National Geographic : 1990 Mar
Shadows sweep Stonehenge in southernEngland (right), where centuries offarmers worshiped the seasons that ruled their lives. Built before 1500 B.C., this circle of stones is thought by some to be the world's oldest observatory. Celestial movements on a vasterscale-the spinningof neu tron stars-inspirescientists at Arecibo Observatoryin Puerto Rico (below). Usinga 20-acre receiving dish, they can measure radio waves from such pulsars to within a microsecond over sev eralyears, a precisionunexcelled even by atomic clocks over simi lar periods.