Modulus in Pascal - c++

Question

I am trying to translate some Pascal code into C++ code. I am stuck trying to figure out how to translate this portion.
Function ThetaG_JD(jd : double) : double;
var
UT,TU,GMST : double;
begin
**UT := Frac(jd + 0.5);**
jd := jd - UT;
TU := (jd - 2451545.0)/36525;
GMST := 24110.54841 + TU * (8640184.812866 + TU * (0.093104 - TU * 6.2E-6));
**GMST := Modulus(GMST + 86400.0*1.00273790934*UT,86400.0);**
ThetaG_JD := twopi * GMST/86400.0;
end; {Function ThetaG_JD}
I am particularly having trouble with the two lines I made bold. How can I translate this to c++? Thank you so much.

Answer 1

In C++ the equivalent functions would be:
fmod to get a floating point modulus
modf to break a floating point item into its fraction and integral parts (equivalent of Frac).

Answer 2

If you want to calculate Julian Day, Greenwich Mean Sidereal Time and Local Mean Sidereal Time, maybe the below can help you - written in PowerShell:
<#
.Synopsis
Astronomy calculations
.Description
Some helper functions to calculate:
- Julian Day,
- Greenwich Mean Sidereal time,
- Local Mean Sidereal Time.
#>
cls
# https://en.wikipedia.org/wiki/Julian_day#Julian_day_number_calculation
function Get-JulianDay
{
param ( [System.DateTime]$dt )
$year = $dt.Year
$month = $dt.Month
$day = $dt.Day
$hour = $dt.Hour
$minute = $dt.Minute
$second = $dt.Second
$a = [System.Math]::Floor((14 - $month) / 12)
$y = $year + 4800 - $a
$m = $month + 12 * $a - 3
$JDN = $day + [System.Math]::Floor((153 * $m + 2) / 5) + 365 * $y + [System.Math]::Floor($y / 4) - [System.Math]::Floor($y / 100) + [System.Math]::Floor($y / 400) - 32045
$JD = $JDN + ($hour - 12) / 24 + $minute / 1440 + $second / 86400
return ($JD)
}
# https://en.wikipedia.org/wiki/Sidereal_time#Definition
# http://aa.usno.navy.mil/faq/docs/GAST.php
function Get-GMST
{
param ( [double]$JD )
$D = $JD - 2451545.0
$GMST = 18.697374558 + 24.06570982441908 * $D
return ($GMST % 24)
}
function Get-LMST
{
param ( [double]$gmst, [double]$longitude )
return ( $gmst + $longitude / 15.0 )
}
# Test above functions
$current = (Get-Date).ToUniversalTime()
$jd = Get-JulianDay -dt $current
$gmst = Get-GMST -JD $jd
$longitude = 17.668487800
$lmst = Get-LMST -gmst $gmst -longitude $longitude
$lst = [timespan]::FromHours($lmst).ToString()
Write-Host "Local mean sidereal time: $lst"