Earth from Moon

[pint]

hm. i'm pretty sure that this question was asked some times, but I can't found any answers.

seeing from the Moon's surface (sync orbit, near the surface), the Earth moves on a strange path on the sky. since Moon always shows the same side to Earth, I expected Earth to stay on one position on the sky. Okay, the Moon's orbit is not an exact circle, so I expected some small movements, but Earth, as Celestia shows, walks a path over 45deg in size.

why?

[granthutchison]

Sounds like you have an older version of Celestia, in which the Moon was wrongly orientated with its equator parallel to the Earth's equator, rather than the plane of the ecliptic.
Since the Moon's orbit is inclined about 5 degrees to the plane of the ecliptic, the Earth appears to shift north and south in the Moon's sky by about five degrees during each month. With the old orientation, the Earth seemed to move 23+/-5 degrees north and south each month.

You can fix the Moon's orientation by replacing its definition in solarsys.ssc. The current definition reads:

Code: Select all

"Moon" "Sol/Earth"
{
   Texture "moon.jpg"
   # Texture "clem-moon-4k.jpg"
   # Texture "moon4k.dds"
   BumpMap "moonbump1k.jpg"
   BumpHeight 0.7
   Radius   1737.53
   Oblateness 0.002

   CustomOrbit "moon"
   EllipticalOrbit {
      Period           27.321661
      SemiMajorAxis    384400
      Eccentricity     0.054900
      Inclination      5.15
   }

   Obliquity      23.45 # mean axis
   EquatorAscendingNode    0.0  # mean axis
   RotationOffset       38

   Albedo           0.12
}


Grant

[jrobert]

Hello pint,

First let me be the first to welcome you to this forum.

The reason the Earth appears to wobble so dramatically across the sky when viewed from the moon is in fact due to your position on the moon. Also, the plane of orbit of the moon doesn't follow along the equator of the Earth and also the moon's orbit is moderately eccentric. All these factors contribute to the appearant "wobble". Likewise, the Sun appears to wobble thru a 23.5 degree 6 month cycle before completing it's cycle in another 6 months.

[Calculus]

I made a screen shot of the path of earth seen from the moon.

Actually, the shape of the trajectory can be very different one month to another.

http://www.shatters.net/gallery/view_photo.php?set_albumName=Calculus&id=ana_earth_from_moon_1_29


Another kind of phenomenon is the moon analemma
Look at this shot:
http://www.shatters.net/gallery/view_photo.php?set_albumName=Calculus&id=Moon_analemma_from_North_East_USA_June_2002_comet_7P_Pons_Winnecke_string

[Guest]

You can fix the Moon's orientation by replacing its definition in solarsys.ssc. The current definition reads:

Many thanks! But if I put that code into the solarsys.ssc, all planets disappear that comes after Earth in the file. something bad with that data, maybe the old version does not recognise some parts. now downloading the newest version.

I'm curious to see what the real situation is with that old Earth.

[granthutchison]

But if I put that code into the solarsys.ssc, all planets disappear that comes after Earth in the file.

Oops. Are using 1.2.4? It was only after that that EquatorAscendingNode was introduced, so that may be the cause of your problem. Sorry, I should have anticipated that.

Grant

[Guest]

Oops. Are using 1.2.4? It was only after that that EquatorAscendingNode was introduced, so that may be the cause of your problem. Sorry, I should have anticipated that.
Grant

absolutely no problem. it was a lesson about backups :)
anyway, i had 1.2.5
now i have 1.3.0
Earth is much more lazy now :))

thx, pal!!

[pint]

grrrrrrr
that Guest thing makes me mad!
it was of course me