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Thread: Planning ahead for when celestial things will be at the angle you want

  1. #1
    Join Date
    Oct 2006
    Posts
    3,778

    Planning ahead for when celestial things will be at the angle you want

    You might already be aware of Horsetail Fall in Yosemite National Park (37.7291N, 119.6284W). It's a waterfall that's mainly known for the fact that, for a short period each year, the setting sun directly hits the water but not the nearby rocks, thus making it look as if the water were glowing, like a liquid version of fire (and even casting light on the nearby rocks), for about 10 minutes each day. People gather at particular spots in the valley below every year to wait for this.

    Predicting when it will happen isn't a complex cosmo-geometry problem; it's just the same time of year that it's always been. Here is an example of a list of dates for 2019 that one photographer acquired somehow, and any variations from year to year are only on the scale of the changes in the dates of solstices and equinoxes. (The sun is at the right angle twice per year, in February and October, but there's nothing to see in October because there's too little water flowing.)

    But it's occurred to me that the same kind of thing should also happen at night or twilight with a full or close-to-full moon. So, how would one predict when the moon will be at the right angle and in the right phase? I know there are programs out there that calculate the geometry for this kind of thing, but I don't have one or know how to use them...

  2. #2
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,499
    I could use Stellarium to find the declination and elevation of the Sun at the time shown in that link, and then search for times the Moon would be at the same position. Such occurrences might be few and far between, because the Moon would often straddle the optimum position from one day to the next instead of hitting it squarely every time.

    That photo reminds me of the "firefall" I observed from the floor of Yosemite Valley back in 1962, when they shoved a roaring bonfire over the side at Glacier Point. I had stood by that fire and looked 3,000 feet straight down into the valley just a couple of hours earlier. My grandparents reported that they had done away with it a couple of years later.

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