Determining the Date For a Phase of Earth's Moon - Printable Version +- HP Forums (https://www.hpmuseum.org/forum) +-- Forum: HP Software Libraries (/forum-10.html) +--- Forum: HP Prime Software Library (/forum-15.html) +--- Thread: Determining the Date For a Phase of Earth's Moon (/thread-12488.html) Determining the Date For a Phase of Earth's Moon - Eddie W. Shore - 02-23-2019 04:47 PM Introduction The program MOONDATE determines when a desired phase of Earth's moon given month and year. The four phases of the moon available are: New Moon (0.00) 1st Quarter (0.25) Full Moon (0.50) 3rd Quarter (0.75) The following equations are used: Let y = year, m = month, t = type (see the above) y' = y + (m -1)/12 k = integer( (y - 2000) * 12.3685) + t/4 t = k/1236.85 J = 2451550.09765 + 29.530588853 * k + (1.337 * 10^-4) * t^2 - (1.5 * 10^-7) *t^3 + (7.3 * 10^-10) * t^4 J is the Julian Date and will need to be converted to the Gregorian Date. With the DATE+ function, the HP Prime makes this easy. Look to Dieter's post on this thread for details, link: http://www.hpmuseum.org/forum/thread-12184.html?highlight=julian+date Caution: This program is designed to work with all dates after January 1, 2000. If you choose any dates before, you may have to adjust the month Code: ```HP Prime Program MOONDATE EXPORT MOONDATE() BEGIN // EWS 2019-02-19 LOCAL P,Y,K,M,T,l,c,s; LOCAL J,D,N,R; l:={"New","First Qtr","Full", "Third Qtr"}; INPUT( {Y, {M,{1,2,3,4,5,6,7,8,9,10,11,12}} ,{c,l}}, "Moon Phase Date", {"Year: ","Month:","Stage:"}); s:=(c-1)/4; N:=M; REPEAT  R:=Y+(N-1)/12; K:=IP((R-2000)*12.3685)+s; T:=K/1236.85; J:=2451550.09765+29.530588853*K +1.337ᴇ−4*T^2-1.5ᴇ−7*T^3 +7.3ᴇ−10*T^4; J:=IP(J)-2451545; D:=DATEADD(2000.0101,J); N:=N+1; UNTIL D≥(Y+M/100); RETURN D; END;``` Note: You can use an alternate of 1/1/2000 (date code 2000.0101) with corresponding Julian Date 2451545. Example: January 2019: New: 2019.0105 (1/5/2019) 1st Qtr: 2019.0113 (1/13/2019) Full: 2019.0120 (1/20/2019) 3rd Qtr: 2019.0127 (1/27/2019) Source: Meenus, Jean. "Astronomical Algorithms" Willmann-Bell, Inc.: Richmond, VA 1991 ISBN 0-943396-35-2 Blog post: https://edspi31415.blogspot.com/2019/02/hp-prime-determining-date-for-phase-of.html