Celestial Grids: Request for Feedback

[chris]

Fun experiment with the new grid:
- Follow Earth and goto to a point near it
- Select Alpha Centauri or some other nearby star and center it
- Zoom in until the field of view is about one minute
- Change the time rate so that on Earth year passes every few seconds

...and watch the movement of Alpha Centauri due to parallax. This was a less interesting before due to the absence of any sort of fixed reference (the grid lines were spaced too widely to be visible at such small fields of view.)

--Chris

[t00fri]

Yes, I think this is pretty good now. The rectangular aspect ratio is ~ constant over many zoom stages (with just very few exceptions). But still the shape is not square, rather 2.5:1 or so. One might heretically ask perhaps why this rather zoom-invarant pattern cannot be rescaled vertically to be just a bit more square, altogether?

This is not heretical at all. Reading your comment prompted me to double check a calculation. I found a stray factor of 2 that was causing too great a vertical spacing to be chosen.

Symmetry,...the secret guide for almost everything...

*** Excellent, spectacular...that's the solution!! ***

Fridger

[chris]

The next question is what additional grid should Celestia support? A horizontal (altitude/azimuth) based coordinate system is an obvious one. I think that the zenith direction should be defined by the surface normal of the currently followed body at the observer subpoint (distinct from the center-to-observer direction when the body is non-spherical.) I see that there are several conventions for azimuth; it appears that the modern one is to measure azimuth in degrees east, starting from the north.

Galactic? This seems like it could be useful.

Ecliptic? Easy to implement if it's useful.

Equatorial system of the followed body? Instantaneous equator, with RA 0 defined as in HORIZONS. It seems like this could be useful in making sense of the positions of objects as seen from the surfaces of other planets.

--Chris

[Cham]

Ecliptic and galactic grids should be implemented. They are very usefull. Each one of them should obviously use a different color (that could be edited in the config file).

[t00fri]

The next question is what additional grid should Celestia support? A horizontal (altitude/azimuth) based coordinate system is an obvious one. I think that the zenith direction should be defined by the surface normal of the currently followed body at the observer subpoint (distinct from the center-to-observer direction when the body is non-spherical.) I see that there are several conventions for azimuth; it appears that the modern one is to measure azimuth in degrees east, starting from the north.

Galactic? This seems like it could be useful.

Ecliptic? Easy to implement if it's useful.

Equatorial system of the followed body? Instantaneous equator, with RA 0 defined as in HORIZONS. It seems like this could be useful in making sense of the positions of objects as seen from the surfaces of other planets.

--Chris

I think the standard grids are a must: Alt/Az, Galactic, Ecliptic and Equatorial. But, I am not so sure yet about the various possible conventions.


Fridger

[chris]

Here's the galactic coordinate grid:

galactic-grid.jpg

In this test, the galactic coordinates are related to the J2000 equatorial coordinates by the following:
North Galactic Pole at RA 192.85858 (degrees), dec 27.1283361
Zero longitude is at position angle 122.932

--Chris

[t00fri]

Here's the galactic coordinate grid:

galactic-grid.jpg

In this test, the galactic coordinates are related to the J2000 equatorial coordinates by the following:
North Galactic Pole at RA 192.85858 (degrees), dec 27.1283361
Zero longitude is at position angle 122.932

--Chris

At last That's really nice and most useful.

Fridger