celestial poles & axis precession

Event Date: March 23rd
Time: 9:00 PM

Brief

   I will continue from yesterday's entry with the Big Dipper, and mention another reason why it is significant, besides being a distinguishable asterism: two of its stars act as excellent path finders.  Such stars are also defined as "hoppers", meaning that a path from one to another leads to a hop to another, or a deep-sky object.  In this case, let us look at the stars Merak and Dubhe.  In that order, a line drawn straight through them leads nearly to both our north celestial pole, and--visibly to our eyes--the star Polaris.  The name says it all, as this is the brightest star within even a wide binocular view of the pole.  At precisely 2nd magnitude, this star with clear dark skies above open seas was--and still can be--a great target for mariners to use to navigate.  After all, 4-500 years ago (and I mean this jokingly of course), GPS wasn't what it is today! 
   For the first image shown below, imagine a line between the aforementioned Dipper stars, and the straight path that almost hits Polaris.  The celestial pole is marked also, although not quite as in line with the stars.  I also included the celestial grid to show how it is 20º incremented towards the pole.  As a reminder, the pole's altitude above the horizon, as seen from any part of the northern hemisphere, is the same as the latitude of viewing.  Therefore, our pole from select location is slightly less than 38º high.

Detailed

Polaris is about 0.7º away from the celestial pole, although a certain characteristic of the Earth's axis will have it seen a little closer to the pole each year until the end of the century: precession.
A rapid, yet interesting animation can be seen at this link http://en.wikipedia.org/wiki/Precession which shows our Earth as if it was spinning very similarly to a top, in that struggling manner before [the top] is about to fall and stop.  This animation shows that motion as continuous, as it will be over the next countless number of years.  As a result of precession, Polaris isn't always our brightest star closest to the pole; the cycle is close to 26,000 years, so alot changes gradually over that long span.  The animation however, bends that duration to one precession per second, or over 800 billion times faster than real time!  Of course, that is to give us the concept of the top-like motion, so understandable for that reason.
   Getting back to Polaris, it will seem to creep closer to the pole and expected to get as close to it this week during the year 2100.  For years after that, look at the precession cycle circle in the upcoming images, for what other unaided eye stars will be close to the pole, before the return of Polaris. 
   Unfortunately for southern hemisphere viewers, their "pole" star for now and over the last several centuries is much dimmer, and wasn't too helpful for navigators over the last several hundred years.
These next two images show both the north and south celestial pole and their precession cycle circles:  first the north, then the south.  For both, the stars vary in magnitude and separation from the circle, while a few of them (such as Vega) are brighter by far, than Polaris.  Notice for the southern hemisphere, that although as far as 8º from the circle, there is a bright enough star that will be that close for a few thousand years: Canopus, as our second brightest star as seen from Earth, in apparent magnitude.  This star can be seen briefly above the horizon in the southern states, and seen longer if viewing from as far south as Mexico and further.  After the star Sirius Sirius (magnitude -1.5), another southern hemisphere star closer to the equator, Canopus appears a little dimmer at magnitude -0.7, yet only slightly noticeable, when both are seen high enough in the sky.

*click on images to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.