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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Happy New Year!
by Geoff Chester, USNO Public Affairs
06 January 2021
The Full Cold Moon (and Orion) rising, Ocean City, Maryland, 2020 December 29
HDR image made with a Canon EOS Rebel SL2 DSLR and EF-S 18-55mm zoom lens @ 18mm, f/8.
The new year finds the Moon waning in the morning sky, coursing her way through the rising constellations of spring. Last Quarter occurs on the 6th at 4:37 am Eastern Standard Time. You’ll find Luna to the northwest of the bright star Spica before dawn on the 6th. By the week’s end she is low in the southeastern sky, passing through the first of the rising summer constellations.
Earth reached perihelion, its closest point to the Sun, on January 2nd. Fortunately for us our planet’s yearly excursion around our star follows a nearly circular path that varies only 5 million kilometers (3 million miles) between apsides. The eccentricity of Earth’s orbit varies over periods of hundreds of thousands of years due to the gravitational influence of the other planets in the solar system. Currently it is trending toward a more circular state, reaching a minimum in about 28,000 years.
We are now at the point in the year where we are seeing the latest sunrises in the Northern Hemisphere. Here in Washington Old Sol crests the horizon at 7:27 am EST. By the end of the week he will rise one minute earlier. However, the time of sunset is now just after 5:00 pm and we have added five minutes of total daylight since the winter solstice. The longest nights of the year are now safely in the rear-view mirror.
The splendor of our winter constellations is best appreciated during the month of January. The stars that comprise the “Great Winter Circle” dominate the evening hours, led by the striding figure of Orion, the Hunter. No other constellation boasts as many bright and colorful stars as this one which is visible from every inhabited part of the globe. Anyone who has done any casual stargazing on a winter night has probably noticed the three perfectly aligned stars that make up Orion’s “Belt” framed by the first-magnitude stars Betelgeuse to the northeast and Rigel to the southwest. All of these stars are easily visible from urban environments, but it is the view from more rural sites that really brings out the splendor of the constellation. From a dark site the first thing you will notice are the colors of the principal stars. Betelgeuse shines with a ruddy tinge while Rigel and the Belt Stars sport an icy blue hue. These colors tell us something about the nature of these stars. Betelgeuse is relatively cool while its companions are very hot. All of these stars shine across enormous gulfs of space, ranging from about 550 light-years for Betelgeuse to 2000 light-years for Alnilam, the middle star in the “Belt”. The latter is one of the most intrinsically bright stars in our galaxy, with some 500,000 times the luminosity of our little Sun.
As midnight falls another brilliant blue star lies on the meridian at the end of a southeasterly line from Orion’s belt. This is Sirius, the brightest star in the entire sky and chief luminary of Canis Major, the Greater Dog. Unlike Orion’s powerhouses, Sirius is bright by dint of its proximity, just 8.6 light-years away and a luminosity of 25 Suns. Both Sirius and Orion figure prominently in the sky lore of many ancient civilizations and led to humanity’s first reckonings of time.
Another planetary gathering takes place in the southeastern sky at dusk this week when Mercury, Jupiter, and Saturn convene in the early twilight. From the 9th to the 11th the three planets will be within three degrees of each other, but they will only be a few degrees above the horizon at 5:30 pm local time. You will need a very clear sky, an unobstructed horizon, and binoculars to easily see them.
Mars is now the sole planet that’s easily visible during the evening hours. You will find his ruddy glow on the meridian at 7:00 pm as he makes his way from the bounds of Pisces into the diminutive constellation of Aires, the Ram. Mars appears slightly brighter than Betelgeuse in Orion, but the two objects share a warm reddish tint. Mars now shows a small gibbous-phase disc in the telescope eyepiece.
You can still spot bright Venus low in the southeast half an hour before sunrise. Look for a very slender crescent Moon near her before dawn on the 11th.
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