今日の「秋分の日」を天文で考える

　今日は「秋分の日」です。地球から見る今日の太陽は天の「秋分点」に到達しました。それでは「秋分点」はどこにあるかと言えば、「乙女座」【写真左】の中の天の赤道と交差する点です。しかし、太陽光の強さで「乙女座」を確認することはできません。「乙女座」は半年後の春によく見えます。特に、この星座に含まれる「スピカ」という星は黄道の近くにある一等星として有名です。

　一方、「秋分の日」の今日、私たちを載せた地球は公転軌道上の、一体、何処にあるかと言えば「春分点」がある「魚座」の方角に来ています。「春分点」は「魚座」【写真右】のすぐ下の天の赤道と交差するところです。もちろん、天には何の印もありません。「魚座」はインドの形にそっくりですから、すぐに確認できます。このように地球の公転上の現在位置を知るには午前０時に真南にある「黄道十二星座」(占いに使われる星座)を見ることです。

　ところで、「秋分の日」に赤道直下の国々では、太陽は真東から昇り、「真上を通過し」、つまり、太陽高度は９０°です。そして真西に沈みます。日本でも太陽は真東から昇り、真西に沈みます。正午の太陽高度は５３．８度【群馬県】です。因みに９０°－５３．８°＝３６．２°であり、この度数は群馬県内の緯度になります。

　実はこの記事は早朝５時台に書いてますが、大分涼しいです。「暑い寒いも彼岸まで」と言われてますが、快い気温の中で「秋分の日」について、太陽や地球の動き、そして星座で考えました。秋分の日をまとめると、今日の宇宙では乙女座～太陽～地球～魚座がほぼ一直線に並んでます。

Dear my Facebook friend as an earthling

　Today is the Autumnal Equinox day. The sun as seen from our Earth has reached the Autumnal Equinox point. Where is the Autumnal Equinox point in the sky? It is the point where the sun crosses the celestial equator in the constellation Virgo (left photo). However, Virgo cannot be seen by sunlight. Virgo is best seen in spring, six months from now. In particular, the star Spica, which is part of this constellation, is famous for a first magnitude star near the ecliptic.

　Meanwhile, on this Autumnal Equinox day, where exactly is the Earth on our orbit? It is in the direction of Pisces, where the Vernal Equinox point is located. The Vernal Equinox is where the sun crosses the celestial equator just below Pisces (right photo). Of course, there is no sign in the sky. Pisces is very similar in shape to India, so it is easy for us to see. In this way, to find out the current position of the Earth on its orbit, we had better look at the Zodiac (constellations used in fortune telling) located due south at midnight.

　By the way, on the Autumnal Equinox day in countries located directly on the equator, the sun rises due east and "passes directly overhead," which means that its altitude is 90°. It then sets due west. on the other hand, in Japan, the sun also rises due east and sets due west. The solar altitude at noon is 53.8° [Gumma Prefecture]. Incidentally, 90° - 53.8° = 36.2°, which is the latitude within Gumma Prefecture.

　I'm actually writing this article at 5am in the morning, and it's quite cool this morning. They say it's hot or cold until the equinox, but in this pleasant temperature I thought about the Autumnal Equinox day in terms of the movements of the sun and earth and constellations. To summarize the Autumnal Equinox day, in the universe today, Virgo - Sun - Earth - Pisces are lined up in almost a straight line.

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