Even though the main purpose of a planisphere is to show the sky as it
appears right now, one of the fun things you can do with it
is to turn the star wheel and watch the procession of rising and setting
stars. The time step arrows next to the clock icon give you a way
to "turn the star wheel". The right arrow turns the wheel forward in time;
the left arrow turns the wheel back.
The default step - 1 hour - is the most basic application of the concept.
Tap the hour step arrow repeatedly and watch the progress of your
chosen star through the sky in hourly increments. The 5-minute step
does the same thing in smaller steps. Zoom in on a star near the horizon
and watch it rise or set slowly.
The month step will show you what the sky looks like at the same time
during successive months. Set the time to your favorite stargazing time
of night and find out which constellations you can see at that time
throughout the year.
But the real fun starts with the advanced time steps: rise,
set and transit. Pick a star or a planet and select one of
these time steps. Then, tap the step arrows and see the sky as it was,
or will be, when your selected star rises, sets or culminates in the sky.
One obvious use of the advanced time steps is to determine sunrise, sunset
and noon (not 12 o'clock, but the exact time when the sun is at its highest
point).
Another interesting activity is watching the sky at sunset on successive
days. Pick Sun/Set and tap the right-hand step arrow repeatedly. This is
a good way to determine the best night to try to spot the elusive planet
Mercury; watch it hover over the horizon for the few precious days
when it is to the east of the setting sun and pick the night when it is at
the farthest point.
When you get tired of playing with the time steps, tap the clock icon
and return to the present.