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70 lines
4.0 KiB
Go
70 lines
4.0 KiB
Go
package basic
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import "math"
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// LunarEclipseShadowGeometry 是食甚时刻的地影几何。
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// 注意单位口径：Gamma 用地球赤道半径，而两个影半径用度——后者是 NASA 月食图上 P./U. Radius 的口径，
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// 换成地球赤道半径要乘以月球处的地球视差（弧度）。
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// LunarEclipseShadowGeometry is the terrestrial-shadow geometry at maximum eclipse. Gamma is in Earth
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// equatorial radii while the two shadow radii are in degrees, matching the P./U. Radius convention of
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// NASA lunar-eclipse charts; multiply a radius by the Earth's parallax at the Moon to get Earth radii.
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type LunarEclipseShadowGeometry struct {
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// Gamma 是月心到地影轴的最小距离，单位地球赤道半径。
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// Gamma is the least distance from the Moon's centre to the shadow axis, in Earth equatorial radii.
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Gamma float64
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// PenumbralRadiusDegrees 与 UmbralRadiusDegrees 是半影、本影在地影轴垂直面上的角半径，单位度。
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// PenumbralRadiusDegrees and UmbralRadiusDegrees are the penumbral and umbral angular radii on the
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// plane perpendicular to the shadow axis, in degrees.
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PenumbralRadiusDegrees float64
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UmbralRadiusDegrees float64
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// MoonDistanceEarthRadii 是食甚时的地心月距，单位地球赤道半径。
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// MoonDistanceEarthRadii is the geocentric lunar distance at greatest eclipse, in Earth equatorial radii.
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MoonDistanceEarthRadii float64
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// AxisDegrees 是食甚时月心到地影轴的角距，即 NASA 月食图上那列 Axis。
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// 它与 Gamma 是同一个量的两种刻度：Gamma 除以月球处的地球视差就是它。
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// AxisDegrees is the angular distance from the Moon's centre to the shadow axis at greatest eclipse,
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// the column NASA lunar-eclipse charts print as Axis. It is the same quantity as Gamma on a different
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// scale: divide Gamma by the Earth's parallax at the Moon.
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AxisDegrees float64
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}
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// LunarEclipseShadowGeometryAt 用 Danjon 影半径模型计算食甚时刻的地影几何。
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// LunarEclipseShadowGeometryAt computes the shadow geometry at maximum eclipse with the Danjon shadow model.
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func LunarEclipseShadowGeometryAt(maximumJDE float64) LunarEclipseShadowGeometry {
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return lunarEclipseShadowGeometryAt(maximumJDE, lunarEclipseShadowDanjon)
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}
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// LunarEclipseShadowGeometryChauvenetAt 用 Chauvenet 影半径模型计算食甚时刻的地影几何。
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// LunarEclipseShadowGeometryChauvenetAt computes the shadow geometry with the Chauvenet shadow model.
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func LunarEclipseShadowGeometryChauvenetAt(maximumJDE float64) LunarEclipseShadowGeometry {
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return lunarEclipseShadowGeometryAt(maximumJDE, lunarEclipseShadowChauvenet)
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}
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// LunarEclipseShadowGeometryAtModel 按结果里记录的影半径模型取地影几何。
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// LunarEclipseShadowGeometryAtModel picks the shadow geometry by the model recorded in a result.
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func LunarEclipseShadowGeometryAtModel(maximumJDE float64, model LunarEclipseShadowModel) LunarEclipseShadowGeometry {
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if model == LunarEclipseShadowModelChauvenet {
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return LunarEclipseShadowGeometryChauvenetAt(maximumJDE)
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}
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return LunarEclipseShadowGeometryAt(maximumJDE)
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}
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func lunarEclipseShadowGeometryAt(maximumJDE float64, shadowModel lunarEclipseShadowModel) LunarEclipseShadowGeometry {
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state := computeLunarShadowState(maximumJDE, shadowModel)
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moonDistanceKM := HMoonAway(maximumJDE)
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// 影半径是以地心为顶点的角量，除以月球处的地球视差就换成地球赤道半径。
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earthParallax := lunarEarthEquatorialRadiusKM / moonDistanceKM
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_, _, _, _, minimumDistance := refineLunarEclipseMaximum(maximumJDE, shadowModel)
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if earthParallax <= 0 {
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return LunarEclipseShadowGeometry{}
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}
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return LunarEclipseShadowGeometry{
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Gamma: minimumDistance / earthParallax,
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PenumbralRadiusDegrees: state.penumbraRadiusRad * 180 / math.Pi,
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UmbralRadiusDegrees: state.umbraRadiusRad * 180 / math.Pi,
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MoonDistanceEarthRadii: moonDistanceKM / lunarEarthEquatorialRadiusKM,
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// 平面 x 向东（黄经差）、y 向北（黄纬和），方位角自北向东量。
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AxisDegrees: minimumDistance * 180 / math.Pi,
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}
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}
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