春分の日について

　春分の日には、反時計回りに公転中の地球から見ると、太陽の位置が「春分点」にあります。「春分点」の位置は黄道十二星座の「魚座」のすぐ南の天の赤道上にあります。もちろん、天には何の印もありません。一方、春分の日の今日、私たち載せ公転中の地球は黄道十二星座の「乙女座」の方角に来ています。図では公転軌道が円になってますが、実際は少し楕円形です。

　ですから、午前０時に真南には「乙女座」が来ています。「乙女座」の一際明るい星は一等星「スピカ」です。この星は２１個ある一等星の中では唯一黄道の近くにあります。

　ところで、地上の位置を示すには緯度と経度ですが、恒星など天体の位置を表すには赤緯と赤経です。例えば、太陽を除いて最も明るい恒星シリウスは赤経 06h 45m、 赤緯 −16° 42′であり、冬のダイヤモンドのカペラは稚内市【北緯４５°】の真上を通過するので赤経 05h 16m 41s、赤緯 +45° 59′です。このように赤緯では北半球上は＋をつけ、南半球上は－をつけます。また、赤経では１５°ずつhをつけ一周２４hです。赤緯も赤経も今日の太陽の位置である春分点の「赤緯±0、赤経0h」を天体の基準点としてます。

　群馬県の今日の日の出は５時４８分、太陽高度は５３°、日の入りは１７時５５分ですから、「昼と夜の長さは同じでなく」、すでに昼の方が７分も長くなってます。私たちは、昼が長くなるこれからの一日一日を有効に使い、自らめざす目標に向かって邁進したいものです。

　

【画像をクリックし、拡大してご覧ください。星座は左が魚座、右が乙女座です。】

Dear my overseas friends who are crazy about astronomy and great music.

　The simultaneous interpretation into English for your understanding will be done after my breakfast Japan time.

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　On the vernal equinox day, today, the Sun is located at the vernal equinox point when viewed from the Earth which revolves counterclockwise. The vernal equinox point is located on the celestial equator, just a little south of the zodiacal constellation "Pisces". Of course, there are no marks in the sky. Meanwhile, on today, the vernal equinox day, the Earth that we are traveling on is in the direction of the zodiacal constellation "Virgo". In the diagram, the orbit appears circular, but in reality it is slightly elliptical.

　So, at midnight, we can see "Virgo" due south. The brightest star in Virgo is the first magnitude star "Spica". This is the only one of the 21 first magnitude stars that is close to the ecliptic which is the way of the sun.

　By the way, as you know, while latitude and longitude are used to indicate positions on earth, declination and right ascension are used to indicate the positions of celestial bodies such as fixed stars. For example, the brightest star other than the sun, Sirius, has a right ascension of 06h 45m and a declination of -16° 42', and one of the the winter diamonds "Capella" passes directly above Wakkanai-City in Hokkaido [45° north latitude], so has the right ascension of 05h 16m 41s and the declination is +45° 59'. In this way, declination is marked with a + in the northern hemisphere and a - in the southern hemisphere. Also, right ascension is marked with an h every 15°, and one revolution takes 24 hours. Both declination and right ascension use the vernal equinox point which is the position of the sun today, as the celestial point of reference that means "declination ±0, right ascension 0h".

　Today's sunrise in Gumma Prefecture was at 5:48, the solar altitude is 53°, and sunset will be at 17:55, so "day and night are not the same length," daytime is already 7 minutes longer. Each days of the longer days from now on, we should work or study hard toward our own purposes.

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[Click on the image to enlarge it. The constellations on the left are Pisces and on the right are Virgo.]