A planisphere (or four) makes a fine addition to your minimalist star-gazing kit. Photo by Gary Seronik
By Gary Seronik
In an earlier blog I mentioned how you could spend a productive night at the telescope armed with just two pieces of “software:” the RASC Observer’s Handbook and Sky & Telescope’s Pocket Sky Atlas. But one more item I’d add to that list is a star wheel, otherwise known as a planisphere. It’s a remarkably handy low-tech computer that does a great job helping you plan a night under the stars.
A planisphere is simply a rotating star map housed inside a sleeve with several windows for showing the date and the appearance of the night sky. They come in different sizes and styles, but they all do the same thing the same way. For greatest accuracy, choose one matched to the latitude of your observing site.
Using a planisphere couldn’t be easier. Simply turn the wheel to line up the current time with the date, and at a glance you can see what the night sky looks like at that moment. Or, you can use it to figure out when stars and constellations rise or set. For example, let’s say I want to figure out when Regulus will rise on December 10th. I simply turn the moving disk of the wheel until the star is parked on the eastern edge of the window, and I see Regulus will rise that night at about 10 p.m., local time. And without changing any of my settings, I can also see that by the end of the month, Regulus will rise well before 9 p.m. You can think of a planisphere as a dynamic star chart.
One modification I make to some of my planispheres is the addition of a line that runs from due north to due south, to indicate the meridian. This gives my planispheres the added capability of indicating when a given region of sky is at its highest, and at its best — a very handy piece of information for planning an observing session.
So what are the downsides of a star wheel? There are two. First, the constellations near the southern horizon are pretty badly distorted. To minimize this, some planispheres are two-sided and include a map just for the far south. The second issue is that by their nature, planispheres can’t plot the ever-changing positions of the planets. But even so, they’re wonderfully handy, inexpensive, don’t need batteries, and will last forever. Every observer should have at least one.