It probably has to do with changing positions of the sun or planets during the year. Does it have any graduation, for example is it marked off with dates or seasons? And how big is it? If it stretches from the tropic of Cancer to the tropic of Capricorn (23 degrees north to 23 degrees south) it's representing the changing position of the sun at noon during the year, and should be marked June/July at the top and December/January at the bottom. indicating that the sun is directly above the tropic of Cancer in late June, northern mid-summer.