March Equinox

                             
event date: March 19th
time: 10:15(.28) PM

Brief

When we think of the first day of spring, people in the northern hemisphere are aware of it happening in late March.  Just the opposite, those residing in the southern hemisphere think of it happening in late September.  There are other terms more commonly used in astronomy for these days: equinox, vernal and autumnal. The vernal equinox refers to spring beginning, while autumnal refers to autumn or "fall" beginning.  As my title displays, to avoid seasonal confusion, the equinoxes are also based on the month of the calendar: March or September.  These two links give more detail, answering a popular question: are day and night exactly the same length on the equinox dates?  If not, is there a term for that too?  The answers are no and yes respectively, as well as where the word is derived from.  Images and other helpful visuals are presented.
http://en.wikipedia.org/wiki/Equinox
http://www.timeanddate.com/calendar/march-equinox.html

As others may, I like to consider the moment of the vernal equinox for the northern hemisphere as the marking of the *astronomical new year, based on the Sun's position in the sky along its annual path: the ecliptic.  To find out more about that, read the detailed section to follow.
*From this point forward, the detailed section will often elaborate on the brief section, therefore, my only mentioning it here as the first entry.
-----------
Detailed

   The following two images from my version of Starry Night Pro-Plus software are perhaps more abstract than those shown from the links in the brief, yet can still make sense with some basic lessons in astronomy.  Remembering that Earth's axis tilt of very close to 23.5º, leads to my showing two different sets of grids offset in the first image: red and green.  If there was no tilt at all, a second grid would not be necessary; the Sun would rise at the same azimuth points relative to east and west, and transit at the same altitude every day if standing at the same terrestrial latitude.  This would lead to us becoming weather-bored because of the lack of seasons; I would become so, anyway!
   Looking at image one, we see four labeled key gridlines: the celestial equator, ecliptic, celestial meridian and ecliptic meridian.  **more text to follow image.

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

The celestial equator represents our equator on Earth projected into space, as is the case with our global latitude and longitude coordinates.  The ecliptic equator, or simply known as the ecliptic, marks the path of the Sun in our sky as a result of the Earth's axis tilt.  The extreme points of the ecliptic north and south of the celestial equator, equal that of the Earth's tilt, at 23.5º.  The grid is really only included this time, to show how much more offset it would be if we had a greater axial tilt.  Think of outer [solar system] planet Uranus in comparison, with its near-90º tilt!
The celestial meridian marks very closely, where the Sun is at the equinox in celestial longitude: zero hours (think 24 hour, or military time on a watch).  Longitude is measured in (h)ours/(m)inutes/(s)econds, and I will show longitude measurements using this notation.  It is also known as right ascension, or r.a. for short.  The ecliptic meridian plays more of a role for measuring the exact time of the equinox; measured in degrees(º)/arc-minutes(')/arc-seconds(") instead, when the Sun is exactly at the 0º 0' 0" mark for ecliptic longitude.  *If an astronomical new year is to exist, this is it, in my opinion!  The ecliptic meridian also marks 180º 0' 0", which is where the Sun is at the time of our autumnal equinox,  and vernal for the southern hemisphere.  The latitude lines for both the celestial and ecliptic grids, are known as declination and ecliptic latitude respectively.  I will talk about both of those more for planets, deep sky targets and the Moon, for future entries.
   Also in image 1, the horizon is hidden, as if we are looking through the ground beneath our feet, to see the Sun below the horizon at this late-evening hour.  The second image shows the Sun zoomed in to a field of view (FOV) of 1º to show more closely how it is centered on certain gridlines.

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

 It also shows how these lines converge towards each other in this part of the sky, very close to the Sun at equinox.
   Finally, the time listed in the images--10:15.28 PM Pacific Daylight Time--indicates that of the equinox from our viewing location, as opposed to if a viewer was standing exactly in the center of the Earth's core, since this is impossible.  As the table from the Wikipedia link shows above, the equinox time is listed about a minute earlier, based on the Earth-center perspective.  Depending on which week of the year, most entries here will use Pacific standard or daylight time.  The former being 8 hours behind Universal Time (UT--Also known as Greenwich Mean Time (GMT)) and the latter 7 hours behind UT.  When clicking on this time link, between the second Sunday in March and first Sunday in November of the same calendar year, we will notice that when comparing the hour difference for Pacific time zone, there is no daylight savings change for UT.
So, happy March equinox, and astronomical new year. :-)