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If March equinox is on 20th March (see the table in the link), according to it, when is the time that the sun enters Taurus? Is it on April 19 or April 20?

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    $\begingroup$ And from Guide9, about 18.00 UT on 13 May is when its leading edge hits the boundary between Aries and Taurus - at least for the astronomical constellations. $\endgroup$ – Dr Chuck Apr 9 at 19:16
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    $\begingroup$ I'm voting to close this question as off-topic because astrology $\endgroup$ – James K Apr 9 at 21:08
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    $\begingroup$ @James K What astrology in that? It is pure astronomy! It is the second time you call my question astrology while I'm interested in astrology at all, but I'm interested in the history of the astronomy. $\endgroup$ – Reckless Glacier Apr 9 at 21:36
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    $\begingroup$ James K has a point, the Sun's λ=30° date is of little interest outside astrology. If there's an astronomical purpose, you need to show what it is, not just claim that there is one. $\endgroup$ – Mike G Apr 9 at 22:21
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    $\begingroup$ My purpose is to understand the differences between the 'previous' astronomy to the current one. Data of astronomy out of date is still astronomy and still studied in astronomy departments at any respected university. $\endgroup$ – Reckless Glacier Apr 9 at 22:31
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Guy Ottewell makes a comprehensive Astronomical Calendar each year. From the 2020 edition:

8959.117 Apr 19 SUN 15    Sun enters the astrological sign Taurus, i.e. its
                          longitude is 30°

That's 2020-04-19 ~15h UT, which falls on April 20 in far eastern terrestrial longitudes and April 19 everywhere else. The next such event is 1 tropical year (365d 5h 49m) later.

8983.315 May 13 Wed 20    Sun enters Taurus, at longitude 53.48° on the eclip-
                          tic

The Sun enters the constellation at 2020-05-13 ~20h UT, which falls on May 14 in Asia, Australia, etc., and May 13 everywhere else. This event recurs 1 sidereal year (365d 6h 9m) later. In that time, the ecliptic longitude of the constellation boundary increases by 0.014°.

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