North Celestial Pole

Event Date: April 22nd
Time: 10:00 PM

Brief

   Several times so far in the 5 weeks that I have done blog entries, I have used a reference latitude of approximately 38º N.  Not only is this for my home-base observatory at Chabot Space & Science Center, but it is also one for overall, ideal viewing throughout the year; more of that is mentioned in the detailed.  Before that, I will show an image looking directly north.  Labeled are the north celestial pole (n-c-p), which at our latitude, is located at the altitude of precisely the same value.  The same scenario would be the case if we were looking in the direction of the n-c-p from the [global] north pole: 90º in latitude, 90º in altitude, which is straight up at zenith.  Therefore, the next time that you are trying to remember where Polaris is, currently about 41 arc-minutes from the pole, this may help.  Polaris is not only the closest easy-to-see star to the axis, but simply, a reminder of where north is when skies are clear enough to see it.

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

Detailed

   38º, give or take a few, is also a good latitude for other reasons: from here, all zodiacal constellations--the ones that the Sun passes the stars of each year--are easily visible above the horizon at their transit (highest) times; the shortest nights in the summer are still over 9 hours, including about 7 hours of dark enough skies; it is far enough from the global equator that humidity is not a large problem.  The latter scenario is important not only for clear sky-viewing, yet for better astro-photography conditions.  When taking photos, clear conditions are important, but we also want as low humidity as possible, for good transparency and seeing conditions.  The next paragraph further elaborates on these terms and other conditions for our latitude
  Good transparency doesn't only mean very clear skies; this is often because of very little water vapor in the air.  There are some times when skies can be very clear yet very damp, normally following a period of rain and when temperatures are cold; this is often the case before fog comes in, although not always the case.  By "seeing" being excellent, features such as the belts and zones on Jupiter can be contrasted in color much more easily, and we can see, as example, the divisions of the rings on Saturn.  Very importantly, we need dry enough skies so camera optics don't fog up. When it is windy, that leads to poor seeing conditions, causing a wavy turbulence visible in the air.  Of course, if a location is too far north, it means that (1) there are too many bitter cold evenings, with or without precipitation; (2) during late spring and summer months, there are not enough dark hours--if any--at the most extreme latitudes at certain times of year.