Hipparchus - Hipparchus (English spelling)

Date of birth and death unknown. Ancient Greek astronomer. Born in Nicaea, Asia Minor. Active in the second half of the 2nd century BC. His view of the universe was geocentric, but he made several important achievements in astronomical observations, the results of which were included in Ptolemy's Almagest , laying the foundations of later astronomy. He measured the parallax of the moon. Parallax is the difference in direction when the same celestial body is viewed from two different places, and in order to accurately calculate it, he created a table of chords and established trigonometry (particularly spherical trigonometry). Calculating this parallax, he determined that the distance from the earth to the moon is about 67 times the radius of the earth, and the distance to the sun is 2490 times the radius of the earth. He also discovered a star in the constellation Scorpio that had not been recorded until then. At the time, the heavens were considered to be eternal and unchanging, and this discovery prompted him to observe more than 1,000 stars in the entire sky, classifying them by brightness in six classes, and creating a star catalog showing their positions in ecliptic longitude and ecliptic latitude. Furthermore, by comparing this catalog with earlier star catalogs, he discovered that the vernal equinox was moving slightly earlier each year (precession). In other words, the length of the sidereal year and the tropical year (solar year) were different. The precession phenomenon occurs when the Earth's axis of rotation changes due to the gravitational forces of the Sun and Moon, but he thought that this was because the North Celestial Pole circles in a very large cycle.

[Hirata Hiroshi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

生没年不詳。古代ギリシアの天文学者。小アジアのニカイア出身。紀元前2世紀後半に活躍。彼の宇宙観は天動説であるが、天文観測でいくつかの重要な業績をあげ、その成果はプトレマイオスの『アルマゲスト』Almagestに採録され、後世の天文学の基礎を築いた。彼は月の視差の測定をした。視差は二つの異なる場所から同一の天体を見たときの方向の違いであり、それを正確に計算するために、弦の表を作製して三角法（とくに球面三角法）を確立した。この視差を求めて地球から月までの距離を地球の半径の約67倍、さらに太陽までの距離を地球の半径の2490倍だとしている。また、さそり座にそれまで記録されていない星を発見、当時は天界は永久不変とされており、この発見をきっかけに全天の1000個以上の星を観測し、明るさを6等級に分類し、その位置を黄経、黄緯で示す恒星目録を作製した。さらにこの目録をそれ以前の星表と比較し、春分点が毎年すこしずつ早くなっていること（歳差現象）を知った。つまり、恒星年と回帰年（太陽年）の長さが違っていた。歳差現象は地球の自転軸が太陽や月の引力作用のために首を振るように変化するために生じるが、彼は天の北極が非常に大きな周期で円を描くためと考えた。

［平田　寛］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例