How can i calculate the percent of full moon on a given date? - date

Question

I need to find the percentage of a full moon on a given date, but i can not figure out how to calculate this. My attempt to do this is wrong, because the percent of to day is about 96.9%. Can anyone see what i'm doing wrong in my delphi-code.
procedure TfrmLag.btnCalcClick(Sender: TObject);
var
whatDate : TDateTime; // Now;
lunarDays : Double; // 29.53058770576
lunarSecs : LongInt; // lunarDays * (24*60*60);
new2000 : TDateTime; // 6.1 2000 18:14
totalSecs : LongInt; // whatDate - new2000
currSecs : LongInt; // totalSecs MOD lunarSecs
currFrac : Double; // currSecs / lunarSecs
currDays : LongInt; // currFrac * lunarDays
perOfFull : Double;
begin
whatDate := Now;
lunarDays := 29.53058770576;
lunarSecs := Round(lunarDays * (24*60*60));
new2000 := EncodeDateTime(2000,1,6,18,14,00,000);
totalSecs := SecondsBetween(whatDate, new2000);
currSecs := totalSecs MOD lunarSecs;
currFrac := currSecs / lunarSecs;
currDays := Round(currFrac*lunarDays);
perOfFull := (100*currFrac);
lb.Items.Add('Date : '+FormatDateTime('dd.mm.yyyy hh:mm:ss',whatDate));
lb.Items.Add('Lunar days : '+IntToStr(lunarSecs));
lb.Items.Add('First full 2000 : '+FormatDateTime('dd.mm.yyyy hh:mm:ss',new2000));
lb.Items.Add('Total seconds : '+IntToStr(totalSecs));
lb.Items.Add('Current seconds : '+IntToStr(currSecs));
lb.Items.Add('Current fraction : '+FloatToStr(currFrac));
lb.Items.Add('Current days : '+IntToStr(currDays));
lb.items.Add('Percent of full : '+FloatToStr(perOfFull));
end;

Answer 1

I suppose new2000 is the first new moon in the year 2000? If so then this code should calculate correctly.
If new2000 is the full moon, you only have to remove the -1 in the cos() function.
uses
DateUtils;
procedure Calculate();
const
MoonPeriod = 29.53058770576;
var
KnownNewMoon: TDateTime;
NowUTC: TDateTime;
DaysSinceLastNewMoon, NumberOfNewMoons, MoonPart: Extended;
begin
KnownNewMoon := EncodeDateTime(2000,1,6,18,14,00,000);
NowUTC := TTimeZone.Local.ToUniversalTime(Now);
//How many moon periods (new moon -> full moon -> new moon) have passed
//since that known new moon date?
NumberOfNewMoons := (NowUTC - KnownNewMoon)/MoonPeriod;
DaysSinceLastNewMoon := Frac(NumberOfNewMoons)*MoonPeriod;
//The "moon part" is a sine/cosine function that starts at new moon with -0,
//reaches 1 at full moon and goes back to 0 at the next new moon.
//Starting at cos(-Pi) gives a -1 as "new moon value". Add 1 to set this to 0.
//Full moon is cos(0) gives 1. With the 1 added before, we have to divide by 2.
MoonPart := (cos((NumberOfNewMoons*2 - 1) * Pi) + 1)/2;
lb.items.Add('Number/amount of new moons: '+ FormatFloat('0.000000', NumberOfNewMoons));
lb.items.Add('Current moon part/position: '+ FormatFloat('0.000000', MoonPart));
lb.items.Add('Days since last new moon: '+ FormatFloat('0.000000', DaysSinceLastNewMoon));
end;
This should give you the visible moon part in MoonPart and the days (incl. fraction) since the last new moon.