Astronomical view from orbit?

[DeafScribe]

I'm going to TRY to explain what I want to do here. I'm not sure if I'm doing it wrong or if it just can't be done with Celestia.

Basically I want to get a better sense of seasonal changes in the night sky by speeding it up and viewing it Celestia.

I want to establish a fixed position over earth's night side, following the earth as it orbits, with a view out away from earth.

With that perspective set up, I want to accelerate time and watch the procession of constellations as they appear through the Earth year.

My idea was to start with a view towards Orion, and see how it cartwheels through the sky through the seasons.

However, when I've tried it, by taking a position over Florida, setting Celestia to follow Earth, and looking out, the view of the sky remains fixed no matter how fast time is sped up. I can look back at Earth and it's spinning madly and the planets are racing through their orbits.

So how can I follow Earth's orbit and at the same time see the stars and constellations sweep past as the Earth moves about the Sun?

[Johaen]

There's probably a simpler way to do it, but here's what I did. I made a fake Earth with-in the real Earth, with the same rotational period as it's orbital period:

Code: Select all

"Earth Fake" "Sol"
{
   Radius 6000

   CustomOrbit "vsop87-earth"
   EllipticalOrbit {   
      Period            1.0000
      SemiMajorAxis     1.0000
      Eccentricity      0.0167
      Inclination       0.0001
      AscendingNode   348.739
      LongOfPericenter 102.947
           MeanLongitude   100.464
   }

   RotationPeriod   8742.2085551664
   Obliquity        -23.4392911
   RotationOffset   280.5    # offset at default epoch J2000
}

Then just sync-orbit the Fake Earth, and enjoy the view. Looked pretty cool to me. Thanks for the idea.

[Johaen]

Looking in the opposite direction, towards the sun is pretty neat too. The sun makes an analemma in the sky. Pretty neat to watch.

[DeafScribe]

Tried it, and that is exactly what I was looking for.

It was also instructive - I started out with the earth (FakeEarth) oriented as it would be on a map, north to south, sped up time and looked about, but it didn't seem quite right.

Then I had that "DUH!" moment and realized I needed to turn the planet on its ear - rotate it 90 degrees - to better approximate the view from the ground.

Very, very illuminating. I'd like to see this done in a planetarium, so there would be a "full-screen" view! It's also useful to turn on the orbit tracks to see how the plane of the ecliptic shifts from our perspective as we roll around the sun. (And the moon whizzing by month after month is just comical!)

I've always known Celestia is a fantastic program, but I haven't had time to really dig into it and become conversant with the scripting capability.

Thanks for taking time to make Celestia even more useful for all of us.

[LordFerret]

I also tried something, but using a cel script instead. Johaen's idea is probably simpler though.

Code: Select all

{

#
# LordFerret's Night Sky script
#

# starting from anywhere, at any point in time...
# move into position facing Earth's day-side (with north up)
   cancel {}

   preloadtex {object "Sol/Earth"}
   preloadtex {object "Sol/Earth/Moon"}

   select {object "Sol"}
   goto {time 5 distance 10}
   wait {duration 5}

   select {object "Earth"}
   goto {time 5 distance 2 upframe "ecliptical" up [0 1 0]}
   wait {duration 5}
   follow {}

# move around to Earth's night side (180 degrees)
   orbit {duration 5 rate 36 axis [0 1 0]}

# lock this position and look back at the night sky
   lock {}
   lookback {}

# turn on any renderflags and labels desired
   renderflags {set "boundaries | constellations"}
   labels {set "asteroids | comets | constellations | galaxies | moons | planets | spacecraft | stars"}

# increase the field of view if desired
   set {name "FOV" value 40.0}

# speed up time and view the night sky passing by as Earth orbits Sol,
# takes approximately 6.5 minutes for 1 complete orbit
   timerate {rate 100000}
   wait {duration 390}

# reset time back to normal
   timerate {rate 1}

# reset the field of view
   set {name "FOV" value 25.5}

# reset renderflags and labels
   renderflags {clear "boundaries | constellations"}
   labels {clear "asteroids | comets | constellations | galaxies | moons | planets | spacecraft | stars"}

# end the script
   lookback {}
   cancel {}
   print {text "End Of Script" row -12 column 28 duration 5}
   wait {duration 5}

}