83 lines
4.0 KiB
JavaScript
83 lines
4.0 KiB
JavaScript
 
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// Where the Sun is, for the globe's day and night (wwwroot/js/globe.js): the Astronomical Almanac's low-precision solar
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// coordinates, good to about 0.01° between 1950 and 2050. Everything here is a pure function of a date.
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const radians = Math.PI / 180
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const J2000 = Date.UTC(2000, 0, 1, 12)//2000-01-01 12:00 UTC
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// The subsolar point (where the Sun is overhead), the Earth–Sun distance and the angles that place the Sun in the year.
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export function sunAt(date) {
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const days = (date.getTime() - J2000) / 86400000
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const meanLongitude = 280.460 + 0.9856474 * days
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const meanAnomaly = (357.528 + 0.9856003 * days) * radians
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const eclipticLongitude = degrees(meanLongitude + 1.915 * Math.sin(meanAnomaly) + 0.020 * Math.sin(2 * meanAnomaly))
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const obliquity = 23.439 - 0.0000004 * days
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const declination = Math.asin(Math.sin(obliquity * radians) * Math.sin(eclipticLongitude * radians)) / radians
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const rightAscension = Math.atan2(Math.cos(obliquity * radians) * Math.sin(eclipticLongitude * radians), Math.cos(eclipticLongitude * radians)) / radians
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const siderealTime = 280.46061837 + 360.98564736629 * days
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const distance = 1.00014 - 0.01671 * Math.cos(meanAnomaly) - 0.00014 * Math.cos(2 * meanAnomaly)
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return {
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date,
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lat: declination,
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lng: longitude(rightAscension - siderealTime),
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distance,
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irradiance: 1 / (distance * distance),
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eclipticLongitude,
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obliquity
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}
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}
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// What the Sun does at a place: its elevation, the phase of the day, the local solar time, the day's length, the season
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// in that hemisphere and the next equinox or solstice.
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export function sunAtPlace(sun, lat, lng) {
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const hourAngle = longitude(lng - sun.lng)
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const elevation = Math.asin(
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Math.sin(lat * radians) * Math.sin(sun.lat * radians) + Math.cos(lat * radians) * Math.cos(sun.lat * radians) * Math.cos(hourAngle * radians)) / radians
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const solarHours = (((12 + hourAngle / 15) % 24) + 24) % 24
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// sunrise and sunset as almanacs give them: the Sun's upper edge on the horizon, refraction included (−0.833°)
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const sunset = (Math.sin(-0.833 * radians) - Math.sin(lat * radians) * Math.sin(sun.lat * radians)) /
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(Math.cos(lat * radians) * Math.cos(sun.lat * radians))
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const dayLength = sunset <= -1 ? 24 : sunset >= 1 ? 0 : 2 * Math.acos(sunset) / radians / 15
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const north = lat >= 0
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const seasons = seasonsOf(sun)
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return {
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elevation,
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phase: elevation > 6 ? "day" : elevation > 0 ? "golden" : elevation > -6 ? "civil" : elevation > -12 ? "nautical" : elevation > -18 ? "astronomical" : "night",
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morning: hourAngle < 0,
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solarTime: `${String(Math.floor(solarHours)).padStart(2, "0")}:${String(Math.floor(solarHours % 1 * 60)).padStart(2, "0")}`,
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dayLength,
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hemisphere: north ? "north" : "south",
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season: north ? seasons.north : seasons.south,
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nextEvent: seasons.nextEvent,
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nextEventDays: seasons.nextEventDays
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}
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}
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// The astronomical season in each hemisphere (from the equinoxes and solstices, by the Sun's ecliptic longitude) and the
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// next equinox or solstice, in days at the Sun's mean pace.
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export function seasonsOf(sun) {
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const quarter = Math.floor(sun.eclipticLongitude / 90) % 4
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const nextBoundary = (quarter + 1) * 90
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return {
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north: ["spring", "summer", "autumn", "winter"][quarter],
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south: ["autumn", "winter", "spring", "summer"][quarter],
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nextEvent: { 90: "june-solstice", 180: "september-equinox", 270: "december-solstice", 360: "march-equinox" }[nextBoundary],
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nextEventDays: Math.round((nextBoundary - sun.eclipticLongitude) / 0.9856)
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}
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}
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// The Sun's direction in the frame of three.js's SphereGeometry, the frame the globe's shader draws in: texture u is
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// 0 at longitude −180°, and a point is (−cos 2πu · cos lat, sin lat, sin 2πu · cos lat).
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export function directionOf(sun) {
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const u = (sun.lng + 180) / 360 * 2 * Math.PI
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const lat = sun.lat * radians
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return [-Math.cos(u) * Math.cos(lat), Math.sin(lat), Math.sin(u) * Math.cos(lat)]
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}
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function degrees(value) {
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return ((value % 360) + 360) % 360
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}
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function longitude(value) {
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const wrapped = degrees(value + 180) - 180
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return wrapped === -180 ? 180 : wrapped
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}
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