Since it is easy to get disoriented when flying between the planets and stars
in Celestia, this essay was written to help with orientation. The XYZ lightyear
coordinates of any star or planet can be found using the script by cpotting :
http://www.celestiaproject.net/forum/viewtopic.php?t=8076
cpotting said:
"The ecliptic frame of reference means that the ecliptic plane also defines
the X-Z plane. The X axis points "horizontally" towards the vernal equinox
and the Y axis points "upward" 90 degrees to the X-Z plane. The Z axis is
90 degrees to the X-Y plane (so it points to the summer solitce point - or
maybe the winter one...). Place the Sun at the origin and you have the
coordinate system used by the script".
Also, a set of .3ds addon rulers were made available by GlobeMaker to use
for frames of reference :
http://www.celestiaproject.net/forum/viewtopic.php?t=8501
Several prism shaped rulers were put in orbit around the Sun,
like this one that is 1 au from the Sun:
"Ruler_1au" "Sol"
{
# Class "spacecraft"
Mesh "f_200_520g.3ds"
# 12345678901234 kilometers
Radius 400000.000
EllipticalOrbit {
Period 10000000.00
SemiMajorAxis 1.0
Eccentricity 0.0
Inclination 0.0
AscendingNode 0.0
ArgOfPericenter 0.0
MeanAnomaly 0.0
}
Obliquity 0.0
RotationOffset 0.0
EquatorAscendingNode 0.0
Albedo 0.34
}
The set of rulers with lengths from 800,000 km to 80,000 lightyears can
be downloaded along with the solarsys_rule.ssc file :
Go to my ftp page by following the first link on this page :
http://www.reliefglobe.com/video.html
Download the solarsys_rule.ssc file and the rulers called f_*.3ds and r_*.3ds,
where * has several wildcard values. Each ruler is less than 700 bytes.
Here is a screenshot of the rulers and the XYZ coordinates of a star almost
exactly on the X axis, HIP 56. It is 939 light years away and has ecliptic
coordinates x 939 y 2 z -3 . It is the star about 2cm below Sol and 3cm to
the right of Sol and the rulers. The green lines are the celestial grid, rendered
by Celestia. Notice that the celestial grid is tilted 23.5 degrees
counter-clockwise from the rulers. The rulers are squared with the XYZ
ecliptic coordinate system.
Notice that the planets are in a line. That is because we are viewing
the plane of the ecliptic edge on. The green lines of the celestial grid
are at 0 hours and 0 degrees. The green line that is almost vertical
is at 0 hours. The line almost horizontal is the 0 degree line of the
celestial grid. (Right ascension and declination, if I remember correctly).
To conclude this first part of the essay, I will clarify cpotting's
uncertainty about : "the Z axis points to the summer solstice point - or
maybe the winter one". Which is it?
I zoomed in on the Earth and the South Pole is pointed up and it is
tilted 23 degrees towards the Sun. It is Summer at the South Pole.
Today is December 21, 2005, coincidentally, today is the Winter Solstice.
That is true in the Celestia view as well as in reality.
The coordinates x 939 y 2 z -3 of HIP 56 give us the clue : since Z=-3
and HIP 56 is to the right of the Sun in the picture, the Z axis points to
the Summer Solstice point direction of Earth.
In conclusion, thanks to cpotting for the xyz.celx script. The rulers
add a frame of reference as follows : this arrangment of rulers points
80,000 light years in the X direction. There are long prism rulers and
flat frame shaped rulers. The frame rulers (too small to see in the picture)
lie in the plane of the ecliptic. The Y axis is perpendicular to the plane
of the ecliptic and the frames. The prisms have two 45 degree angles
and one of them points in the -Z direction, parallel to the frames and
the plane of the ecliptic. Future work can improve on the .3ds frames
of reference, relative to the XYZ coordinate utility script. The frames
and prisms have a small file size and give 3 dimesional clues to the
orientations when viewed from within 1 light year of Sol. The frames and
prisms disappear when the Celestia Camera is more than 1 light year
from Sol.