Time Step

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.