graidan - 10 months ago 53

Javascript Question

As part of a bigger project, I'm developing a JavaScript library that will help me generate ephemerides for pretty much whatever you want. The current piece I'm working on is coordinate conversion. As a tester project, I'm generating a list of degrees ON the ecliptic (that is, longitude 34, latitude 0) and their corresponding horizontal azimuths for a given time and place. The problem I'm getting is that in the middle, between ecliptic degree 155 and 334, all the azimuth jump into the 20000 range. What's going wrong?

The HTML script:

`function main() {`

var i, e, h, op;

var olon = -121.46888;

var olat = 38.55556;

var jd = julian(1971,3,21,6,38,0);

var ecl = getNuta(jd).ecl;

op = "Equinoctal Az by Ecl Degree<br><br>"

for (i = 0; i < 360; i++) {

e = eclEqu(i, 0, ecl);

h = equHor(e.ra, e.dec, olat, lst(jd, olon));

op += "Ecl: " + i + " = Hor: " + h.az.toFixed(3) + "<br>"

}

document.write( op);

}

And the conversion functions:

`function eclEqu(lambda, beta, ecl) {`

// Converts ecliptic to equatorial

// -> 3x float in degrees

// <- {ra:, dec:} 2x float in hours, degrees

var l, b, e, a, d, up, dn;

l = lambda * co.dtr;

b = beta * co.dtr;

e = ecl * co.dtr;

up = Math.sin(l) * Math.cos(e) - Math.tan(b) * Math.sin(e);

dn = Math.cos(l);

a = Math.atan2(up,dn);

d = Math.asin(Math.sin(b) * Math.cos(e)

+ Math.cos(b) * Math.sin(e) * Math.sin(l));

return {ra: a * co.rth, dec: d * co.rtd};

}

function equHor(alpha, delta, phi, lst) {

// Converts equatorial to horizontal

// -> 4x float, in hrs, deg, deg (obs. lat), and hrs

// <- {az:, alt:} 2x float in deg

var d, p, ha, A, h, up, dn;

d = delta * co.dtr;

p = phi * co.dtr;

ha = (lst - alpha) * co.htr;

up = Math.sin(ha);

dn = Math.cos(ha) * Math.sin(p) - Math.tan(d) * Math.cos(p);

A = norm(Math.atan2(up, dn), 360);

h = Math.asin(Math.sin(p) * Math.sin(d)

+ Math.cos(p) * Math.cos(d) * Math.cos(ha));

return {az: A * co.rtd, alt: h * co.rtd};

}

`norm`

`norm(n, x) = n - ( x * int(n/x))`

I applied

`norm`

`Ecl: 153 = Hor: 2.537`

Ecl: 154 = Hor: 0.557

Ecl: 155 = Hor: 105.103

Ecl: 156 = Hor: 103.214

and

`Ecl: 333 = Hor: 289.018`

Ecl: 334 = Hor: 287.038

Ecl: 335 = Hor: 178.622

Ecl: 336 = Hor: 176.734

I'm also concerned - it seems there should still be an even, smooth distribution (even if it's not linear) around the horizon, but it doesn't seem to be the case.

Answer

I think the problem is the statement

```
A = norm(Math.atan2(up, dn), 360);
```

in the function `equHor()`

. The result of `atan2`

is an angle in radians. You wouldn't probably normalize it with 360.