at maximum zoom, most of the stars are too small to show a visible disc in Celestia when viewed from the Earth.
What "size" are you talking about?

Grant

Are you talking about scaled disc option?
This size is inaccurate.

Anonymous wrote:Are you talking about scaled disc option?
This size is inaccurate.

Of course, but in the other way; they are too large.

jamarsa wrote:

Anonymous wrote:Are you talking about scaled disc option?
This size is inaccurate.

Of course, but in the other way; they are too large.

But the scaled disk display is just a familiar means in star charts to compensate for the limited dynamics range of displays. The scaled disk display is not at all inaccurate in the following sense:

The luminosity of stars is mapped onto the disk size of the displayed stars. While not corresponding to an entirely natural view, it represents concise information about a star's magnitude in a star chart.

I said that because I did a little experiment when this feature was released. I went to the Sun (or another star, it happens the same), and started hitting the 'End' key to move away from it. In the moment Celestia switches from rendering to scaled disk, I notice a sudden increase in size. So, either the rendering view is inaccurate, or it is the scaled disc.

Well, perhaps this is fixed by now (I don't have the last release yet).

Javier,

You're comparing apples with oranges

While you're first travelling away from the sun, Celestia is showing you the optical diameter of the Sun: its physical diameter is being projected onto your viewscreen.

If you have "Scaled discs" enabled, then you have asked Celestia to draw circles with sizes that are proportional to how bright an object is, not how big it is. When you have travelled beyond a certain distance, where the Sun's optical diameter is smaller than a certain amount, Celestia changes from showing you an image proportional the physical size to showing you a circle with a diameter proportional to its luminosity (or proportional to its albedo if it's an object that doesn't glow).

Celestia's builtin calculation for the maximum diameter to draw for an object with high visual luminosity happens to produce a circle that's very slightly larger than what it draws for the Sun's optical diameter at the transition distance.

That maximum size is strictly arbitrary. Chris could have chosen something larger or something smaller. It'd be nice if one could adjust the scaling factor and limits for the luminosity diameters, but it's hardwired inside Celestia.

Does this clarify things?