FRIDAY, APRIL 17 ■ Vega, the Summer Star, already twinkles low in the northeast these April evenings. Find it five or six fists at arm's length lower left from Arcturus, the brightest star shining high in the east. They're both magnitude 0. How high or low will Vega shine for you after dark? That will depend on your latitude. The farther north you are, the earlier Vega rises. If you live in the latitudes of the southern U.S., you'll have to wait longer for Vega to make its appearance. ■ New Moon (at 7:52 a.m. EDT on this date). SATURDAY, APRIL 18 ■ Low in the west-northwest in twilight, a hair-thin crescent Moon, just 1½ days old, hangs 4° or 5° to the right or lower right of Venus. That's about three finger-widths at arm's length. Binoculars should give a lovely view of earthshine softly illuminating the Moon's whole night landscape. Venus and the Moon both set soon after complete dark. The waxing crescent Moon forms up with bold Venus and the delicate Pleiades on the weekend of the 18th and 19th.…