Cyrus gave the Jews permission to return to their land and rebuild
the Temple in the first year of his reign, which was about 50 years
after Nebuchadnezzar destroyed the First Temple. But because of
opposition from the Samaritans, the Second Temple wasn't completed
until about 20 years later, 70 years after the destruction of the
First Temple. Daniel (9:2) says that the land was desolate for 70
years, but the 70 years either began before the destruction of the
first Temple, or ended after the return. Rashi on Daniel 9:2 says
that the 70 years began when Nebuchadnezzar subjugated Yehoyakim,
which was at least 20 years before the Temple was destroyed.
2 Chron. 36:21-22 says that a 70-year period ended when
Cyrus became king; but the commentators (e.g., Rashi) take this
as referring to a 490-year period in which the Jews failed to
observe 70 Sabbatical years, and the total of 490 years includes
only 52 years after the destruction of the First Temple.
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