You get it tilted. 
-- 
Americans fought Germany in WWI and WWII 
because they were forced to by the draft. 
	-- The Iron Webmaster, 401

Post a reply to this message

Attachments:
Download 's400a.jpg' (17 KB)

Preview of image 's400a.jpg'

> You get it tilted.

looks better, did you really tilt it or just tilt the camera?


--
Rick

POVray News & Resources - http://povray.co.uk
Kitty5 WebDesign - http://www.kitty5.com
Hi-Impact web site design & database driven e-commerce
TEL : +44 (01625) 266358 - FAX : +44 (01625) 611913 - ICQ : 15776037

PGP Public Key
http://pgpkeys.mit.edu:11371/pks/lookup?op=get&search=0x231E1CEA

Post a reply to this message

"Rick [Kitty5]" wrote:
> 
> > You get it tilted.
> 
> looks better, did you really tilt it or just tilt the camera?

	Nope, a real tilt of the earth moon system. Note the moon is also
tilted. I'd have to look it up but I think the moon is something likle
+/- 7 degrees to the earth equator at all times. 

-- 
The greater moral good is that which 
further the liberal political agenda.
	-- The Iron Webmaster, 361

Post a reply to this message

I hate to keep beating on this point, but I still think the geometry is
confused.

I will try to explain it as clearly as I can and you can tell me where I go
wrong.

Lets define the earth's orbit as a plane.  The sun is also on this plane,
and in 99% of the illustrations of the system the camera is also in the same
plane.  So far so flat.

The earth's spin axis (through the north and south poles) is not
perpendicular to this plane.  It is tilted by 23.5 degrees.

The moon is also not in the solar plane, but I will take your word for the
angle.

So I guess what a photo would show is a vertical terminator that does not go
through the north and south poles.  I hope that makes sense.  I have
attached a tiny bitmap to illustrate the geometry.

In June everything north of the arctic circle is closer to the sun than the
day/night terminator, which is why the sun doesn't set in the arctic in the
summer.

In the December you get the exact opposite effect so the antarctic is in
perpetual daylight and the arctic has perpetual night.  Spring and fall are
obvious interpolations.

Enough geometry, on to POV :-)

If you model the earth as a single sphere with a single texture and light it
from the "sun" you should be able to produce still shots and animations
easily.

If you choose to use the "two textures" method things will be much trickier.
You might be able to produce a snapshot by careful splicing of the day/night
textures on the sphere, but a one-day animation (spinning earth) would
require a geometry genius.

I hope you will take all of this in the spirit it is intended.  I really
like your picture and I want to encourage you to produce one that combines
the artistic merit you already show with an accurate geometry.  I wish I
could do so well.

"Matt Giwer" <jul### [at] ijnet> wrote in message
news:3A3D788B.EF61EA76@ij.net...
> You get it tilted.
> --
> Americans fought Germany in WWI and WWII
> because they were forced to by the draft.
> -- The Iron Webmaster, 401


----------------------------------------------------------------------------
----

Post a reply to this message

Attachments:
Download 'earth.bmp.dat' (9 KB)

Do the lights really start that abruptly in RL?

--
David Fontaine  <dav### [at] faricynet>  ICQ 55354965
My raytracing gallery:  http://davidf.faricy.net/

Post a reply to this message

Spock wrote:
> 
> I hate to keep beating on this point, but I still think the geometry is
> confused.

	First off, as I have posted here, my geometry is faked for earth and
moon as the separation is 1/10th real. Real distances and screen pixel
pixels just don't mix. That may also explain why NASA has produced so
few pictures of the two in the same image. 

	http://giwersworld.org/artiii/elevat23.jpg is an image with the correct
distance and diameters. (Don't hold me to anything else in it. I was
still learning to spell povray back then.) Not the white blob that in
fact is an image map of a moon photo. 

> I will try to explain it as clearly as I can and you can tell me where I go
> wrong.
> 
> Lets define the earth's orbit as a plane.  The sun is also on this plane,
> and in 99% of the illustrations of the system the camera is also in the same
> plane.  So far so flat.

	And the orbital plane is within a degree or so of the sun's equatorial
plane. I have to look that up also. 

> The earth's spin axis (through the north and south poles) is not
> perpendicular to this plane.  It is tilted by 23.5 degrees.

	And that is the tilt I dropped in for the image. 

> The moon is also not in the solar plane, but I will take your word for the
> angle.

	That is memory. I haven't come across the number in a long time. 

> So I guess what a photo would show is a vertical terminator that does not go
> through the north and south poles.  I hope that makes sense.  I have
> attached a tiny bitmap to illustrate the geometry.
> 
> In June everything north of the arctic circle is closer to the sun than the
> day/night terminator, which is why the sun doesn't set in the arctic in the
> summer.
> 
> In the December you get the exact opposite effect so the antarctic is in
> perpetual daylight and the arctic has perpetual night.  Spring and fall are
> obvious interpolations.
> 
> Enough geometry, on to POV :-)
> 
> If you model the earth as a single sphere with a single texture and light it
> from the "sun" you should be able to produce still shots and animations
> easily.
> 
> If you choose to use the "two textures" method things will be much trickier.
> You might be able to produce a snapshot by careful splicing of the day/night
> textures on the sphere, but a one-day animation (spinning earth) would
> require a geometry genius.

	C'est Moi! It might still be on p.b.a but vertical. 

earth-moon-sun-stars = union {earth + moon} + sun + stars

earth-moon = union {tilt}

===

	There should also be several stills here of 

	sphere {day_image} + sphere {night_image translate 0.0001*x}

> I hope you will take all of this in the spirit it is intended.  I really
> like your picture and I want to encourage you to produce one that combines
> the artistic merit you already show with an accurate geometry.  I wish I
> could do so well.

	If it is ever perfect I'll be the first to say so. ;) 

> "Matt Giwer" <jul### [at] ijnet> wrote in message
> news:3A3D788B.EF61EA76@ij.net...
> > You get it tilted.

-- 
I am never effected by my environment or anything else
for that matter. 
	-- The Iron Webmaster, 125

Post a reply to this message

David Fontaine wrote:
> 
> Do the lights really start that abruptly in RL?

	Course not. Still working on that part. But at least I got the daylight
to shade into night at the terminator. That is one improvement. I'm
trying to avoid a "fake" solution at the moment. 

	The "improvement" is very sneaky -- I just shifted the view point to a
realistic position. It was there all along and corrects a prior
criticism. 

-- 
Even eternal gratitude doesn't last forever. 
	-- The Iron Webmaster, 211

Post a reply to this message

"Matt Giwer" <jul### [at] ijnet> wrote in message news:3A3EA778.48B6BA0D@ij.net...
> Spock wrote:
> >
> > I hate to keep beating on this point, but I still think the geometry is
> > confused.
>
> First off, as I have posted here, my geometry is faked for earth and
> moon as the separation is 1/10th real. Real distances and screen pixel
> pixels just don't mix. That may also explain why NASA has produced so
> few pictures of the two in the same image.

Yep, the photo needs a long focal length similar to a telescope; or a telephoto
camera lens, as you probably know, in order to see both Moon and Earth at
relative sizes.  Almost like orthogonal view in POV-Ray or at least a small
'angle' in the perspective camera.
The well-known picture made by of the Galileo spacecraft was taken from about
15 X the Earth/Moon distance.  Voyager took one from almost twice as far away,
and NEAR (Near Earth Asteroid Rendezvous spacecraft) photographed them looking
toward the south poles so they were at the same distance but from only about
the Earth/Moon distance away.
Pictures and text I've found and read :-) on those are here:
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-earthmoon.html

Bob H.

Post a reply to this message