# eqtime: Equation of time In insol: Solar Radiation

## Description

Computes the equation of time for a given Julian Day.

## Usage

 `1` ```eqtime(jd) ```

## Arguments

 `jd` Julian Day.

## Value

Equation of time in minutes.

## Author(s)

Javier G. Corripio

## References

Meeus, J. 1999. Astronomical Algorithms. Willmann-Bell, Richmond, Virginia, USA.

Reda, I. and Andreas, A. 2003. Solar Position Algorithm for Solar Radiation Applications. 55 pp.; NREL Report No. TP-560-34302, Revised January 2008. https://www.nrel.gov/docs/fy08osti/34302.pdf

## Examples

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28``` ```# plot the equation of time for 2013 at daily intervals jdays = seq(ISOdate(2013,1,1),ISOdate(2013,12,31),by='day') jd = JD(jdays) plot(eqtime(jd)) abline(h=0,col=8) # Analema plot(eqtime(jd),declination(jd)) # Analema from Greenwich Observatory latGwch = 51.4791 x = 180+eqtime(jd)*15/60 y = 90-latGwch+declination(jd) plot(x,y,ylim=c(0,90),xlab=expression(paste('Azimuth (',degree,')')), ylab=expression(paste('Elevation (',degree,')'))) ## Add the solstices and equinoxes (nearest day, see Meeus ch. 26 for more precision) decl = declination(jd) wintersolstice = which(decl==min(decl)) summersolstice = which(decl==max(decl)) ## spring equinox: when declination becomes zero in the first part of the year springeqx = uniroot(declination,jd[c(1,180)])\$root springeqx = daydoy(JD(springeqx,inv=TRUE)) autumeqx = uniroot(declination,jd[c(180,360)])\$root autumeqx = daydoy(JD(autumeqx,inv=TRUE)) nodeseqx = c(springeqx,summersolstice,autumeqx,wintersolstice) points(x[nodeseqx],y[nodeseqx],pch=19,col=3) abline(h=c(90-latGwch,90-latGwch+max(decl),90-latGwch+min(decl)),col=8) ```

insol documentation built on Jan. 16, 2019, 5:04 p.m.