There are at least 3 corrections that are needed to convert between watch time and sundial time: equation of time, longitude correction, and daylight saving time (if applicable). Latitude also plays a part in positioning the sundial.
From what I understand, it looks like the 1793 Old Farmer’s Almanac edition used the longitude for the central meridian of the Eastern time zone, 75 degrees west. However, the 2017 Almanac (and our current editions) calculate based on Boston, which is east of the central meridian, at about 71 degrees 3 minutes west. If I’ve done the calculations correctly, that is a +15.8 minute adjustment (since Boston is east of the central meridian, add the minutes). If this is right, then the August data published in the 2017 Almanac is correct, once the correction for longitude is added to the equation of time.
August 1, 2017:
-6 minutes (equation of time, sun slow) + 15.8 minutes (longitude correction) = 9.8 minutes (or roughly 9, as in the Almanac)
August 31, 2017:
0 minutes (equation of time) + 15.8 minutes (longitude correction) = 15.8 minutes (or roughly 16, as in the Almanac)
There are at least 3 corrections that are needed to convert between watch time and sundial time: equation of time, longitude correction, and daylight saving time (if applicable). Latitude also plays a part in positioning the sundial.
From what I understand, it looks like the 1793 Old Farmer’s Almanac edition used the longitude for the central meridian of the Eastern time zone, 75 degrees west. However, the 2017 Almanac (and our current editions) calculate based on Boston, which is east of the central meridian, at about 71 degrees 3 minutes west. If I’ve done the calculations correctly, that is a +15.8 minute adjustment (since Boston is east of the central meridian, add the minutes). If this is right, then the August data published in the 2017 Almanac is correct, once the correction for longitude is added to the equation of time.
August 1, 2017:
-6 minutes (equation of time, sun slow) + 15.8 minutes (longitude correction) = 9.8 minutes (or roughly 9, as in the Almanac)
August 31, 2017:
0 minutes (equation of time) + 15.8 minutes (longitude correction) = 15.8 minutes (or roughly 16, as in the Almanac)
Hope this helps!