FRIDAY, MAY 22 ■ We have a first-quarter Moon both this evening and tomorrow evening. The Moon is exactly first quarter at 7:11 a.m. EDT tomorrow morning, about halfway between the two evenings (for the Americas). ■ Spot 1st-magnitude Regulus just a couple degrees to the Moon's upper left after dark. From Regulus, the Sickle of Leo extends toward the right now, with its cutting edge facing down. It's a little more than a fist at arm's length long. SATURDAY, MAY 23 ■ With the Moon just a half day past exact first quarter (for the Americas), you'll see the terminator between lunar day and night still running almost, but not quite, straight down the Moon's face as shown below. In a telescope the terminator will cross interesting terrain. In the north (top) it's just beginning to uncover Mare Imbrium, filling the crater pair Aristillus and Autolycus with black shadow on Imbrium's flat floor (left of Mare Serenitatis in the picture below).…