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Celestial sphere · equatorial coordinates
Celestial sphere · equatorial coordinates
赤经 RA/赤纬 Dec · 天赤道/天极/黄道 · 球面投影
Right ascension RA and declination Dec locate a star on the sky
Horizon system · altitude & azimuth
Horizon system · altitude & azimuth
高度角/方位角 · 地平圈/天顶 · 观测者视角
Altitude above the horizon and azimuth around it locate a star for an observer
Star trails · rotation about the pole
Star trails · rotation about the pole
星绕天极 · 弧∝赤纬+曝光 · 15°/小时 · 北极星近不动
A long exposure smears each star into an arc centred on the celestial pole
Sidereal day vs solar day
Sidereal day vs solar day
恒星日~23h56m vs 太阳日24h · 公转~1°/日多转
Earth moves ~1° along its orbit each day, so it must rotate a little extra to face the Sun again
Axial precession & the changing pole star
Axial precession & the changing pole star
~25772年画锥 · 北极星 Polaris↔Vega · 二分点移动
Earth's tilted axis sweeps a cone every ~25,772 years — the pole star drifts among the stars
The analemma — the Sun's figure-8
The analemma — the Sun's figure-8
同钟点太阳一年画8字 · 倾角=南北 · 偏心率=东西时差
Same clock time each day for a year: axial tilt swings the Sun north–south, eccentricity east–west
Apparent retrograde motion of a planet
Apparent retrograde motion of a planet
外行星视逆行环 · 地球内圈超越 · 日心投影背景星空
Faster Earth overtakes the outer planet → apparent longitude reverses near opposition
Planetary conjunctions & the synodic period
Planetary conjunctions & the synodic period
两行星连线 · 会合周期 1/S=|1/P1−1/P2| · 合/冲
Two planets line up (same heliocentric longitude) every S years, 1/S = |1/P₁ − 1/P₂|
Sidereal vs synodic period
Sidereal vs synodic period
恒星周期 vs 会合周期 · 1/S=1/P内−1/P外 · 追赶一圈
The faster body laps the slower one every S years: 1/S = |1/P_planet − 1/P_earth|
Tidal bulges · why two tides a day, spring vs neap
Tidal bulges · why two tides a day, spring vs neap
月球差异引力=两隆起 · 日约2次高潮 · 大潮/小潮
The Moon's differential gravity heaps the ocean into two bulges. Spin the Earth through them; line up the Sun for spring tides.
Exoplanet transit · the light curve of a passing planet
Exoplanet transit · the light curve of a passing planet
系外行星凌星 · 深度=(Rp/Rs)² · 亮度下凹
As a planet crosses its star, the brightness dips by exactly (Rp/Rs)². Watch the transit and read the light curve.
Twilight phases · civil, nautical, astronomical
Twilight phases · civil, nautical, astronomical
民用0–6°/航海6–12°/天文12–18° · 太阳俯角 · 过18°全黑
After sunset the sky darkens in three stages set by how far the Sun has sunk below the horizon.