Should you observe The Big Dipper continuously from nightfall to dawn, you will notice that it appears to shift its position in the heavens.  The Big Dipper is not really moving, rather it is you who are changing position due to the rotation of the Earth.  Furthermore, if the sun’s glare did not wash out the light of The Big Dipper during the day, you would see it make a complete circle around the North Star in one full 24 hour day. 

At one particular time of year, you might see The Big Dipper in the northeast after sunset, nearly over your head at midnight, and in the northwest at sunrise.  Six months later, you may see it start in the northwest, at or near the northern horizon at midnight, and in the northeast at dawn.  Its positions at sunset and dawn seem to change during the course of a year because the Earth is moving around the Sun in a circuit that takes one year to complete. 

These changes in position from one month to the next also cause The Big Dipper to appear at different angles.  In one season it will be horizontal with respect to the viewer’s horizon, in another season it will appear vertical.  If you do not understand what I am talking about here, a look at Figure 13 (next page) should quickly end your confusion.  The arrow in the illustration indicates how two stars at the end of The Big Dipper always point toward The North Star (whose proper name is Polaris).  Once you know where Polaris is, locating other things is much easier. 

Indeed, all stars in the sky will change position from month to month for the same reason The Big Dipper does.  This seasonal change is the reason why most constellations and asterisms rise and set at different times as the year progresses.   However, a few extreme northern star groupings such as The Big Dipper may never set for viewers who live a far distance north away from the Earth’s equator. 

While we’re on the subject of Polaris, I would like to make a point before you undertake to find this star for the first time.   I have noticed that many people not very familiar with astronomy seem to think that the North Star is supposed to be the brightest star in the sky.  Nothing could be farther from the truth!  You will find that it is a star of rather ordinary brightness: not super bright, but not dim either.  It will be about the same brightness as the brightest stars in The Big Dipper. 

            I have already both mentioned and illustrated (in Figure 13) how you can use two stars in the bowl of The Big Dipper to find Polaris.  However, in late winter through late summer you may use other stars in this asterism to point you toward other sites of interest.  But before I begin to talk about this aspect of The Big Dipper, there is an important point that I need to make.  Hence forth, unless stated otherwise,  I shall assume that you are going to do your sky gazing in the evening.  Even so, keep in mind that the same sights may be visible later at night or early morning in different seasons.

 

copyright 2004 Singularity Scientific, all rights reserved.

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