Winter Solstice triggers poinsettias

Poinsettias, a favorite of backyard gardeners, are now exhibiting their scarlet, petal-like bracts, just in time for the holidays. The onset of 14-hour-long nights triggers their behavior: In San Diego this condition is met just before the date of winter solstice, December 21.

Winter Solstice, the moment when the sun reaches its southernmost point on the celestial sphere, occurs this year at 2:24 a.m., Wednesday, December 21. This event signals the beginning of winter in the Northern Hemisphere and the beginning of summer in the Southern Hemisphere. With only about ten hours of daylight, San Diego must endure its darkest and gloomiest nights this week and next. During the day, the sun struggles to an altitude of only 34 degrees (as seen from San Diego) and then quickly sinks toward the horizon.