The Rotating Sky and Motion of the Stars
Ever noticed how the stars move? The entire sky appears to rotate around the North Star above us. (Of course, we’re actually the ones moving!) You should be able to observe this movement by looking up at the constellations. We’ll show you how, along with a sky map of the mid-December sky.
Just click here or on the image below to open the printable map—then bring outside!
Note: You’ll need to do your star gazing from a very dark location to see many of the sights described here.
This Month’s Highlight: The Stars Go Round and Round
The sky is always in motion. Or, to be more accurate, the sky *appears* to be in constant motion due to the rotation of the Earth on its axis. For observers in the northern hemisphere, this motion is most obvious when you look to the north on a dark night.
The apparent motion of the sky can be compared to the movement of the hands of a clock. The center of the “sky clock” in the northern sky is Polaris, the North Star. Polaris lies very near the North Celestial Pole, the point in the sky that lies directly above Earth’s north pole. Polaris always remains nearly stationary, while all of the other stars and constellations appear to slowly revolve around it. Just as the hands of a clock revolve around the center of the clock’s face, so the sky appears to revolve around the Celestial Pole as the Earth rotates on its axis.
Stars Revolving Around Polaris
The stars of the northern sky appear to rotate counter-clockwise around Polaris. And just as with a clock, the movement is slow. Here is an example.
- Note the Big Dipper, located at the bottom of the map, directly below Polaris at 8:00 p.m. If you waited six hours—until 2:00 a.m.—and look at the northern sky again, the Big Dipper will have revolved to a position directly to the right of Polaris. All of the other stars on the map will have moved, too. This movement is caused entirely by the rotation of the Earth on its axis.
A few constellations are near enough to Polaris that they never drop below the horizon, as do constellations farther from the North Star. The constellations close to Polaris are thus visible all night and every night of the year for observers at mid-northern latitudes, such as the 48 United States and much of Europe, and are known as circumpolar constellations. There are six of them, highlighted in green on our map:
- Ursa Minor, the Lesser Bear (including Polaris);
- Ursa Major, the Greater Bear (including its Big Dipper asterism);
- Camelopardalis, the faint Giraffe;
- Cassiopeia, the Queen (with her distinctive Big W or M shape);
- Cepheus, the King (looking like a child’s drawing of a house); and
- Draco, the Dragon (whose four-sided head is especially distinctive).
These six circumpolar constellations perpetually chase one another around Polaris.
On a cosmic time scale, different stars take turns as our North Star. It turns out that the Earth wobbles slightly as it spins through space, just as a spinning toy top wobbles as it slows down. This means that the axis of the Earth does not always point at Polaris. Over a period of about 26,000 years, the Earth’s axis traces out a huge circle on the sky, and over time it points toward bright stars other than Polaris, as shown on the inset on our map.
- For example, between 4,000 and 6,000 years ago, the Earth’s axis pointed at Thuban, a star in the body of Draco, and so Thuban was then the North Star.
- Looking ahead, about 13,000 years from now, the Earth’s axis will point toward the bright star Vega, which will then be our North Star.
- And in 26,000 years, the axis will once again point at Polaris, making it our North Star once again.
For much of the 26,000-year period, the Earth’s axis points to no bright stars. So, for thousands of years at a time, we have NO North Star at all!
December Sky Map
Click here or on map below to enlarge (PDF).
Sky map produced using Chris Marriott’s Skymap Pro
How to Read the Sky Map
Our sky map does not show the entire sky which would be almost impossible. Instead, the monthly map focuses on a particular region of the sky where something interesting is happening that month. The legend on the map always tells you which direction you should facing, based on midnight viewing. For example, if the map legend says “Looking Southeast,” you should face southeast when using the map.
The map is accurate for any location at a so-called “mid northern” latitude. That includes anywhere in the 48 U.S. states, southern Canada, central and southern Europe, central Asia, and Japan. If you are located substantially north of these areas, objects on our map will appear lower in your sky, and some objects near the horizon will not be visible at all. If you are substantially south of these areas, everything on our map will appear higher in your sky.
The items labeled in green on the sky map are known as asterisms. These are distinctive star patterns that lie within constellations. When getting your bearings under the stars, it’s often easiest to spot an asterism and use it as a guide to finding the parent constellation.
The numbers along the white “Your Horizon” curve at the bottom of the map are compass points, shown on degrees. As you turn your head from side to side, you will be looking in the compass direction indicated by those numbers. The horizon line is curved in order to preserve the geometry of objects in the sky. If we made the horizon line straight, the geometry of objects in the sky would be distorted.
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