User`s guide

Adding Your Own Data 165
pericentre. Recall that Kepler’s laws of
motion then allow Starry Night to
calculate the new object’s position in
its orbit at any other time.
• Elements Epoch: The date (expressed
as a Julian day) for which this set of
orbital elements will be most accurate.
As you move farther away from this
date, the accuracy of the object’s
position will decline, although the
change in position is often too small to
be noticeable. Nevertheless, for precise
work, it is advisable to use a set of
orbital elements with an epoch
relatively close to the date you are
interested in.
Element Style (Near-Circular): The near-
circular style is usually used to specify the
orbital elements of planets, asteroids, and
planetoids. Only those fields different
from the pericentric style are described
below.
• Mean Distance: Objects in circular
orbits would travel at a constant
distance from their parent body, but
since most planetary orbits are slightly
elliptical, the distance between an
object and its parent is constantly
changing. The common practice is to
average this distance, and record it as
Mean Distance. It is usually measured
in AU’s.
• Mean Anomaly (M) and Epoch:
Together, these fields describe exactly
where in its orbit a new object is
located at the specified time. The
“specified time” is the epoch (given as
a Julian Day) and the position is the
mean anomaly, the angle between the
object’s position and the pericentre of
the object’s orbit, as measured from the
centre of the parent body.
Tip: Notice that “epoch” has a different
meaning in the pericentric and near-
circular formats. In the pericentric format,
changing the epoch will not change an
object’s position, but in the near-circular
format, it will.
Element Style (NASA Two Line): NASA
Two Line Elements (TLE’s) are often used
to describe the orbit of Earth-orbiting
artificial satellites. Describing the TLE
format is beyond the scope of this User’s
Guide. However, you do not need to
understand TLE’s to use them! If you find
the TLE for a satellite, simply copy the
element to your clipboard, then press the
Paste TLE From Clipboard button.
Starry Night will paste in the new
elements. Not all of the fields in a TLE are
used by Starry Night. Fields that are not
used will be marked with ‘xxx’. See
“Where can I get orbital elements for new
objects I want to add using the Orbit
Editor?” on page 197 for information on
finding satellite TLE’s on the Internet.