小満における「地球」の公転上の位置

　今日は二十四節季の一つ「小満」です。二十四節季は私たちを載せた地球が反時計回りに１５°公転するごとに名称が変わります。「小満」の今の季節では麦が実り、梅が収穫の時期を迎え、農家は「小さな満足」の季節となり、また、沖縄県では梅雨に入る頃です。

　太陽の周囲を約１億５千万㎞の距離を保って、ほんの少し楕円形で公転してる私たちを載せた地球は、今日、公転軌道上の何処を周遊しているでしょうか。と言えば、上図の通り、太陽と黄道十二星座の一つ「さそり座」の間に差し掛かっています。それは深夜０時に南の空を見れば分かります。ご存じの通り、「さそり座」は尾をくねらせた独特の姿で、その中心にある橙色した一等星アンタレスが地上を睨みつけてるかの如く非常に個性的な星座です。

　一方、「小満」の太陽は、今日、地球から見てどこにあるかと言えば、春分点から６０°東に移動した赤経４hの天空にあります。この遥か後方には冬の星座で有名な「スバル星団」と「おうし座」の中間に来ています。実際は日中、眩しくて確認できなくても、半年前の午前０時に真南にあったことで、その事実が分かります。

「おうし座」は、日本では真冬に頭上に見られる「冬のダイアモンド」の西の一角に位置し、赤味を帯びた1等星アルデバランが目印です。アルデバランは「後に従うもの」という意味ですが、これはスバル星団「プレアデス星団」の後に東の空に上ってくることから名づけられました。私は子供のとき、冬に頭上に星がたくさん集まってる不思議な光景が今でも印象に残ってます。これは「スバル星団」だったのです。

Dear my friends abroad who are interested in astronomy

　Today is "Shoman" which is one of the 24 solar seasons in Japan. The name of the 24 solar seasons changes every time the earth revolves 15 degrees counterclockwise. In the current season of "Shoman" the wheat has ripened and the plums are ready to be harvested. Therefore it is the season of small satisfactions for farmers, and it is about to enter the rainy season in Okinawa Prefecture.

　By the way, Where is the Earth revolving today? Our earth is in a slightly elliptical shape, keeping a distance of about 150 million km around the Sun. Speaking of which, as shown in the figure below, the earth is approaching between the sun and Scorpio which is one of the twelve constellations of the zodiac. You can understand by looking at the southern sky at midnight. As you know, "Scorpio" is a very unique shape constellation with a twisted tail, as if the orange first magnitude star named Antares in the center is glaring at our earth.

　On the other hand, the sun of "Shoman" today is located in the sky at right ascension 4h, which is 60 degrees east of the vernal equinox point. Far behind this is the midpoint between the famous winter constellations of the Subaru Cluster and Taurus. Even if you can't see it in the daytime because it's too bright, you can recognize because it was due south at midnight half a year ago or later.

　"Taurus" is located in the western corner of "Winter Diamond", which can be seen overhead in the middle of winter in Japan and the temperate zone, and is marked by the reddish 1st magnitude star Aldebaran. Aldebaran means "follower", so it was named because it rises in the eastern sky after the Subaru star cluster "Pleiades". When I was a child, the mysterious sight of many stars gathered overhead in winter still left an impression on me. This was the "Subaru Star Cluster" that is to say "Pleiades".

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