by tonytran2015 (Melbourne, Australia).
Figure: Illustration of a rotatable Sky map with an overlaid grid mask showing azimuths and latitudes of stars for a user at 40 degree latitude.
It is an advantage to know the arrangement of stars for the nights before engaging in nightly activities such as going to the country side or navigating your way by stars. It is difficult to have a good display of the night sky with current commercially available circular star maps as they are equidistant-azimuthal and have a lot of distortion for visualization whereas easy visualization requires that groups of stars should appears with the same shape as actually observed in the sky and the constant altitude (also called elevation) curves should be nearly circular around the zenith point.
The device given in this posting gives the desired displays with low distortion for the night sky. I give it the name Slide Sky-Disk (which is similar to the name Circular Slide Rules of similar looking mathematical devices used before the age of calculators).
It is made of two maps of stars and of interchangeable viewing grids to give elevation and azimuth angles of stars to observers located near to 0 degree, 20 degrees, 40 degrees and 60 degrees in latitude.
It will be useful to people who want to learn the stars by themselves or need to refresh their nightly detailed knowledge of the sky before going out. It is low cost, light weight, small, flexible, durable and quite portable. If made from waterproof materials, it may also be used as a low cost standby star maps for pilots, travelers, hikers and seamen (My is made from CD discs, flexible CD cases and plastic films, they are all waterproof).
The device is made by following instructions in the next 4 steps. PLEASE READ THROUGH ALL STEPS BEFORE STARTING ANY CONSTRUCTION.
Step 1: Making the Base Maps for the Slide Sky-Maps.
Figures 1, 2: Two base maps.
The two maps of the North and South regions of the Celestial sphere made by Inversion Projection (Stereoscopic Projection) are used for the Northern and Southern hemispheres respectively.
The maps are to be printed on both sides of a thick sheet of A4 paper to make a base disc. Alternatively they can be printed on ordinary A4 papers and pasted on the opposite sides of a thick disk used as the base disc. I used 2 CD discs and print the maps as their labels.(A small circular area will be lost around each pole but that does not matter much as the areas are not critical and can be redrawn with ease by most people).
Step 2: Making Rotatable Overlaying Masks Giving Azimuth and Elevation.
Figures 1, 2, 3, 4: The grid masks for observers at 0 degree, 20 degrees, 40 degrees and 60 degrees in latitude.
A grid mask is placed on top of the base map to read the azimuth and the elevation of the stars drawn on the map. The grid masks must match the type of coordinates used for drawing the Celestial sphere. An observer must use the mask drawn for his latitude.
Description of curves on grid masks:
The smallest circle of each grid is graduated into 12 intervals of 30 degrees each to show the azimuth angle of the star or direction from True North (or True South for Southern latitudes). The curves radiating from the center represent the great circles from the zenith to the terrestrial points of 0 degree (North), 30, 60, 90 degree (East) , 120, 150, 180 degree (South), 210, 240, 270 degree (West), 300, 330 (The North line points toward the map’s center in Northern hemisphere and away from it in Southern hemisphere.). The red circular arcs represent the constant elevation circles in the sky. They are placed at 30, 60 and 90 degrees from the zenith. The circle at 90 degrees from the zenith represents the horizon on flat locations. The graduation can also be read from the horizon circle toward the center to show the elevation angle of the star. The position of any star in the sky can be read against the grid.
Four grid masks are given here for use with both Celestial spheres. You have to select one that is based on a latitude nearest to your current latitude.
For latitude between 0 degree and 10 degree select the mask based on 0 degree latitude.
For latitude between 10 degree North and 30 degree select the mask based on 20 degree latitude.
For latitude between 30 degree North and 50 degree select the mask based on 40 degree latitude.
For latitude between 50 degree North and 70 degree select the mask based on 60 degree latitude.
You can make all four masks as each can be easily fit into and removed from the device as you move to a location with a different latitude.
Make each mask with the CORRECT size and print it at the CENTER of an uncut A4 waterproof transparent film by a photocopier. If this cannot be done you may have to print the mask on an ordinary piece of paper, place a transparent film on top of it and trace the grid lines onto the waterproof transparent film using a pen with waterproof ink.
Step 3: Making the Grid Holder
Figures 1,2: The grids holder is made from a flexible CD case.
The grid holder is made from a flexible plastic CD box. The front circular window has been cut for viewing the map. A small rear window is cut for moving the map. A grid is drawn onto a square transparent film and fitted to the front cover. It is to be held in place by the four plastic lugs (visible in the picture). The base map will be fitted on to the holding stub on the back cover and it can be rotated relative to the case and the grid on the front cover.
Step 4: Final Assemblage.
Figure 1: Photograph of an actual Slide Sky-Disk fitted with a mask for 40 degree. Figure 2: Photograph with Sky-Disk rotated anti-clockwise by about 25 degrees.
Push the CD with the picture of the chosen hemisphere onto the holding stub of the CD case. Make sure that it can be easily rotated inside the holder. Close the case and the Slide Sky-Disk is ready for use. The disc is rotated by access through the small window on the back cover.
Step 5: Usage.
Figures 1,2: Night sky on Dec 21st at latitudes of 60°N, 40°N. Figs. 3,4: of 20°N. Fig 5,6,7: of 0°. Fig 8,9: of 20°S. Fig 10, 11: of 40°S and 60°S.
The sky at night is represented by the circular sky map centered on the corresponding Celestial pole under the transparent window carrying the grid showing azimuth and altitude (that is the disc rotates under the viewing window).
1/- Check that the center cross of the grid is on the declination line corresponding to your required latitude.
2/- Rotate the map to place the current date on the opposite side of the window. The map and the grid gives the view of the mid-night sky for the date.
3/- Then rotate the core map by half a division (15 degree on the equator or half a month) to decrease or increase the Right Ascension for every hour ahead of or after midnight. R.A. increases in the clockwise direction for Northern and counter-clockwise for Southern hemisphere.
4/- As the latitude for the grid is not being exactly that of the observer and the true time at the location is not being equal to the zonal time the slide star disk may not give very accurate values of elevation and azimuth angle for the stars within 30 degree of the zenith. However the lines joining these stars still give accurate directions and they help identifying other stars near the horizon. The stars near the horizon can be read from the Slide Sky-Disk with more accurate values of azimuth and elevation angles.
The sky of December 21st can be visualized for any latitude using these Slide Sky-Disks in combination with a Slide Sky Map  . You may have to click on each image to have a clearer view.
. tonytran2015, Finding North and time by stars,
www.instructables.com/id/Find-North-direction-and... Posted on August, 2015
. tonytran2015, Finding North and time by stars in the tropics.
https://www.instructables.com/id/Finding-North-and-... Posted on May, 2016
. tonytran2015, Slide Sky-Map for displaying tropical stars,
https://www.instructables.com/id/Slide-Sky-Map-for-... posted on October, 2016