03/19/2024
Happy Spring!
Hi Gang,
Tonight at 11:01 PM EDT spring will begin.
But how is it that the start of of a season, defined by the motion of the sun, can occur when the sun is below the horizon? And how we can nail this down so precisely? Why isn't it defined by the sunrise or sunset times?
The beginning of spring, the equinox, is defined not by the day on which the sun rises due east and sets due west, but by the exact moment that the center of the sun crosses the celestial equator- the imaginary line in space that extends outward from Earth's equator.
As the Earth revolves around the sun, our home star appears to drift northward and southward over the course of the year. Today it the day that it crosses the middle of that north-south motion. And, yes, it is the day where the sun happens to rise directly (or almost directly) east and set directly (or almost directly) west- and aligns nicely with the "equinox" sun slots in the JGAP plaza.
This time is also the time where the sun, by definition, crosses the "Celestial Prime Meridian"- the "Zero degree" point on the sky, when one measures east and west. (Astronomers call the East/West measurement of the sky "Right Ascension", BTW.) The zero degree point (or, as astronomers say, 0 hours Right Ascension) is sort of like the "Greenwich Meridian" of the sky.
On Earth the Greenwich Meridian and the Equator cross in the Atlantic ocean south of Ghana (Africa) and west of Gabon.
In the sky the Celestial Equator and the "Celestial Prime Meridian" cross in the constellation Pisces.
This won't always be the case, however. Due to a strange wobble in the motion of the Earth, that takes 25,765 years to make a complete loop, the position of sun's crossing of the celestial equator will slowly drift (or "precess" as astronomers say) westward across the sky, entering the constellation Aquarius in about 600 years or so. After 25,765 years, it will have made a complete loop through all the constellations of the zodiac (and Ophiucus) back to where it started.
This is called "Precession of the Equinoxes."
Happy spring!
Even MORE bonus detail:
While the celestial Prime Meridian is defined by the east-west position of the sun when it crosses the celestial equator, astronomers don't generally measure the positions of objects relative to that exact point. Instead, they pick a year, usually a nice round number, and define the coordinates based on the crossing of the equator then.
Right now, most printed maps chart the sky using the year 2000 as the base. (They do this to avoid having to re-print maps, and republish coordinates every year as the position of the spring equinox shifts.) The year chosen is is called the "Epoch".
Back when the "official" IAU defined constellations that we use today were mapped out, many of the borders were based on the nice round numbers on charts made out in the epoch of that time. In other words, at that time, they mostly lined up nicely with the prominent north-south or east-west lines on maps.
But you'll notice that on newer maps that the constellation boundaries are kinda' wonky. They don't line up nicely with nice round numbers and are slightly "un parallel" from the coordinate grid. This is because the epoch that the newer maps use is very different than that that was used when the constellation boundaries were laid down by international agreement 102 years ago.
Now you know...
