Paper Plate Education
"Serving the Universe on a Paper Plate"

Video Excerpts: Platisphere

The following pictures and narration are excerpted from the Paper Plate Astronomy video.

A plaNisphere is a device that reduces the sphere of stars to a plane surface. Around the perimeter you find dates and times for setting the dial.

In this activity we will make a Platisphere, which reduces the circumpolar stars to a paper plate. With this device, you can determine the position of the circumpolar constellations for any given time or date as they rotate around Polaris.

When you are facing north, as the hours pass you see the stars move starwise, or counterclockwise. If you denote due north with a red meridian line, you see the pivot point around which they rotate is Polaris, the north star.

Use astronomy software to make a star chart. While centered on Polaris, set the sky to January 1 at midnight. Make the field of view equal to 50 degrees. And if you live with light pollution, set the limiting magnitude to 3.5.

In addition to the red meridian...

...enable the constellation outlines and the equatorial grid.

Then print the star chart.

Here is the red meridian line. Polaris is at 90--then there are 80, 70, 60, 50, and...

...40 degrees of declination.

Cut out along the 40 degree circle of declination, being sure to include the major stars Capella, Vega, and Deneb.

Center and affix the star chart on a stack of 9-inch black plates. We will use the following technique to mass produce plates for a group.

On an area where there are few stars, put a clamp over an entire stack of plates.

At a drill press or with a hand drill you will use three different-sized drill bits.

Use the smallest bit to drill through the fainter stars and a slightly larger bit to drill through the brighter stars, as indicated on the star chart.

The Platisphere really works well when you limit the number of constellations depicted. You can always add them later as your observing progresses. Also, if you are in light polluted skies, omit the four fainter stars in the middle part of the Little Dipper. It is recommended that you do include the three bright guide stars, even if the printed chart shows them to be below the horizon at the moment.

Of course, use caution with the drill near your fingers.

With a large bit in the drill, move up along the north meridian line and drill a hole at the top of the plates near the edge.

This will be your indicator hole.

When you remove the star chart you have a stack of black starfield plates remaining. Here is the Big Dipper.. which leads you to Polaris...and beyond to the W of Cassiopiea....Down low are Deneb...Vega...and Capella.

Now, to make a horizon for your latitude, cut off the the semi-transparent horizon from the bottom of the printed star chart...

Center the truncated star chart on a larger 10-inch white plate and draw a line (woops) onto the white plate as shown.

Add a few horizon features--a house, a tree--and cut off the rest of the plate above that.

To make individual horizons, put this horizon template on a plate, trace out the horizon line, and cut.

To mass produce horizons, copy the template onto a stack of white 10-inch plates and cut the whole stack at a band saw or scroll saw.

You now have a black starfield plate, a 10-inch background plate with a small hole in its center, and a cut out horizon. Turn the horizon upside down and staple it to the big white plate, giving it a 3-D appearance.

Slip the black one inside and secure them with a paper fastener. With the large indicator hole on top--set for January 1 at midnight-- you’re Platisphere is ready for use.

Beginning at January 1, every day the stars shift about 1 degree “starwise” or counterclockwise...

...until after one year, or just over 360 days, the stars have shifted through 360 degrees...hence, the 1 degree per day.

From January 1 we can subdivide the plate into 12 months--February 1, March 1, April 1, May 1...

October 1 November 1, December 1, and back to January 1--all being at midnight.

Concurrently, every 24 hours the stars will rotate about once around...so half a plate is 12 hours, a quarter plate is 6 hours...If we start on January 1 at midnight, a quarter turn takes us to 6 a.m., then noon...

...then here you see the hole on the right edge at 6 pm (again, all for January 1) and finally by midnight the hole has returned to about its starting point.

So let’s try an example--set the sky for April 1 at 9 pm.

From January 1 at midnight, move forward 3 months...to February 1, March 1, and April 1 at midnight. Then set the time, moving forward 21 hours from midnight--to noon, this is 6 p.m., plus three more hours to 9 p.m....So we’ll move the indicator hole from the April 1-midnight position ... forward 21 hours to April 1 at 9 p.m.

So this how the stars will appear when facing north on April 1 at 9 p.m.

Another way to get to April 1 at 9 p.m....From April 1 at midnight you could go forward 21 hours...or, from April 1 at midnight go backwards in time by 3 hours to 9 p.m. So again this is how the north circumpolar stars appear on April 1 at 9 p.m.

In this second example you’ll see how two different times and dates can have the same starfield. From January 1 at midnight, move “starwise” until the indicator hole is on the right edge, or at October 1-midnight.

From here you can move it forward either 2 hours or one month.

This could be either October 1 at 2 a.m. or November 1 at midnight--same starfield, two different circumstances.

Notice how at these two opportunities the Big Dipper may be hard to find simply because it is low on the horizon and near obstructions. And since the end star of the Big Dipper, called Alkaid, dips below the horizon from this latitude, it is technically not a circumpolar star.

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