Fine Points

A further complication is that a planisphere works correctly for only one narrow range of latitudes on Earth. Fortunately, many models come in several editions, each for a particular latitude range.

Then there's the matter of daylight saving time ("summer time"). When this is in effect, remember to "fall back" to standard time by subtracting an hour from what your clock says before you set the planisphere's dial.

Actually, planispheres don't work in standard time either, but rather in something called local mean time. The difference between the two, which depends on where you live in your time zone, can amount to 30 or 40 minutes or more. It results from the fact that the Earth is round — and that the curvature of the Earth's surface is noticeable even when you go a few miles east or west. On average, the Sun, Moon, and stars rise, transit, and set an hour later on the western side of your time zone than on the eastern side of it.

The local mean time correction (in minutes) for various U.S. cities is given in the following table. Apply the numbers in the table to your standard time to get local mean time, which is planisphere time. Fortunately, even a half hour one way or the other doesn't really matter for most star finding, so you can probably get away with ignoring this refinement if you want.

Anchorage	–60	Kansas City	–18
Atlanta	–38	Los Angeles	+7
Bismarck	–43	Memphis	0
Boise	–45	Miami	–21
Boston	+16	Minneapolis	–13
Buffalo	–15	New Orleans	0
Chicago	+10	New York	+4
Cincinnati	–38	Philadelphia	–1
Cleveland	–27	Pittsburgh	–20
Dallas	–27	Richmond	–10
Denver	0	Rochester, N.Y.	–10
Detroit	–32	St. Louis	–1
Durham	–16	Salt Lake City	–28
El Paso	–6	San Francisco	–10
Helena	–28	Santa Fe	–4
Honolulu	–31	Seattle	–10
Houston	–21	Tucson	–24
Indianapolis	–44	Tulsa	–24
Jacksonville	–27	Washington, D.C.	–8

In fact, if you just want to know which constellations are up and where they are, a planisphere's limitations can largely be overlooked. It's remarkable that such a simple working model of the sky can work so well.null