mercoledì 20 dicembre 2017
THE DECEMBER SOLSTICE: Thursday, Dec. 21st, will be the shortest day of 2017 for anyone living north of the equator. It's the December solstice, when the sun reaches its lowest point on the celestial sphere. Northern winter begins at 11:28 a.m. EST. Because seaons are reversed on opposite sides of the equator, southern summer beings at the same time. Happy Solstice!
A GASH IN THE SUN'S ATMOSPHERE: Magnetic maps of the sun's atmosphere reveal a long gash, more than 700,000 km long, where solar wind may be spraying into the solar system. This image of the structure is based on data from NASA's Solar Dynamics Observatory:
Astronomers call this a "coronal hole"--a region when the sun's magnetic field opens up and allows solar wind to escape. Coronal holes are common, appearing every few weeks. This one, however, is uncommonly long and narrow. A fan-shaped spray of gaseous material flowing from the gash could reach our planet as early as Dec. 23rd. Arctic sky watchers should be alert for auroras when the solar wind arrives. Free: Aurora Alerts.