The Sun's year is a pure function too, checked against the almanac

sun.js gains analemma(date, days): where the Sun is overhead at the same UTC time on each coming day, the figure-8 the
globe will draw. tools/globe/check.mjs checks the pure functions against almanac values with Node's own test runner
(dev only, nothing installed): the 2026 June solstice's declination, the March equinox, perihelion's distance, Turin's
longest day, the season in October, and the analemma spanning the tropics and the equation of time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

tools/globe/check.mjs

@@ -0,0 +1,45 @@
+// Checks of the globe's pure functions against almanac values, dev only (Node's own test runner, nothing installed):
+//   node --test tools/globe/check.mjs
+// sun.js is the Astronomical Almanac's low-precision formulas, good to about 0.01° between 1950 and 2050.
+import { test } from "node:test"
+import assert from "node:assert/strict"
+import { analemma, seasonsOf, sunAt, sunAtPlace } from "../../wwwroot/js/sun.js"
+
+const near = (actual, expected, tolerance, what) =>
+	assert.ok(Math.abs(actual - expected) <= tolerance, `${what}: ${actual} is not within ${tolerance} of ${expected}`)
+
+test("the June solstice of 2026 puts the Sun over the Tropic of Cancer", () => {
+	near(sunAt(new Date("2026-06-21T08:24:00Z")).lat, 23.435, 0.01, "declination")
+})
+
+test("the March equinox of 2026 puts the Sun over the equator", () => {
+	near(sunAt(new Date("2026-03-20T14:46:00Z")).lat, 0, 0.02, "declination")
+})
+
+test("perihelion on 2026-01-03 is the year's nearest Sun", () => {
+	near(sunAt(new Date("2026-01-03T17:15:00Z")).distance, 0.98329, 0.0002, "distance in AU")
+})
+
+test("Turin's longest day is about 15 h 38 min", () => {
+	near(sunAtPlace(sunAt(new Date("2026-06-21T12:00:00Z")), 45.07, 7.69).dayLength, 15.63, 0.05, "day length in hours")
+})
+
+test("early October is autumn in the north, and the December solstice comes next", () => {
+	const seasons = seasonsOf(sunAt(new Date("2026-10-06T12:00:00Z")))
+	assert.equal(seasons.north, "autumn")
+	assert.equal(seasons.south, "spring")
+	assert.equal(seasons.nextEvent, "december-solstice")
+})
+
+test("the analemma spans the tropics and the equation of time", () => {
+	const start = new Date("2026-10-06T12:00:00Z")
+	const points = analemma(start)
+	assert.equal(points.length, 366)
+	assert.equal(points[0].lat, sunAt(start).lat)
+	near(Math.max(...points.map(p => p.lat)), 23.44, 0.05, "northernmost")
+	near(Math.min(...points.map(p => p.lat)), -23.44, 0.05, "southernmost")
+	// at 12:00 UTC the Sun is overhead near the prime meridian, early or late by the equation of time (about ±16 min, ±4°)
+	const longitudes = points.map(p => p.lng)
+	near(Math.max(...longitudes), 3.7, 0.4, "easternmost")
+	near(Math.min(...longitudes), -4.1, 0.4, "westernmost")
+})

wwwroot/js/sun.js

@@ -65,6 +65,15 @@ export function seasonsOf(sun) {
 	}
 }
 
+// Where the Sun is overhead at this same UTC time on each of the coming days (the first is the date itself): traced on
+// the globe, the analemma, its figure-8 the year's declination and the equation of time.
+export function analemma(date, days = 366) {
+	const points = []
+	for (let day = 0; day < days; day++)
+		points.push(sunAt(new Date(date.getTime() + day * 86400000)))
+	return points
+}
+
 // The Sun's direction in the frame of three.js's SphereGeometry, the frame the globe's shader draws in: texture u is
 // 0 at longitude −180°, and a point is (−cos 2πu · cos lat, sin lat, sin 2πu · cos lat).
 export function directionOf(sun) {