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Thread: Equinox 2010

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    Smile Equinox 2010



    We observe the Sun rising in the east and setting in the west every day, something that never changes.

    At noon if we keep observing the shadows that are cast by the Sun every day, we see that on March 20th and September 22nd, if we are at the equator, there will be no shadow cast at noon. The reason is very simple, the Sun should be exactly overhead at noon.

    We can conclude that, after observing the length and the direction of the shadows during one year, the Sun will be exactly overhead at noon in the months of March and September around the dates 20th and 22nd respectively at the equator. In the month of June, the Sun will reach a maximum angle in north and in December, the Sun will reach its maximum angle in south.

    The day when there are no shadows at noon, which as we observed, happens on March 20th and September 22nd, at the equator is called the Equinox. We can also say that, the point where path of the Sun called the ecliptic, cuts the equator circle.

    On this day of March 20th the Sun will be on the equator. In following days the Sun will move towards north in the sky. The angle keeps increasing and on June 21 the Sun reaches the maximum angle of 23.5 degrees. This is called Summer Solstice.

    But, in Bangalore, where the latitude is 13 degrees north of the equator, we can still see the shadows on March 20th and also on September 22nd. Therefore we have to wait till April 24th for the Sun to be at the Zenith at noon, we know what will happen to the shadows when the Sun is exactly overhead, they disappear!

    The Sun again reaches Zenith for Bangalore latitude on August 19th , making shadows disappear.

    From June 21st the Sun starts moving towards south and the angle keeps decreasing till it reaches 0 degrees, reaching back to Equator on September 22nd . This is called autumnal equinox.

    Now the Sun starts its journey towards south, the angle again steadily increasing, only this time the direction has changed, towards south. The Sun reaches the maximum southern angle of 23.5 degrees on December 21st, this is called winter solstice.

    After reaching maximum south, the Sun starts moving back towards the equator. The angle keeps decreasing again till it comes back to 0 degrees when the Sun reaches the equator. This happens on March 20th, and is called Vernal equinox.

    Astronomers have special interest for the Vernal equinox. We know that on the day of Vernal equinox, the Sun will be on the equator. The Sun on this day travels on this imaginary circle extended from the earth's equator. This circle is called the declination circle.

    This point, where the Sun cuts the equator circle, when the Sun's angle is zero degrees, is also called the first point of Aries. From this point the circle is divided into 24 hours, with hours increasing towards west, 1 hour is equal to 15 degrees, if we draw circles on every hour or minute perpendicular to declination circle, and making sure all the Right ascension circles pass over the north pole and south pole, we have a co ordinate system just as latitude and longitude. The entire sky can be mapped using this system. This system is called Equatorial coordinate system.

    Due to Precession of the Equinoxes the first point of Aries keeps shifting at a rate of 50.3 seconds of arc per year. Astronomers have to take this into account when assigning 0 hours on the Right ascension circle. If not corrected, it becomes almost impossible for astronomers to find the faint objects in the sky.

    First point of Aries is not in the Aries constellation anymore. The First point of Aries now lies in the constellation of Pisces, close 4.5 magnitude star lambda Piscium.

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    Thank you and well written.
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    North and south of the equator, at latitude 23.5 degrees, the angle of the plane of the equator to the plane of Earth's orbit, are the tropics of Cancer and Capricorn. Between these , the sun will pass directly overhead twice during the year, but above and below, the sun will never reach the zenith. This never occurred to me until I read the informative post by Akshay above. I decided to investigate with Stellarium to see if I could figure out why these two lines were so named. I was not successful. Perhaps Akshay has some insight into that.

    I also noted that the sun is in Pisces on this equinox day and yet we are at the beginning of the "month" of Aries. This puzzles me as well. I had always assumed a correlation between the sun and the 12 constellations of the Zodiac.
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