NavList:

Rafael, you wrote:
"My local amateur astronomy club offers occasional outreach presentations for middle and high school children and boy scout / girl scout troops. I thought that showing how this globe may be built would lead to a far better hands-on understanding of the apparent motions of the celestial sphere than just presenting a commercially available celestial globe. Stellarium is wonderful, but a do-it-yourself object has a special quality. Therefore, details about its construction (e.g., how to show the effect of the observer's latitude) would be most welcome.  Of course, full credit will be given to the originator of this specific design."

Yes, in addition to its minimal practical value, it certainly has educational value, and I agree that the process of making it adds greatly to the educational benefit. Spinning a star globe at different latitudes has enormous value to anyone trying to understand the sky, and that alone makes it a valuable tool for would-be celestial navigators.

Start with the cheapest toy ball you can find with minimum printing. Try to select one that's spherical (many are "spheroidal"). Look for one that has a seam that appears to split the ball into equal hemispheres to serve as a convenient equator. Grab one marker with water-soluble ink and one or more permanent markers. Now you can start drawing... 

You need three great circles on your "globe" to get the "ball" rolling... First, draw an equator. There are tricks for this, but it's not too complicated. Use the seam on the ball if possible. Next draw a "prime meridian" circle along 0°/180° and another meridian along 90°/270°. That splits the globe up into eight identical "octants" each 90° on three sides and each with 90° corner angles, too. Label a North pole, South pole, and a zero of SHA. Next you need a scale. So grab a strip of paper, line it up along any 90° side (and check all of them while you're at it to make sure you're work so far is consistent). Mark 0° and 90° at the ends of the strip and then by measuring the length and dividing by nine you can scale off every 10°. You can see that strip of paper in my photos. It's a general scale of degrees along any great circle or any arc of a great circle. It can measure SHAs or Decs or Altitudes (or degrees of ecliptic longitude, etc., etc.).

Suppose I want to plot the star Vega. Its SHA is 81° and its Dec is 39°. So I go to the equator and measure off 81° to the right from the prime meridian (or safer, measure 9° back to the left from the 90° line of SHA). Put a little mark on the equator there with your water-soluble pen. Now turn your scale. Line it up along the arc from the equator to pole from your SHA mark and measure up 39°. Make your dot! And on to the next star... Do 20-25 stars that way. You can add as many other fainter stars, like, for example, the Big Dipper, more of less manually (without careful measurement). It's helpful to draw brighter stars with bigger dots. For a group of students, it may be helpful to make a "good globe" and then photograph each 90° octant. Then "students" could copy that or at least compare to make sure they're not making egregious errors.

Here's a list of SHA and Dec for brighter stars:

• Aldebaran: 291, 17
• Altair: 62, 9
• Arcturus: 146, 19
• Betelgeuse: 271, 7
• Capella: 280, 46
• Deneb: 49, 45
• Pollux: 243, 28
• Procyon: 245, 5
• Regulus: 208, 12
• Vega: 81, 39
• Pollux: 315, 89
• Achernar: 335, -57
• Acrux: 173, -63
• Canopus: 264, -53
• Fomalhaut: 15, -30
• Hadar: 149, -60
• Rigel: 281, -8
• R.K.: 140, -61
• Sirius: 258, -17
• Spica: 158, -11

You should add another great circle: the ecliptic. Just remember that any great circle is perfectly "straight". It doesn't curve (when seen from directly above). It's equivalent to a "tilted equator". Of course it should have extreme points at 90° and 270° of SHA where it should reach 23.5° from the equator, and it should cross the equator at 0° and 180° SHA. I also recommend marking off 30° points along the equator. Each of those corresponds to a date (around the 21st day of each month in the year --maybe label those by date).

You may want to add a "constant latitude" circle for your home latitude. On my star globe, that's a dashed line. Its distance from the north celestial pole is equal to your latitude. If you build a star globe with a polar frame of some sort so that it can turn, this is un-necessary.

The globe sits in a common cardboard shipping box. If you have a 7-inch ball, a 9"x12"x4" box works well. Cut a 7.5 inch circular hole in the top and drop the ball in. If the hole is too deep, tape a few layers of bubble wrap to the bottom of the box (inside) so that the ball rides high enough (any of the great circles should correspond with the "horizon" created by the hole in the box). You can label azimuths using the scale you made to plot the stars, or you can do it by lining up the ecliptic along the horizon and copying from there. Note that the direction of azimuth, clockwise or counter-clockwise, will depend on whether you plotted your stars "inside out" or "outside in". Does the Big Dipper look like the Big Dipper in the sky? Then your azimuths will run counter-clockwise (mirrored compass rose). Or does the Big Dipper look like a mirror image? Then clockwise azimuths (normal compass rose).

Orient the globe by placing the north pole point (Polaris, in other words) directly above the north azimuth on the box (reverse in the southern hemisphere), and tilt the globe up so that the altitude of Polaris is equal to your latitude (near enough). The SHA on the opposite compass direction is the "SHA of the meridian". One can calculate this or simply observe it. Better yet, for evening twilight, you can rotate the globe until the Sun (positioned by dates on the ecliptic) is roughly 6-12° below the horizon in the west. Similarly in the east for morning twilight. Need the stars at midnight? Pick your globe up out of the box and turn it until today's "date" on the ecliptic on the globe is "down". Drop the globe in place and you're then seeing the stars for (local apparent) midnight. After the latitude and SHA orientation are set properly, you can read off the azimuths and altitudes of any of the plotted stars.

Old "professional" star globes include special arches and devices for measuring altitudes. You don't need that. Just use the same strip of paper that you have used for plotting (or now that you have beaten that one up, draw a better one on card stock at this point). Just measure up from the horizon, holding the scale against the globe as you go. The azimuth is found directly below the star. If you place your star globe on the floor and look straight down on it, you're looking at the zenith at the center of your view of the globe. That can be useful for general orientation.

I'm going to try a few design enhancements on my toy star globe, and I would be happy to hear ideas. For my own interest I want it to remain dirt cheap and, as much as possible, a DIY home project.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA