astro/eclipse/observation_helpers.go

package eclipse

import (
	"time"

	"b612.me/astro/basic"
	"b612.me/astro/tools"
)

func moonSunLoDiff(date time.Time) float64 {
	jde := basic.TD2UT(basic.Date2JDE(date.UTC()), true)
	sunLo := basic.HSunApparentLo(jde)
	moonLo := basic.HMoonApparentLo(jde)
	return tools.Limit360(moonLo - sunLo)
}

func solarAltitude(date time.Time, lon, lat float64) float64 {
	jde := basic.Date2JDE(date)
	_, loc := date.Zone()
	return basic.SunHeight(jde, lon, lat, float64(loc)/3600.0)
}

func solarCulminationTime(date time.Time, lon float64) time.Time {
	jde := basic.Date2JDE(date.Add(time.Duration(-1*date.Hour())*time.Hour)) + 0.5
	_, loc := date.Zone()
	timezone := float64(loc) / 3600.0
	calcJde := basic.CulminationTime(jde, lon, timezone) - timezone/24.0
	return basic.JDE2DateByZone(calcJde, date.Location(), false)
}

func lunarAzimuth(date time.Time, lon, lat float64) float64 {
	jde := basic.Date2JDE(date)
	_, loc := date.Zone()
	return basic.HMoonAzimuth(jde, lon, lat, float64(loc)/3600.0)
}

func lunarAltitude(date time.Time, lon, lat float64) float64 {
	jde := basic.Date2JDE(date)
	_, loc := date.Zone()
	return basic.HMoonHeight(jde, lon, lat, float64(loc)/3600.0)
}

func lunarCulminationTime(date time.Time, lon, lat float64) time.Time {
	if date.Hour() > 12 {
		date = date.Add(-12 * time.Hour)
	}
	jde := basic.Date2JDE(date)
	_, loc := date.Zone()
	culmination := basic.JDE2DateByZone(basic.MoonCulminationTime(jde, lon, lat, float64(loc)/3600.0), date.Location(), true)
	// MoonCulminationTime returns the nearest root to its internal midnight
	// seed, which can land one civil day later when the input is local noon.
	// Visibility scans need the root belonging to the supplied local day.
	if culmination.Year() != date.Year() || culmination.YearDay() != date.YearDay() {
		// Preserve the solved local clock time while moving it onto the
		// requested civil date. This also avoids assuming every DST day is
		// exactly 24 elapsed hours.
		culmination = time.Date(
			date.Year(), date.Month(), date.Day(),
			culmination.Hour(), culmination.Minute(), culmination.Second(), culmination.Nanosecond(),
			date.Location(),
		)
	}
	return culmination
}