fix julian date calculation

lib/julian.js

@@ -1,12 +1,12 @@
 "use strict";
-const J1970 = -10957.5;
+const J2000 = Date.parse("2000-01-01T11:58:55.816Z");
 
 function to(ms) {
-  return J1970 + ms / 86400000;
+  return (ms - J2000) / 86400000;
 }
 
 function from(t) {
-  return (t - J1970) * 86400000;
+  return t * 86400000 + J2000;
 }
 
 exports.to = to;

lib/lunar.js

@@ -60,7 +60,7 @@ function _distance(t) {
 function _right_ascension_direct(lat, lon, obl) {
   return Math.atan2(
     Math.sin(lon) * Math.cos(obl) - Math.tan(lat) * Math.sin(obl),
-    Math.cos(lon)
+    Math.cos(lon),
   );
 }
 
@@ -71,7 +71,7 @@ function _right_ascension(t) {
 function _declination_direct(lat, lon, obl) {
   return Math.asin(
     Math.sin(lat) * Math.cos(obl) +
-    Math.cos(lat) * Math.sin(obl) * Math.sin(lon)
+    Math.cos(lat) * Math.sin(obl) * Math.sin(lon),
   );
 }
 
@@ -92,7 +92,7 @@ function _transit_direct(t, ha) {
 function _hour_angle(t, lon) {
   return _hour_angle_direct(
     sidereal.local(t, lon),
-    _right_ascension(t)
+    _right_ascension(t),
   );
 }
 
@@ -139,7 +139,7 @@ function _elevation(t, lat, lon) {
 
   return Math.asin(
     Math.sin(lat) * Math.sin(decl) +
-    Math.cos(lat) * Math.cos(decl) * Math.cos(ha)
+    Math.cos(lat) * Math.cos(decl) * Math.cos(ha),
   );
 }
 

lib/solar.js

@@ -106,7 +106,7 @@ function _altitude(sin_h0, rise, t, lat, lon) {
   const cos_dec = Math.sqrt(1 - sin_dec * sin_dec);
 
   return _transit(t, lon) + rise * Math.acos(
-    (sin_h0 - Math.sin(lat) * sin_dec) / (Math.cos(lat) * cos_dec)
+    (sin_h0 - Math.sin(lat) * sin_dec) / (Math.cos(lat) * cos_dec),
   );
 }
 
@@ -116,7 +116,7 @@ function _altitude(sin_h0, rise, t, lat, lon) {
 function _azimuth(sin_lat, cos_lat, tan_dec, sin_hour_angle, cos_hour_angle) {
   return Math.PI + Math.atan2(
     sin_hour_angle,
-    cos_hour_angle * sin_lat - tan_dec * cos_lat
+    cos_hour_angle * sin_lat - tan_dec * cos_lat,
   );
 }
 
@@ -187,7 +187,7 @@ class Location {
     cos_hour_angle,
     sin_elevation,
     sin_azimuth,
-    cos_azimuth
+    cos_azimuth,
   ) {
     this.sin_hour_angle = sin_hour_angle;
     this.cos_hour_angle = cos_hour_angle;
@@ -247,7 +247,7 @@ class Time {
       cos_hour_angle,
       _sin_elevation(sin_lat, cos_lat, sin_dec, cos_dec, cos_hour_angle),
       cos_hour_angle * sin_lat - tan_dec * cos_lat,
-      sin_hour_angle
+      sin_hour_angle,
     );
   }
 }
@@ -265,7 +265,7 @@ function position(ms) {
     l,
     e,
     sin_l * sin_e, // sin declination
-    Math.atan2(sin_l * cos_e, cos_l) // right ascension
+    Math.atan2(sin_l * cos_e, cos_l), // right ascension
   );
 }
 

test/lunar.js

@@ -89,7 +89,7 @@ describe("lunar", () => {
     }
     else {
       expect(
-        calculated_value
+        calculated_value,
       ).to.be.closeTo(result, 10 * 60 * 1000); // Allow error of 10min
     }
   }

test/solar.js

@@ -23,27 +23,27 @@ describe("solar", () => {
 
   it("should return the nearest civil dawn to a given time", () => {
     expect(solar.dawn(ms, lat, lon)).
-      to.be.closeTo(Date.parse("2006-03-20T05:45-0700"), 120000);
+      to.be.closeTo(Date.parse("2006-03-20T05:45-0700"), 60000);
   });
 
   it("should return the nearest sunrise to a given time", () => {
     expect(solar.rise(ms, lat, lon)).
-      to.be.closeTo(Date.parse("2006-03-20T06:10-0700"), 120000);
+      to.be.closeTo(Date.parse("2006-03-20T06:10-0700"), 60000);
   });
 
   it("should return the nearest solar transit to a given time", () => {
     expect(solar.transit(ms, lat, lon)).
-      to.be.closeTo(Date.parse("2006-03-20T12:14-0700"), 120000);
+      to.be.closeTo(Date.parse("2006-03-20T12:14-0700"), 60000);
   });
 
   it("should return the nearest sunset to a given time", () => {
     expect(solar.set(ms, lat, lon)).
-      to.be.closeTo(Date.parse("2006-03-20T18:18-0700"), 120000);
+      to.be.closeTo(Date.parse("2006-03-20T18:18-0700"), 60000);
   });
 
   it("should return the nearest civil dusk to a given time", () => {
     expect(solar.dusk(ms, lat, lon)).
-      to.be.closeTo(Date.parse("2006-03-20T18:43-0700"), 120000);
+      to.be.closeTo(Date.parse("2006-03-20T18:43-0700"), 60000);
   });
 
   it("should look up the position of the sun for an observer", () => {
@@ -55,7 +55,7 @@ describe("solar", () => {
     // (NOTE: I cheated a little here! Syene is now Aswan, and Aswan is no
     // longer on the Tropic of Cancer (since the tropics migrate about 15 m/yr).
     // So I took Aswan's longitude but used the modern latitude.)
-    const pos = solar.position(Date.parse("2019-06-21T11:50:07+0200"));
+    const pos = solar.position(Date.parse("2019-06-21T11:49:03+0200"));
 
     const syene = pos.observer(23.43679, 32.899722);
     expect(90 - syene.elevation * 180 / Math.PI).to.be.closeTo(0, 0.01);