Make a Platisphere

A Platisphere is a hand-held device that reduces the sphere of stars to a simple paper plate. From a starfield showing January 1 at Midnight, you can predict where the north circumpolar stars will be for any date and time.

Below, for a group activity you will drill star holes to make a stack of functional Platispheres (with optional 3D holders) . Instructions and examples are also on a video, with more excerpts and images here.

Print and cut the starfield.

Print a star chart near your latitude:

Latitude = 40 degrees North (mid-United States)
Other latitudes linked from here.

Cut out circle that is centered on Polaris and extends slightly beyond the red circle of circumpolar stars. Those stars are up all night every night. The lower the latitude the smaller the red circle.

Secure chart and drill holes.

Center and affix (with glue stick) the chart to the top of a stack of 9-inch black paper plates. Clamp the stack without covering any stars.

Drill a hole through the stack of plates at each of the stars. For faint stars use a small drill bit; for brighter stars, use a slightly larger bit.

Drill a big indicator hole in the outermost rim of the plate near where the top of the paper would have been (above the meridian line).

Predict the star positions.

Your upright plate is set for January 1 at Midnight. Each night the circumpolar stars appear to rotate counter-clockwise (or "star-wise").

To correct for desired date, rotate the top hole to a given date, going around once every 12 months (30 degrees per month) . E.g., rotating plate 180 degrees would set the date for July 1 at midnight.

To correct for desired time, rotate the plate through 24 hours per day (15 degrees per hour). E.g., six hours is one quarter turn. From midnight on given date, you can also rotate backwards (clockwise) in time 1/24th of a turn for each hour.

Make optional holder.

On the center of a stack of larger white plates, drill a hole. Label the left edge West (you'll be facing north) and the right edge East. P

On a second white plate, draw the outline of a local horizon with a building or a tree. The horizon should be high enough from the bottom of the white plate to cover the bottom of the red circle when in place.

Repeat on unused half of plate, too, to lessen waste. Using a band saw, cut out a stack of horizon pieces.

Assemble the holder.

Secure the optional foreground horizon to the white background plate so as to have a three-dimensional look. Slide the black starfield plate between them and secure the black plate with a paper fastener through Polaris and through the hole on the background plate.

Consider plastic plates.

Black and white plastic plates of the same size will interlock along the ridge, and better withstand the rigors of damp outdoor use.

Here you can see the stars have rotated "starwise" from the starting point, with the Big Dipper overhead and Cassiopeia near the horizon.

See video and variations.

A video from Paper Plate Education shows procedures to make and use the Planisphere. Variations of the activity linked from the original Platisphere activity page include a children’s version, a tactile version for visually impaired users, and the Photographic Plate, which predicts the smear of stars (star trails) produced from a long duration exposure.

NOTES

From Polaris, drop a plumb bob to the horizon, then swing it in an arc until you complete a circle of the sky around Polaris. Those are the circumpolar stars, up all night every night. The lower the latitude (i.e., the further south you go) the smaller the red circle of circumpolar stars.

When the indicator hole on the perimeter is centered on top, the Platisphere sky is aligned for January 1 at Midnight. Relative to this starting point, you will align your dial with the desired sky by positioning the indicator to the desired date and time. See video for examples.

The optional white background plate and foreground horizon will remain stationary and upright as you rotate the black starfield plate through the hours and months.

For the 40-degree-latitude plate, the foreground horizon plate should be low enough to cut off the star Alkaid (at the end of the Dipper's handler) from view as the black plate is rotated through 360 degrees. This suggests that Alkaid is technically not a circumpolar star from much of the US.

Circumpolar stars appear to rotate counterclockwise around the north pole, which is conveniently marked in the sky by Polaris, the North Star. Unlike the seasonal stars seen toward the south, circumpolar stars and their respective constellations are visible throughout the year. From the northern mid-latitudes region, the major circumpolar constellations are Ursa Minor (The Little Bear), Ursa Major (The Great Bear), Cepheus (The King), Cassiopeia (The Queen), and Draco (The Dragon).

Because the earth rotates once per day, the circumpolar stars appear essentially to travel around Polaris every 24 hours. Each hour the stars sweep through 15 degrees of sky. Therefore if you were to note the position of a circumpolar constellation at, say, 9:00 PM, you would find that six hours later at 3:00 AM the constellation would have rotated 90 degrees around Polaris. A long-duration photograph (in one night) depicts this stellar motion as a smear of concentric arcs called star trails.

Similarly, because the earth revolves around the sun once per year, the circumpolar stars appear essentially to travel around Polaris every 12 months. Each day the stars shift about one degree of sky (360 degrees in 365 days), revealing a new one-degree sliver of sky in the east. Therefore if you were to note the position of a circumpolar constellation at, say, June 1 at a given hour, you would find that two months later at the same hour the constellation would have rotated about 60 degrees around Polaris.

The original Platisphere activity appeared in the GLPA Proceedings, 2000, pp. 67-68.

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