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    Re: Corrections for latitude when taking sights
    From: Brad Morris
    Date: 2019 Mar 13, 16:56 -0400
    Hi Frank

    No Stellarium, no joy.

    I don't doubt for an instant that the object will be on the prime vertical, at some instant in time.  That's intuitive.

    The conundrum I am dealing with is the assertion that the object (sun) will be 90° in azimuth.  That is the part I am having trouble with.  If I am at any latitude greater than 24°, then the azimuth to the sun can never be 090 or 270.  Unless I have lost my mind (* several people are typing *), you will have to look to the N from the S hemisphere and to the S in the N hemisphere.  I will never look due east (90) or due west (270) from my position. What am I missing?

    Brad

    On Wed, Mar 13, 2019, 4:17 PM Frank Reed <NoReply_FrankReed@fer3.com> wrote:

    Brad, you wrote:
    "I have resorted to HO71, Azimuths of the Sun."

    There's an app for that. :)

    This is something that's hard to believe when you hear it in words, but it's really easy to see in a planetarium or in a planetarium "app" like Stellarium. I also think it's easier to see when you pick a star at night rather than the Sun.

    Try this in Stellarium: for today's date and your lat/lon, put Arcturus low in the eastern sky at exactly 90° azimuth. Tonight in southern New England / southern New York, around 41°N latitude, that would occur around 3:00 UT or about 10pm EDT. Think of Arcturus as sitting on top of a lever arm extending vertically up from the east point right on the horizon. Now, without changing the time, travel south by 10° (see PS). Arcturus will rotate on its arm toward the north away from the prime vertical. The length of that arm, about 30°, will stay the same, but the whole sky will rotate around that east point on the horizon. The whole sky turns counter-clockwise as if pinned at the point with altitude 0° and azimuth 90°. Keep going... If you continue travelling south, rotating for a full 90°, that arm with Arcturus at the end will now be resting horizontally. You're at 49° S, and Arcturus is resting right on the horizon about 30° to the north of due east. See how that works? You haven't changed the time. These are simultaneous views. The implication then is that the time of prime vertical passage and the time of rising (at the true horizon, no refraction) are the same for two points separated in latitude by 90°. It's a "fun fact". Tabulated sunrise and sunset times are really not good enough to use this trick except as a rough estimation (as outlined in my earlier post), but it's still fun.

    Frank Reed
    PS: There's a quick way to change latitude by whole degrees in Stellarium. Call up the location window. Move that window over to the side so it's not blocking your view. Now click on the degrees of latitude field. Press the down arrow key on your keyboard. The latitude will fall by one degree, and the sky will instantly update to match. Press and hold to cycle quickly and smoothly through a change of 90° in latitude.

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