Coordinate Grids     When studying objects in the sky it is helpful to use a coordinate system to keep track of where things are. The galactic coordinate system has latitude and longitude lines, similar to what you are familiar with on Earth. In the galactic coordinate system the zero degree latitude line is the plane of our galaxy, and the zero degree longitude line is in the direction of the center of our galaxy, towards the constellation Sagittarius.The activity below is designed to help you visualize the galactic coordinate grid.

Materials

• Printout of the coordinate grid on paper or clear transparency.
• Printout of the Galaxy on paper.
• tape
• scissors
• pencil or thin marker

Cut out the double image of the Galaxy leaving the two images connected where they touch. Fold along the connecting edge and tape the edges together in a few places to make a two sided galactic disk.

Trim down the coordinate grid printout by cutting along the dotted lines. Cut a slit at the sections marked A and B. You are going to make a tube, with the galaxy disk inside. Keep the printed grid on the outside, and the printed side of the A and B tab labels on the Galaxy facing up. Insert the galaxy disk A tab into the A slit on the coordinate grid, bend the tab down and hold it in place with tape. Wrap the grid around and insert the galaxy disk B tab into the B slit on the coordinate grid, bend the tab down and hold it in place with tape. The printed grid should be on the outside. Line up the vertical cut edge of the coordinate grid with the first longitude line and secure the wrapped tube in place with tape.

The latitude lines have been marked for you, with a marker or pencil mark the longitude lines. Start at 0 degrees and go around to the left in 30 degree increments. The finished models should look like those shown here. The location of our solar system in marked on the galactic disk. You can imagine that you are in the galaxy at the tip of that arrow, looking up at the sky around you. There would be stars in every direction, this grid is a construct to help identify the location of the stars in the sky. Imagine still that you are at the tip of that arrow in the galaxy. The 0 degree latitude line, or the galactic equator, is in the plane of the galaxy. If you look from the tip of the arrow towards the middle of the galaxy, the 0 degree longitude line is in that direction. This is a cylindrical model of galactic coordinates. In reality the universe around us is spherical and the coordinate grid would close on the top and bottom at 90 degrees from the galactic equator. By representing it this way, you can remove the tape holding the cylinder together, and also the galaxy inside, and then flatten the grid out to see a two dimensional represenation of the coordinate system.

This activity demonstrated a cylindrical representation of galactic coordinates. The universe around us is actually spherical. The next page will show how spherical coordinates are represented on a galactic coordinate grid.