A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Sean C
Date: 2018 Jul 19, 22:54 -0700
It probably is very difficult to directly calculate the JD from a calendar date on paper. That's why I calculate it in advance for the first day of each year* and then simply add the number of days since then. A table which lasts a decade can fit in your shirt pocket without even folding it.
The formula on the USNO website isn't very easy to commit to memory, either. But it is very easy to enter into a calculator or include in a spreadsheet or program. If you'd rather not use electronics altogether - might I suggest HO 249 vol. 1? On page 340 of the current edition, there is a handy table for finding the GHA of Aries for any time between 2011 and 2019 (inclusive).
*Actually, I calculate it for the "zeroeth" of each year - or December 31st of the previous year. That way, I can add the days starting with January 1st as "one".