11 a.m., Oct. 17
Venus's Elongation and the Effect of Thunderstorms on Ocotillos
Venus, the lanternlike planet shining in the west after sunset, reaches greatest elongation from the sun on Thursday, August 19. That means the planet is illuminated from the side by the sun, as seen from Earth’s perspective. Turn a small telescope on Venus, and you will probably be able to discern its current phase or shape, which looks like a perfect capital letter D.
Thunderstorms have visited the Imperial Valley and parts of eastern San Diego County over the past several weeks. The seasonal arrival of moisture from the east and south, more or less typical for late summer, contributes to the greening of certain kinds of vegetation. On the desert floor, for example, the spidery ocotillo can grow an instant crop of green leaves after a storm, only to drop them two to three weeks later if no further rain arrives. The distribution of green-leaved ocotillos tells not only where rain has recently fallen; it also indicates where the runoff has collected and remained for enough time to be absorbed by the ocotillo's root system.