Re: [guide-user] Artificial Satellites In Eclipse

Bill J Gray May 28, 2004

Hi Don,

This is a possibility; I've considered adding some sort of "in
shadow" data to the trails, so Guide will know at each point in the
trail if the object is sunlit or not. (This could be useful with
satellites of Jupiter and Saturn, too: you'd see the trail change
at points where the satellite went behind the planet.) Ideally, at
the same time, Guide would be storing the lat/lon of the point
below the satellite.

That would lead to the following benefit. Suppose you had just
asked Guide to generate a trail showing where the satellite will be
in the sky for a given time span. You could then switch to "geo mode"
(the one used for eclipses/occultations/transits) and see the _ground
path_ of the satellite for the exact same time period. It would
inherit some of the properties of the sky trail: you could click on
it to find out what time corresponds to a given point on the track,
or delete it, or change its color. (That is, it would act like an
overlay.) If the trail in the sky changed color/line style to indicate
when the object went into shadow, then the trail on the earth would
do so, too.

All of this isn't immediately around the corner, though. (Sorry,
I probably got your hopes up a bit in the previous paragraph!) So let
me suggest this immediate method you can use today:

Click on the satellite, then on "More Info". In the resulting
table, you'll see (among other things) the time when the satellite
will enter/leave shadow. Make a mental note of roughly how long
the satellite will be in and out of the earth's shadow, and click
on the entering/leaving shadow time.

Guide will reset its time/date to that of the shadow transition,
and recenter on the target satellite. Now go to "Animation... Add
a Trail." Make one trail going "forward" in time, far enough to
cover the time until the object sets, and another going "backward"
in time (using a negative step size), until the object rises.
Make sure that they're of different colors, so you can tell them
apart.

Okay, you should now be able to zoom out and see the entire
trail, changing color at the point where the object goes from
sunlight to darkness. It so happens that we'll be getting a pass
of Mir this evening in Maine (in bright twilight, unfortunately),
and this is what it'll look like, with the shadowed section
in orange:

http://www.projectpluto.com/trail.png (about 18 KBytes)

The only little problem with this idea is that you can't get
a dashed/dotted line. Aside from that, it'll do what you're
looking for, without having to wait for me to get Really Good
Satellite Functions into Guide.

-- Bill