December 21st marks the year’s longest night and shortest day, with daylight lasting only 9 hours and 20 minutes on the Western Slope. This is our winter solstice, the first official day of winter.
Earth’s northern hemisphere is now tilted strongly away from the sun, and the sun is shining at low angles to the ground, greatly diminishing the energy we receive from it. For many weeks around winter solstice, the sun follows a southerly course through the sky, rising late and setting early.
After sunset, a bright, gibbous moon lights up our evening sky this week, making the fainter stars invisible. If you go outside at 7 PM each evening, you will see the waxing moon becoming steadily brighter each night, as it moves eastward in its orbit around the Earth. We say the moon is waxing when it appears to grow as the sun progressively illuminates more of the moon’s face on successive nights. The moon becomes full on December 28th.
Just after evening twilight, you may see ruddy colored Mars very low in the southwestern sky. Mars has been steadily fading since early March, as we on Earth have raced ahead of Mars in our faster orbit around the sun. Jupiter, king of the planets, rises high in the east as darkness falls. Jupiter is now the brightest object in the evening sky, except for the moon.
For those braving the morning cold, the planet Saturn rises in the east before 4 AM. And brilliant Venus, less bright Mercury, and the reddish star, Antares, form an interesting triangle that rises in the southeast around 6:40 AM from December 21st through the 26th.
Western Slope Skies is produced by members of the Black Canyon Astronomical Society.