A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2019 Mar 13, 16:56 -0400
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.
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.