Latitude/longitude grid overlays

[granthutchison]

I've used the new texture overlay feature to create some IAU latitude/longitude grids for the planets and major moons. Once you've installed the textures and ssc's in your \extras folder, you'll find a couple of new Alternate Surface options - "Grid Overlay" which lays a lat/long grid over the default Celestia texture, and "Blank Grid" which converts the planets into rather pretty white spheres ruled in lat/long lines. I can't now recall which pre-release introduced overlay textures and alternate surfaces - but if at present you can bring up the alternate surface menu with a right-click on a planet or moon, then this add-on should work for you.
Overlays don't work on 3ds objects at present, but I'm hoping Chris is going to fix that RSN, in his copious free time . At which point I'll add grid overlays for irregular objects with defined prime meridians.

Find the add-on and some more descriptive text at:
http://www.lns.cornell.edu/~seb/celesti ... grids.html
Thanks, as ever, to Selden for hosting it.

Grant

[Darkmiss]

I didnt know there was a new overlaytexture command
Very handy having a grid over the texture

Thanks Grant

[selden]

I just discovered that if I turn on "Grid Overlay" for one planet and GoTo another planet, grids are visible on that planet, too. Selecting other surface textures, like "limit of knowledge," turn it off for the planet being viewed.

I suspect that turning it on for all planets is a bug.

Here's a picture of several planets showing a grid overlay. I only enabled it from the Earth's menu.

(this is a link to a larger picture)
(For those who wonder: I use ImageMagick's "montage" program to create thumbnails like this.)

[granthutchison]

Selden:
Turning on any alternate texture affects all other objects with the same alternate texture - the same occurs with LOK textures. I thought this was a feature, rather than a bug - you want to look at LOK, you only need to set the option once for the whole solar system.
Calling up LOK masks necessarily obliterates the grid, since only one LOK texture exists and that's default+LOK mask, so there's no way to add the grid in there, too.

Grant

[billybob884]

I've had a grid made for a while very similar to yours, except it has the torpics and the circles. Thanks for finally finding a use for it!

[selden]

Grant,

Your description seems to make sense that I can accept although when I enabled LOK for Pluto, the images of the other planets in the multiview didn't change. I think these are some of those minor limitations that're easy to live with.

[granthutchison]

... when I enabled LOK for Pluto, the images of the other planets in the multiview didn't change.

I think that's because the default texture for Pluto is the LOK. There was a plan to have Fridger's LOK paired with an interpretive texture based on the LOK, but that seems to have fallen through at present .

Grant

[chris]

Hmm . . . perhaps there should be an option to turn off lighting for the overlay texture. It'd be nice if the grid was visible on the dark side of the planet too.

--Chris

[selden]

Chris,

I'm not quite sure what you mean.
Glow-in-the-dark, perhaps?

The grid is readily visible if you enable some level of "ambient lighting".

[HankR]

Grant,

Your new grid textures are really great! This is something I've wanted to see for a long time. Thanks very much for your efforts. I do think, though, that long term it would probably be better if the grid display were built-in (using vectors rather than textures).

- Hank

[fsgregs]

Grant, thanks so much for your grids. I will be using them extensively in my science classes to teach latitude, longitude and seasons. For reasons which I cannot yet fathom, 8th to 10th graders collectively resist learning the concept of latitude and longitude (may apply to adults as well). Even though I do not have lazy students, they tend to not want to make the mental effort to envision the relationships between the tilt of Earth's axis and the seasons, the Tropics of Cancer and Capricorn, the positions of lat/long cordinates on the globe, etc. We do two separate labs on the topic, I make extensive use of globes, and I still have over 30% of them not get it. I think this will make things easier, since I can put the Earth in motion, change the dates to see the changes in the position of the terminator, etc.

I have created a new grid png file just for the Earth in which I've drawn some additional colored lines on it to highlight the Tropics of Cancer and Capricorn. I've also labeled the Equator and Prime Meridian. You can get it at my website link below. Its called Earth Grid TC.

Thanks again.

Frank

[TERRIER]

Grant
A great idea, and good use of the alternative textures feature.

fsgregs
I like your additions to the earth grid. Is it too much to ask for you to add the Arctic and Antarctic circles?

regards
TERRIER

[selden]

Frank,

Well, sometimes it helps to mention that there are books by those titles that they aren't allowed to read I seem to recall that association helping me somewhat to remember the names.

[fsgregs]

Terrier:

Will do. Look for it in a few days.

Selden: You are right, of course ... but not something that would get by the parent committee

Frank

[Brendan]

Recently, I wondered if the other planets happened to have north and south stars that could be of use to future explorers if we ever send people to other planets. Since the axial tilts can be different, I used the go to object feature to go to 90 degrees north and south and the "*' key to look back to see if there were any such stars. Then I sped up time to see which visible star appeared to make the smallest circle. Zooming in helped. Plenty were faint, and of course, pole stars would be of little use on gas giants. Interestingly, Uranus's north star is in the outer parts of Orion.
I thought that maybe it would be an interesting way to show that Polaris has nothing special about itself and it is by chance that our axis points towards it. Oh since there is a keyword for precession that can be used in ssc files, I'll try to simulate the 25800 year cycle for the earth and see if it works in Celestia. I read that Vega will be our north star in like 12 thousand years and a star in Draco was the north star in the times of the ancient Egypt. So, that could be used as a test to see if it actually works in Celestia.
But of course, Celestia doesn't have the proper motions of the stars.
And of course, it would be neat to make addons for nove and supernove using Beginning and Ending times to teach about stars.

Addition:
I tested the precession of the earth in Celesita by using:

Code: Select all

PrecessionRate   6.67E-7  # 2pi radians in 9420000 days or 25800 years


The PrecessionRate is in radians per day. It does have an effect. To see it, use goto object to go over the earth's north pole and use sync orbit and '*' to look at Polaris. Then speed up time. The earth's axis does end up pointing to the general region of Vega, but it seems to take too long to happen. Did I do anything wrong?

Brendan

[granthutchison]

The PrecessionRate is in radians per day.

It should be in degrees per day, for Celestia.
Only reason precession of the Earth hasn't been implemented in Celestia is a technical one - because the Moon's orbit is presently defined relative to the Earth's current equatorial plane, it gets dragged out of alignment if the Earth precesses :(.

Grant

[fsgregs]

OK, I have added the Arctic and Antarctic Circles to Grant's Earth Grid. It now includes labeled Equator, Prime Meridian, Tropics of Cancer, Capricorn, Arctic and Antarctic Circles.

Get it at my website below. Its name is Earthgrid TCA. Make sure you change the ssc file to reflect its new name.

Enjoy.

Frank