pla-ni-sphere, n. [plane + sphere]

A projection of the celestial sphere on a chart equipped with an adjustable overlay to show the stars visible at a particular time and place.

A planishpere is a device, usually made out of cardboard or plastic, consisting of two wheels stacked on top of each other, possibly also joined at the center. The bottom wheel is a circular map of the sky, and the top wheel has an off-center oval cut out of it, representing the horizon and everything above it. As you turn the top wheel, different parts of the sky map become visible through the oval, representing different parts of the sky being visible at different times of the day. The two wheels usually have months, days and hours marked around their edges, so it is possible to align them with some precision.

A real planisphere device has a few shortcomings. [1] Any given planisphere only works in a limited range of latitudes. You need to buy a different one if you travel too far south or north of where you live. [2] The positions of the Sun, the Moon and the planets among the stars change constantly, so a real planisphere does not show them at all. [3] The wheels of a real planisphere have to be aligned manually. [4] A real planisphere is hard to read in the dark.

A computerized planisphere such as SkyChart solves all these problems.

A planisphere shows you the sky as you would see it lying flat on your back, looking straight up. If your feet are pointing south, then south is down, north is up, east is left and west is right. (On a regular map, east is right and west is left, but that’s because you’re looking down, not up.)

If you make the small mental adjustment and just pretend to be lying on your back, things become a little easier. Hold the planisphere in front of you, up toward the sky. Stand facing in the direction that is at the bottom of the planisphere (the view). The bottom half of the planisphere now shows you the part of the sky that is in front of you. The top half is the sky behind you.

The Sun, the Moon and the planets move among the stars from day to day. The Moon actually moves fast enough to change its position noticeably within the same day. The daily calculations are always made for Greenwich, England, but then have to be adjusted to compensate for the position changes that happen between any given time at Greenwich (GMT) and the same time at your location.

Longitude is important for more accurate calculations within a single time zone. Let’s say you live at the eastern edge of your time zone. You might see the Sun set at 6pm, but someone at the western edge of the same time zone would have to wait until 7pm for the same sunset.

In short, the time zone is needed to convert your local time to GMT, where the calculations are made. The longitude is needed to translate the GMT calculations back to your actual location.

Add one hour to your offset from GMT. For example: -7 + 1 = -6. In other words, normally you are 7 hours behind Greenwich Mean Time, but in the summer you move one hour closer to it, because you “spring forward“ and GMT doesn‘t.

You can determine not only sunrise and sunset, but the rise and set of any object.

From the Step menu (1 hour, 1 day, etc.), select ...rise or ...set. The step icons in the bottom corners of the screen will change to <R and R> (or <S and S>). Then, select the object you want, e.g., the Sun. Finally, tap one of the step icons, and the program will rotate the sky to the previous/next sunrise or sunset, and the time display will also change accordingly.

By tapping the step icon repeatedly, you can observe the subtle changes in the sky’s appearance at sunset on successive days.