I spent the weekend in the Elqui Valley. I’ll write a full report when I’ve finished the mass of grading I did not complete during the trip. First, I must tell you a little about last night.
I’ve seen a lot of our universe in the 40 or so years I’ve taken notice of the night sky. Many of these observations have been at the very edge of my vision after hours, or perhaps years, of preparation and would not impress anyone who has not spent significant time at the eyepiece. Others are more obvious and easier to share: the Milky Way standing high in the sky; a thin crescent moon, illuminated by Earth next to a bright planet or two against a darkening sky tinged with orange; Saturn on a crisp, steady night.
On Saturday night in a little town called Rivadavia in the Elqui Valley, I saw something for which I was completely unprepared. I had set up early because I wanted to image an object that would culminate just after sunset. As often happens, when you have plenty of time to set up, everything goes smoothly. So it was that I was sitting next to my gear, set up alongside the picturesque, but loud, Rio Claro, a tributary of the Elqui, surrounded by the precordillera. I noticed that the moon, a thin crescent a couple of days past new, was sinking toward the mountain ridge to the northwest. I turned the little Dob on it and enjoyed a fine view. As it continued to sink, the ridgeline came into view. It was upside down, of course, inverted by the telescope. Against the dark blue sky, I could make out the silhouette of boulders and cactus. The cacti were like whiskers, pointing down from the inky black outline of the ridge.
The illuminated sliver of the moon was sinking fast toward the ridge and then, just before the bright moon got to it, the outline of the cacti lit up, dazzling white against the dark background, with the dark silhouette of the cacti wrapped in gleaming ivory. In a few seconds it was over and the cacti and ridge again cast their silhouette against the bright moon. But the show wasn’t over yet, for as the illuminated crescent of the moon slipped completely behind the ridge, the remaining gibbous portion revealed the extent to which Earthshine can light the lunar landscape. No longer needing to compete with solar illumination, the twice reflected light of Earthshine made lunar features stand out easily. Again, as the moon fully sank behind the ridge, the cacti were briefly illuminated by a paler white outline.
What happened was this: the reflective needles of the cacti scattered the moonlight so that before the cactus was directly in front of the moon, the needles formed a brilliant white envelope for the darker cactus body. It was a stunning sight I will long remember.