Why did Rav Pappa say that the aris can tell the time of year signalling the end of the planting season, when KIMAH (in the tail of the constellation T'LEH) is overhead at noon (the 6th hour) of the day, which occurs around Adar/Feb. Stars in the daytime are lost in the glare of the sun! Isn't there an easier sign for the farmer to actually see when he is permitted to stop planting?
Look at the sketch of the celestial sphere that I've attached (enlarging it to 100%), It can be seen that at the time of the year when the sun is in the constellation of T'LEH, at alos ha'shachar (6 AM) (the constellation is still somewhat visible in the east as the sun starts to show itself). Then 6 hours later (at noon), the tail of the T'LEH will be directly overhead (since the sun travels across the sky with the invisible constellation of T'LEH in its background).
So the farmer goes home at 10 hours (4 PM). Then, if he looks at the sky just after sh'kiah (6PM), he can again begin to see the constellation of T'LEH in the gathering darkness of the sky. Please note: If he prefers, Rav Pappa could have suggested to the farmer to wait for the time of year when the sun is just leaving the constellation of SHOR. Then the T'LEH's tail (KIMAH) would be exactly overhead at noon.
Again, my question is: Is there a good reason why Rav Pappa did not give this user-friendly tip to the aris, that the farmer should look to see when the constellation of T'LEH can be seen just before dawn or just after sunset? (In my opinion, he indeed knew this fact. Why did he choose to give his sign in such an obscure manner?)
B'chavod,
Gershon Rubin
Ophthalmologist
Netanya
PS: please note that the artist's drawing is a little off. He should have rotated the zodiac a bit more so that the Ram (T'LEH) is behind the sun as observed by a person on earth in Feb.
Your question on Rashi is asked by Tosfos on that page (DH v'Kaima). Tosfos argues with Rashi for this very reason, and offers another approach to the Gemara, whereby it is referring to the beginning of Shevat (when Kimah is actually overhead at nightfall, and not by day).
Rashi would seem to have understood that the Gemara is not giving something to look for in the sky (since it is not evident to the observer, as you wrote). Rather, Kimah was a popularly known constellation (since it was a harbinger of the change in seasons), and people normally could tell you (astronomically) where that constellation should be found. They therefore would know when it was above their heads even if they couldn't see it. This is the simplest understanding of Rashi.
However, RADVAZ (2:821) suggests another approach. He points out that the Gemara, which was interested in naming a constellation at the end of Ram (Tleh), could have said "Ash," the name for the trailing stars of the Ram - as Tosfos there notes. Kimah is actually a bit off from the end of Ram.
The Radvaz therefore suggests that the stars in Kimah are particularly bright, and can be seen by the keen-sighted even by day (like Vega, Spica and magnitude 1 stars). The Gemara specifically chose Kimah as the sign because of this quality. This also answers your question; sharp-eyed workers were used to picking out stars overhead even during the hours shortly preceding sunset. (Take a look at the sky while you are out in the country, one day, and you will see this is true. Of course, you have to know where to look.)
According to some, Kimah is the Pleiades (which are indeed between Aries and Taurus).
P.S. Thank you for the graphic that you scanned for us! Where was it from?