今日課程內容：冥王星何以被除名、Solar Nebula為什麼是扁平的（九大行星剛好位於同一平面；角動量守恆）、Bode's Law
It was noticed in the 19th century that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets (notably by the French mathematician Le Verrier). It was hypothesized that another planet might exist in an orbit even closer to the Sun to account for this perturbation (other explanations considered included a slight oblateness of the Sun). The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place great faith in this explanation, and the hypothetical planet was even named Vulcan. However, in the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds per century, therefore it requires a little over twelve million orbits for a full excess turn. Similar, but much smaller effects, operate for other planets, being 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus
祝融星（又稱火神星）是一個假設在太陽與水星之間運行的行星，是早期為了解釋水星實際的近日點進動與計算出的數值上的差異而被假設存在的一顆行星。按古典力學的方法計算，水星在受到太陽和其他行星的引力攝動下，其近日點在每世紀會東移574角秒，但實際觀測的數字是531角秒，與預期的相差43角秒，於是人們便假設水星軌道以內，尚有一顆未被發現的行星。
後來這個19世紀的假設被愛因斯坦的廣義相對論排除。
後來這個不存在的行星被用到電影「星艦迷航記」中
Vulcans are a humanoid species in the fictional "Star Trek" universe who hail from the planet Vulcan, and are noted for their attempt to live by reason and logic with no interference from emotion.
以上資料出自中英文wiki
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