I notice that the date June 21st corresponds with the time VI hours. Now, on June 21st the sun's longitude, expressed as a time, is 6 hours. Therefore, the scales show the sun's longitude on any date, which could be used to convert solar time to sidereal time.
(BTW, when I first looked at it, I thought it gave July just 30 days, but
then realised the dates in August are 4, 9, 14 etc., not 5,10.. as I
thought.)
Chris Lusby Taylor
