THE USM SOUTHWORTH PLANETARIUM

207-780-4249     www.usm.maine.edu/planet

70 Falmouth Street     Portland, Maine  04103

43.6667° N                   70.2667° W 
Altitude:   10 feet below sea level

Founded January 1970

Julian date:  2458647.5

                  "                                      " 

                            -Marcel Marceau

THE DAILY ASTRONOMER
Thursday, June 13, 2019

North

What is life's deeper meaning?

How did the Universe form?

Will the cosmos last forever?

Is it just one of myriad Universes?

Such profound questions garner a great deal of attention because they veer so precariously close to the concept of physical reality they almost bridge the divide separating physics and metaphysics. Sometimes, though, astronomy pertains to the more straight forward queries such as "How do I find north in the night sky?"

We've heard that question often because many of our patrons are actuated by the strange desire to garner practical knowledge.    Being Northern Hemisphere dwellers, we can guide you to finding north quite readily:  all you need to do is find Polaris, the north star. This star is almost aligned precisely with Earth's North Celestial Pole.  For this reason, it maintains a constant position while the other stars appear to describe circles through the sky due to Earth's rotation. 

Unfortunately, despite popular belief, Polaris is not the night sky's brightest star.   In fact, on the list of the sky's brightest stars (including the Sun), Polaris ranks 49th!*   Also contrary to popular belief, Polaris is not directly overhead, at least not for us.  Polaris occupies the zenith around the North Pole.  Its altitude in an observer's sky depends on the observer's latitude.     Here at 43 degrees North latitude, Polaris is just about 43 degrees above the northern horizon.  

How can one find the 49th brightest star in the sky that is about midway between the horizon and zenith?    Well, one could first locate the "Big Dipper," the seven star asterism comprising the central structure of the constellation Ursa Major.    The Big Dipper's outer two bowl stars, Dubhe and Merak,point directly to Polaris and are therefore known as the "pointer stars."

The Big Dipper can help one find Polaris, the remarkably unremarkable star that is almost directly aligned with Earth's north celestial pole.   Dubhe and Merak, the outer two "bowl" stars, point directly toward Polaris.   As the Big Dipper is circumpolar, one can always find it above the horizon at any time of night and at any time of year.     

As the Big Dipper is circumpolar, meaning that it will never set at this latitude, one can always use it to find Polaris.  This time of year (late theoretical spring), the Big Dipper begins the night almost directly overhead.  All one need do is look straight up toward the zenith just after dark to find the Big Dipper looming high in the sky.    Follow its outer bowl stars about one "dipper's length" away from these stars to find Polaris.     Even though the Big Dipper is always above the horizon, at some times it is more difficult to observe owing to its position.    When lowest, the dipper appears to rest almost directly on the horizon and could be blocked from view by trees, houses or other obstructions.     

Polaris, though, maintains a constant position.   No matter where one ventures in the northern hemisphere one can determine their latitude merely by observing Polaris.   Of all the night sky complexities that confound us, finding north is, at least, one of the simplest complexities, if there is such a thing. 

*This designation is a bit tricky as Polaris exhibits slight variability.  At maximum brightness, mag 1.86, it would rank 40th. At minimum brightness, 2.13, it would rank 63rd.