The Big Dipper

Chart created by Eric Fitzgerald. The Big Dipper is one of the largest and most recognizable constellations in the Northern Hemisphere.

Other than Orion, The Big Dipper is probably the most recognizable constellation in the northern hemisphere. This asterism is among the oldest of constellations and its mythology extends into prehistory. It has been viewed as a dipper, a bear, and a plow. In fact, its official name is Ursa Major, or The Big Bear.

To find Ursa Major in the evenings of the spring, look north just after dark and then look up about 45 degrees. The Big Dipper will be upside down. Ursa Major is a circumpolar constellation. Like the name suggests, this means the constellation never fully sets from this latitude; it merely circles the celestial pole. The celestial pole is the point in the night sky that our Earth’s axis points at.

The elevation of the celestial pole above the north horizon in degrees is the observer’s latitude on Earth. This point is fortuitously marked by the star Polaris, or the North Star. All the stars in the northern skies appear to revolve around Polaris as the Earth turns. As the Earth makes her way around the Sun and the seasonal stars proceed through the year, the Big Dipper appears to “dip” into the northern horizon in the fall.

I’ve always imagined that The Big Dipper is filling up during our dry season and spilling out in the winter as it rises again. Now, in the spring, The Big Dipper has emptied its load and it swings back down in the summer.

You can find Polaris by drawing a line through the far side of the Dipper’s bowl (see chart) and extending it out roughly the length of the Dipper itself

To see The Big Bear, imagine the handle of the Dipper is the tail of the Bear. The bowl of the dipper marks the hindquarters. The ancients must have had quite the imagination to picture a bear with a tail that long!

Mizar is the name of the star that is in the bend of the handle of the Dipper. It is a visual double star. The fainter star is called Alcor. Mizar and Alcor are a “visual double”— as opposed to a spectroscopic double, which requires a spectroscope to resolve. The ancients used this double star as a test of eyesight. Can you split the pair with your naked eye? If you have a small telescope, try looking at Mizar and Alcor. The brighter star, Mizar, reveals itself to be a double star again! Mizar is among my favorite double stars. All told, this is a system of six suns that all do a very complicated dance around one another.

Another star you can find with the aid of the Big Dipper is Arcturus, an orange-colored star that is the brightest star in the constellation of Bootes, The Oxman. If you make an arc out of the handle of the Dipper follow the arc off the end about three Dipper lengths and you will find Arcturus. This star is in our neighborhood at 36.7 light years away, one reason why it is the fourth-brightest star in the sky. The light from Arcturus was used to trip a switch that kicked off the 1933 World’s Fair in Chicago. That light left the star approximately at the same time the previous World’s Fair was held in Chicago in 1893.

Take a stroll after dinner and enjoy our celestial companions in these mild spring evenings. Take a little time to get to know their names. You might find it surprising that such a simple thing can be so rewarding.

 

If you have any questions or comments for The Backyard Astronomer, contact me at: thebackyardastronomer365@gmail.com.

Illustration is from Uranographia 1690, by Johannes Hevelius
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