Milky Way Rising — Understanding Our Place in the Galaxy
In the northern hemisphere, the perfect time to get a sense of our position within the Galaxy and the effect of that position on our view of the night sky is during a summer evening when the Milky Way first rises. At this time, the plane of the Galaxy is parallel with our own horizon, making it particularly easy to visualize our location embedded in the galactic disc with it spreading out all around us. If you live exactly at 27º N latitude, the North Galactic Pole will be at our zenith and the Milky Way's band of light will ride low along the skyline for the entire 360º. 27º is the magic number because that is the vertical angle at which the Earth's axis is inclined with respect to the galactic plane. At other latitudes, the Milky Way will rise with a slight tilt corresponding to the difference between your latitude and 27º N. As the Milky Way rises, all the deep sky objects you see above the glowing band will lie in the northern hemisphere of the Galaxy.**
Picture yourself standing in an unobstructed field as the constellation Sagittarius rises. The core of our galaxy will appear in the southeast, located about 4º north of the teapot's spout. If you live at that magical 27º N, the core will rise at exactly 123º azimuth (where 90º is due East and 180º is due South). This corresponds to the angle within the galactic plane that the Earth's axis makes with respect to a line in the direction of the galactic core. Another way to think about this is to realize the North Celestial Pole, where the Earth's axis points, is located at 27º N latitude, 123º longitude in the alt-az based galactic coordinate system.
Now consider M13, the great globular cluster for which Where is M13? is named. The image below was generated by Where is M13?. Using paired face-on and edge-on views, it shows M13's location relative to the Galaxy's core and the Sun. M13 is represented as the blue-outlined, yellow-filled circle and the Sun as the small orange dot to the left of the galactic core.
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The Object List within Where is M13? indicates that M13 is located at 59º galactic longitude and 41º galactic latitude. This means it is well above the plane of our galaxy and about 59º counter-clockwise from the galactic core when measured around the galactic plane. As the Milky Way rises, you can study the above image and truly see the correspondence to what you see in the night sky. M13 is at altitude 41º and is about 59º to the north in azimuth from where the galactic core is found.
To take a second example, consider M29, the open cluster that appears in Cygnus. In the picture below we have zoomed in because M29 is much closer than M13 is to the Sun (6,000 light-years compared to 25,000 light-years).
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The Object List indicates that M29 is located at 77º galactic longitude and 0.6º galactic latitude. The almost 0º galactic latitude means this cluster lies deep within the spiral arms themselves. The object's galactic longitude implies that it is 77º counter-clockwise from the galactic core. Again, as you are standing in the field you can verify this. M29, and Cygnus in general, lies directly within the glowing band we call the Milky Way, which are the Galaxy's spiral arms seen edge-on. The cluster will appear in the sky 77º north in azimuth from the spout in Sagittarius' teapot.
As the above image illustrates, when looking towards M29 we are looking down the Orion-Cygnus spiral arm in the direction in which the spiral arm winds inward. The fact that Cygnus lies along this arm explains why so many open clusters and dark nebulae are found in this region of the sky.
You can continue to develop your galactic frame of reference by using Where is M13? to plot other object locations and match them with what you see in the night sky. After a while you will develop an intuitive sense of how our celestial perspective depends on the galactic structure and where Earth and the deep sky objects reside within it.
** If you live in the southern hemisphere, you can play this same game. Here, 27º S latitude is the magic number and you will want to watch the winter Milky Way rise. All deep sky objects you see at his time will lie in the southern galactic hemisphere.