User`s guide

108 Starry Night User’s Guide
five options. The first is the familiar
“Surface Of” option. The other four
options are described below. These options
are quite specialized and you may not ever
need to use them.
The Centre Of: This puts you at the centre
of the planet or moon in question. It is
handy when you want to watch the motion
of a celestial object while ignoring the
rotation of the planet you are based on. For
example, assume you want to see how a
comet will move relative to the fixed stars
over a night. Placing yourself at Earth’s
centre allows you to watch the comet’s
actual motion and ignore the effects of
Earth’s rotation during the night.
Position Hovering Over: If you are
positioned at a high enough elevation
above a planet’s surface, you can “hover”
at that location. Hovering allows you to
stay at a constant position above any
object and, as time passes, watch as it
rotates beneath your feet.
Note: Hovering is only useful if you are
relatively high above a planet’s surface.
Therefore, if you select “Position
Hovering Over” when you are on the
surface of a planet or at a low elevation,
Starry Night will automatically increase
your elevation to a height that is suitable
for hovering. This height will vary
depending on the size of the planet. For
moons and small planets, you can hover at
a lower elevation than you can for large
planets such as Saturn and Jupiter.
Position Moving With: This option allows
you to travel together with a planet as it
moves through its orbit, but at a fixed
distance from the planet (this fixed
distance is your elevation). The “Position
Moving With” option differs from
“Position Hovering Over” because it
always keeps you in the same orientation
with respect to Earth and the Sun, whereas
“Position Hovering Over” keeps you in the
same orientation with respect to the
background stars. Assume you choose
“Position Hovering Over” and set yourself
high above Earth looking at the bright
(sunlit) side of Earth. If you play time
forward for six months, you will end up
viewing the dark side of Earth, because
Earth has moved 180° around the Sun, but
you are still facing in the same direction,
relative to the background stars. If you
instead set your location choosing
“Position Moving With” and select the
“Beside (Sun Side)” option, you will
always view the bright side of Earth, no
matter how long you play time forward.
Stationary Location: This option places
you at a fixed location in space, relative to
the Sun. Stationary locations can be
entered as heliocentric ecliptic Cartesian
co-ordinates or heliocentric ecliptic
spherical co-ordinates. For Cartesian
co-ordinates, the position on the ecliptic