As you begin a new year of stargazing, a simple paper plate dial can help you discover highlights of the northern sky. For this activity, January 1 at midnight is the starting point from which you rotate the paper plate to find northern constellations for any date and time throughout the year. Make your own Platisphere, which can be adapted for children, for visually challenged persons, and for photographers doing long exposure star trails.
Basically, you create a starfield of northern constellations to scale on a paper plate. A hole on the outer perimeter, when centered on top, indicates how the stars appear for January 1 at midnight. Each day the plate would rotate approximately one turn; that is, you see the stars rotate once through 24 hours. Each year the plate rotates approximately one turn, too; that is, the view changes by 360 degrees through 365 days. Once the plate is made, set the plate for the starting point of January 1 at midnight, then adjust for date and time.
To use the Platisphere, first set the dial to the current date. Because the earth revolves 360 degrees around the sun in 365 days, the sky seems to shift about 1 degree per day. Mentally subdivide the plate into 12 pieces of a pie to mark the 12 months of the year. Rotate the indicator counterclockwise, or “starwise,” the appropriate amount from January 1.
For example, if your current date is September 1, spin the indicator through ¾ of a year, or 9/12 of a plate, or 270 degrees. The dial now depicts the sky as it appears on September 1, but at midnight. Next you must set the time relative to midnight.
Again, the stars appear to rotate counterclockwise, or “starwise,” 15 degrees every hour. If the current time is before midnight, spin the dial the appropriate amount clockwise to “reverse” time. If the current time is after midnight, spin the dial forward in time to “catch up” to actual time.
For example, if the time to which you want to set the dial is 9:00 p.m., spin the indicator hole clockwise (back in time) through 3/24ths of a day, or 1/8th of a plate, or 45 degrees. Your Platisphere would then show the stars aligned to the real sky for September 1 at 9:00 p.m.
The Platisphere video (10:43) gives detailed (perhaps tedious) background and instructions for mass producing a stack of dials. It's old school, but it works.