insert one dash where needed. the astronomer, with his eye at the telescope, said, \i see mercury about ten…

Brief Explanations:

The dash should be inserted to create a break in the astronomer's statement for emphasis or to separate a sudden - change in thought. Here, it should be inserted before "wow" to mark the abrupt shift from the description of Mercury's position to the exclamation about the comet.

Answer:

The astronomer, with his eye at the telescope, said, "I see Mercury about ten degrees above the - wow, look at that comet!"