[Aztlan] E-Groups and Sundogs

[Craig Berry]

On Thu, Feb 21, 2008 at 6:27 AM, Scott <harview at montana.com> wrote:
>  Sorry, but could you explain to the list, or at least to me, in
>  astronomical terms, what exactly the "nadir passage of the sun" is? A
>  quick 'google' for it reveals mainly astrological or new-age sites.

The "zenith" is the point in the sky directly overhead.  A zenith
passage of the sun occurs when the sun is directly overhead at noon at
a particular location.  It can only happen in the tropics, since
outside the tropics the sun will always stay south or north of the
zenith at noon.

The "nadir" is the (invisible) point in the sky opposite the nadir,
straight down from any given location.  A nadir passage occurs when
the sun at midnight is at the nadir.

>  Most importantly, if I were to go out in my backyard, what
>  observations would I need to make to determine when this 'nadir
>  passage' occurs at my latitude?

Zenith passages are easy to observe; put a vertical stick in the
ground (using a plumb line to make sure it's accurately vertical) and
watch its shadow over the course of a year.  When it has absolutely no
horizontal shadow at all, you're looking at the moment of zenith
passage.  Again, this will only happen if your latitude is in the
tropics.

Zenith passages come in pairs, roughly the same number of days before
and after the solstice.  Imagine a point in the northern-hemisphere
tropics.  On the first day of spring, the sun is over the equator, so
the sun at noon is south of the zenith.  The sun moves north during
the course of the spring, and eventually you get zenith passage.  It
then continues north until the day of solstice, when it's overhead at
the tropic line (around 23.5 degrees north).  Then it starts heading
south again, with another zenith passage occurring along the way.  The
closer you are to the tropic line, the closer together the two zenith
passages will be.

Nadir passages occur around the opposite solstice from zenith
passages.  Once you know your zenith-passage offset around the summer
solstice (say, 20 days before and after), you can apply that same
offset to the winter solstice to figure out when nadir passages will
be (20 days before and after the winter solstice).

Knowing that even though you can't see them directly, you can derive
nadir passage dates from zenith passage dates requires understanding
of the spherical geometry involved.  Did the ancient mesoamerican
cultures work out this geometry and thus know about nadir passages?
I'm not a specialist in the field, so I'll leave that question to
those who are.

-- 
Craig Berry - http://www.cine.net/~cberry/
"So we struggle and we stagger, down the snakes and up the ladder, to
the tower where the blessed hours chime."
  - Leonard Cohen, "Closing Time"