Re: Equatorial monument in Brazil
That's a horse of a different color. Then you have a geometrical image of the hole rather than a pinhole image of the sun. The lack of "perfection" remains, but is no longer so apparent. Now you might want to ask for how many days around the equinox the image stays "pretty much" on the line. The deviation will be largest near dawn and dusk. Let's use 7 AM and 5 PM as reference times and 65' as the height, so the horizontal projection distance is 65'/tan(15 deg) = 240'. An offset of a quarter diameter -- 2' -- should be quite noticeable. That corresponds to an angle of 2/240 = 8.3 mrad = 29 arc-min, nearly equal to the change of declination in one day. It is also nearly equal to the angular diameter of the sun, so the image will be rather washed out at that distance. All in all, you should be able to determine the exact day of the equinox. That's close enough to perfection to leave me impressed. I'd love to see it in action. --Art
Re: Equatorial monument in Brazil
Josef, Art, and All, Yes, this was the monument I saw on the TV show. Thank you Josef for tracking down the link! It was very interesting to be able to see it from the air, to really see its scale and consider its functionas opposed to seeing it from plaza level forjust aminute or soduring a show about an unrelated subject. I am always excited when interesting solar devices pop up unexpectedlyon TV! As you view the photo on the website, the football stadium I mentioned is behind the cameraand to the right along the meridian, and the long street (it looks like it might actually be a very straight dirt road)extends to the left for quite some distance. I have no idea which is East or West...which way would be best when laying out a monument like thisdo you suppose? Would it be better to have the projected disc hit the visible lineon the plaza in the morning or in the afternoon, or does it matter? Art, I would love to see it in action as well...it would be interesting to see just how welltheprojected disc actuallyshows up, andhow dramatic the overall effect is. It would certainly be a special place to spend an equinox! Jim Tallman Sr. Designer FX Studios 513.829.1888
Re: Equatorial monument in Brazil
Hold still, I think I see a nit! The rate of change of the declination at the equinoxes is 24 arc-minutes per day, approximately equal to the size of the image of the solar disk, so if the spot is perfectly centered on the line at dawn, by dusk it should be nearly off one side. Actually, this is cool, because it means with appropriate and painstaking observations you can determine the moment of the equinox to within a few hours. Art Carlson
Re: Equatorial monument in Brazil
Hello Art, "The rate of change of the declination at the equinoxes is 24 arc-minutes per day, approximately equal to the size of the image of the solar disk, so if the spot is perfectly centered on the line at dawn, by dusk it should be nearly off one side. Actually, this is cool, because it means with appropriate and painstaking observations you can determine the moment of the equinox to within a few hours." Art Carlson This is a very good point. Unfortunately they were on site in May, I believe, and so there was no way to see the thing in action. It was hard to estimate the size of the hole in the top of the stone towerbecause it was very high up and probably shot with a wide angleTV camera lense, but I'll bet the aperture was at least 8' in diameter. The monumentwas interesting in its context as well. On the one side (along the East/West axis) about 250-300 yards away was a football stadium with theEquator runningexactly through midfield. There were no bleachers on the monument side of the field. On the opposite side of the towerwas a very long street or walkway also following the equator. The pedestrian plaza around the monument included ablack stone strip inlayed into the pavement to indicate the exact position of the Equator. I am sure the effect on the specialdays would be quitestunning, as long as the sun was shining in the rainforest! I am surprised that I was able to find so little about it on the web! Jim Tallman Sr. Designer FX Studios 513.829.1888
