Update to Tosefta 6:10, note 8
8. Since in ancient times sun was a very common item of pagan worship, including among the Romans and Greeks, Rebbi Yehudah felt that it was inappropriate to make a Beracha on any phenomena that has to do anything with the sun, since it may appear to look like the Jews worshipping the sun. It seems Rebbi Yehudah was not concerned so much about the Berachot on the moon and the planets, although in Greek and Roman paganism all of them had gods and goddesses associated with them. It is possible that public worship of the sun during the Talmudic times in the Land of Israel was more wide spread than public worship of the moon and other planets and that is why Rebbi Yehudah felt that only the Beracha on the sun is inappropriate, but not on the moon and other planets, although this is not really clear.
Professor Saul Lieberman in an article (Saul Lieberman, “Light on the Cave Scrolls from Rabbinic Sources”, Proceedings of the American Academy for Jewish Research, Vol. 20 (1951), pp. 395-404.) provides an alternative explanation. He translates the words Derech Acheret to mean “heterodoxy”, which means a view that does not follow established, or orthodox opinions, such as the opinion of the Rabbis, but rather a dissenting view of a radical group of Jews who wanted to be stricter than the Rabbis. Liebrman cites 2 additional places in the Tosefta where this phrase, Derech Acheret, appears: Tosefta Berachot 6:26 and Tosefta Terumot 7:12, where the practice of saying a Beracha on the sun is condemned in conjuction with a practice of filtering wine and oil from larvae that grows in them. He explains that Rebbi Yehudah is not referring to the Beracha on the sun that is said once in 28 years as the Gemara explains it, but rather to some kind of a benediction over the sun that was said every day. Such a benediction was pronounced by the Essenes as recorded in Josephus (Jewish War, II.8.5, 128.). Josephus writes, “And as for their piety towards God, it is very extraordinary; for before sunrise they speak not a word about profane matters, but put up certain prayers which they have received from their forefathers, as if they made a supplication for its rising.” (from William Whiston translation) Lieberman suggests that Rebbi Yehudah was condemning this practice of the Essenes, just like he was condemning the practice of some extra pious groups who filtered their wine and oil from larvae that grows in them despite the fact that such larvae is permitted for consumption. Although Lieberman’s explanation fits very well in the context of the Tosefta in Terumot it does not fit so well in this Tosefta, because the Tosefta is clearly talking about Berachot established by the Rabbis and not by radical groups such as the Essenes. It is possible to reconcile the Gemara’s explanation of the Beracha on the sun with Lieberman’s by explaining that Rebbi Yehudah disagreed with the general practice of saying the Beracha on the sun even if it was once in 28 years due to this practice of the Essenes who placed special emphasis on the rising of the sun and not necessarily due to general pagan worship of the sun since it was no different than worship of the moon or other planets, as I already mentioned earlier.