2024-04-21 Sunday

06:00PM

I love this hobby. The primary reason is its ability to help keep me humble and grounded. This is done in two ways:

- Observing under the night sky

- The learning curve

Observation achieves this easily by the sense of awe and "smallness" that come with every session exploring under the night sky. Being able to enjoy the fruits of our creative, loving, and relational God who's own creativity brings Him glory and draws us to Him only adds to the sense of awe that comes from the beauty and reality of the objects themselves. Even on a night of failures, when every planned target falls through, this sense of awe is still strong!

The second is done through the complexity of this hobby. Even after years of learning how to find a target, identify it correctly, and observe it well; I still make naive mistakes that make me laugh at myself and sets me on a course to learn something new; three nights ago, this happened again...

That night, for fun, the kids picked my next lunar target for me: Rupes Recta, a "straight wall" scarp. Despite the near full-moon I set about getting ready for the night. Once I got everything setup, and turned my scope towards the lunar surface, I could not identify anything. Despite the decent conditions I could hardly even make out some of the largest craters. For the first time since I began this hobby my problem was that my target was TOO bright. I've spent so long considering the faintness of an object, that when deciding to proceed with the lunar observations I assumed: "at least I'll never have to worry about not being able to see something on the lunar surface since the moon is always bright enough". Silly naive mistake! It seems so obvious now.

It didn't take long to find one of the newbie tips for lunar observation is to observe targets along the lunar terminator. The angle of the sun to the lunar surface along the terminator creates long and easy to see shadows that allow the surface to pop. I soon realized that I could probably track the location of the terminator automatically and use this as a filter in an org-mode agenda to help me sort and pick my lunar targets. I set about modifying my moon.org list adding LAT and LONG properties and filling them with the proper coordinates of each target. I then threw together this helpful elisp function:

(defun my/calc-lunar-terminator-longitude-search (&optional days)
  "calculate lunar terminator longitude"
  (interactive)
  (let* ((days-since-new-moon (if days days
				(/ (abs (org-time-stamp-to-now "2024-04-08 11:23:21" t)) 86400)))
	 (terminator-age (mod days-since-new-moon 14.765295))
	 (terminator-pct (/ terminator-age 14.765295))
	 (lunar-face-degree-with-offset (+ (- 180 (* terminator-pct 180)) 270))
	 (lunar-terminator-start (mod (- lunar-face-degree-with-offset 30) 360))
	 (lunar-terminator-end   (mod (+ lunar-face-degree-with-offset 30) 360))
	 )
    (format "Long<%02d+Long>%02d" lunar-terminator-end lunar-terminator-start)
  ))

This function gives me a rolling 60° window that I can use in filtering agenda views. I'm sure there are probably some errors in there, but some basic testing produced accurate enough results and this should really help me ensure that my lunar targets will be visible.

I believe that the sense of awe from observing under the night sky will never diminish. And here's to hoping that there is no end of making silly mistakes to learn from too!

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