order for a sighting of a satellite to be made, there are 4 conditions
that must be satisfied. First, the satellite must be above the horizon
at the observer location. Second, the satellite must be in daylight.
Third, the side of the satellite in daylight must be roughly facing
the observer. And finally, the observer must be in darkness. When
all constraints have been achieved, a sighting is possible.
NASA SkyWatch allows the
user to alter these sighting constraints. It is important to remember
that not all satellites have the same type of viewing possibilities.
The default parameters are typical but somewhat optimistic based on
recommendations of experienced watchers.
Minimum Solar Separation
This parameter determines if the lit side of the satellite is facing
in the rough direction of the observer. Generally, the observer needs
to be between the sun and the satellite in order for a sighting to
take place. It is assumed that the satellite can only reflect light
back toward the sun and not ahead of itself. This is usually true
for most satellites. However, for larger targets with solar arrays,
like the International Space Station, light can actually be reflected ahead
which makes more viewing potential. For this reason, it is reasonable
to lower the maximum solar separation to as low as 60 degrees.
Maximum Solar Elevation
This parameter defines what "darkness" is at the observer location.
Since one of the constraints for sighting is for the observer to be
in darkness, it is required to define how dark darkness really is.
From past experience, the sun needs to be approximately 6 degrees
or more below the horizon in order not to wash out the sky.
It is possible to acquire a satellite right when it crosses the horizon.
This is likely for electronic acquisition. However, for actually seeing
a satellite, haze and ambient light make viewing almost impossible
below about 15 degrees above the horizon. For exceptionally clear
nights, picking up the sighting at below 15 degrees can be challenging
When using the "Next Sighting", "SkyWeek", or "Super SkyWeek" buttons on
the "Table" tab, NASA SkyWatch will look for a sighting until it finds
one or it reaches this maximum number of days.