Seasonal changes on the horizon

Friday January 25, 2013

Special to The Eagle

As sunset time approaches 5 p.m. we sense the upswing toward spring. Today’s sunset will be at 4:58 p.m. and the sun will set later by a steady one to two minutes every day until around the summer solstice in June.

The only disappointment in shaving off minutes from nighttime in favor of daylight is that our window into the cosmos is shorter.

Civil twilight, when the center of the sun is 6 degrees below the horizon, begins at 5:28 p.m. this evening, At civil twilight, the brightest stars can be seen but objects on the ground are still clearly distinguishable. Astronomical twilight, when the center of the sun is 18 degrees below the horizon, begins at 6:35 p.m. tonight. At that time, darkness is complete.

The Summer Triangle, the grand isosceles (equal on two sides) triangle that has appeared every evening since last spring, now sits atop the west-northwest horizon as twilight deepens, soon to disappear from the evening sky for the season.

Find an unobstructed view to behold Vega, the brilliant star that marks the right corner of the commanding shape. Vega sets at 7:22 p.m. tonight and 6:58 p.m. at month’s end.

A stretch above Vega is less bright Deneb, the furthest star from Earth visible with the naked eye.

The third corner is Altair, due west. From right to left, Altair, Mars and Fomalhaut challenge the naked eye in the sunset glare.

On the opposite horizon, in the east-southeast, Sirius, the brightest star visible from Earth, rises at 5:18 tonight.

Jupiter, appearing to be Sirius’ big brother, is high above. Tomorrow, the full Wolf Moon lifts over the east-northeastern horizon at 4:54 p.m., preceding Sirius, which will appear to the right of the moon.

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