I’ve been playing with navigational star charts, photo maps, star databases, processing for about a year now. With lots of help, I’ve learned how to project an image onto a sphere/globe and plot points onto the image. My coding style is very sloppy and needs improvement, but I have managed to learn how to manipulate the placement of points(stars) some.
Here’s what I found odd. I went to https://aa.usno.navy.mil/data/docs/Nav_Star_Chart.php to get a star chart that I could project onto a sphere. I used paint to crop the image and eliminate extraneous stuff, leaving just the essential chart to be projected. This seemed to work, cropped pretty accurately. I did have to use paint to reverse the image so that when inside the sphere, letters and words were not backwards. I plotted a few stars and some circles centered on these stars and noticed something. The location of everything was off by 180 degrees, but the words weren’t printed backwards.
I went here https://staratlas.com/ to see how the stars for my location and time are oriented. Yup, my cropped USNO chart is off by 180 degrees. I wrote a little if then else and manipulated my circles and star points, adding 180 degrees and my stars landed on the chart’s positions for those stars.
My sketch rotates according to sidereal time for my location. If I plot my circles and stars on a NASA image, it agrees with the staratlas.com.
Does anybody have an explanation?
Here are two screenshots illustrating.