The Sky This Week
U.S. Naval Observatory's Weekly Blog

Time to try out that new telescope!

by Geoff Chester, USNO Public Affairs
12 January 2021
The Moon, 2020 March 2, 02:51 UT
The Moon, imaged from Alexandria, Virginia on 2020 March 2, 02:51 UT
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor and a ZWO ASI224MC color imager.

The Moon returns to the evening sky this week, with New Moon occurring on the 13th at midnight, Eastern Standard Time.  On the evening of the 14th you can spot her slender crescent low in the southwest during evening twilight.  Look for the elusive planet Mercury about two degrees to the right of Luna’s crescent that evening.  As the week progresses the Moon climbs higher in the evening sky, drawing a bead on ruddy Mars.

The waxing crescent Moon offers fine views for owners of small telescopes.  Judging from the back-ordered inventories of popular telescope vendors it looks as if many people received telescopes as gifts during the holiday season, and Luna is a great target for first-time telescope owners.  I often say that the Moon is “looked over, then overlooked”, but spending time exploring her many varied surface features can be a wonderful retreat from the constant barrage of the 24-hour news cycle.  As our closest neighbor in space even small instruments will show an abundance of detail, and it helps to have an atlas of the Moon handy for your exploration.  There are many of these available online.  The Moon’s larger features bear names that were for the most part assigned long ago by the first astronomers to gaze on her with crude telescopes.  Her darker areas are known as “seas” (“maria” in Latin), “lakes” (“lacus”), “bays” (“sinus”), etc. and bear names like Mare Imbrium (Sea of Rains) and Sinus Iridum (Bay of Rainbows).  These large-scale features are the remnants of collisions with large asteroids early in the history of the solar system’s formation.  Individual smaller craters are named after famous people from classical astronomical literature as well as more modern contributors to lunar science.  Each successive night reveals new features along the “terminator” which is the sunrise/sunset line that creeps slowly eastward from our point of view.  The low Sun angle along the terminator throws features into stunning relief as ink-black shadows give way to dazzling sunlit terrain.  A 3- or 4-inch aperture telescope will show many hundreds of features and terrain textures.  Spend some time taking good long looks at our fair neighbor and you’ll want to return each month.

As the Moon moves farther along the ecliptic her light begins to wash out the faint stars that characterize the late autumnal constellations.  To challenge her, the bright stars of winter roll into the evening hours, offering more treats for the novice telescope user.  By the late evening the bright constellation of Orion, the Hunter is well up, dominating the southern sky.  Surrounding him is a large circle (or hexagon) of bright stars.  Point your telescope at these luminaries and you will see their colors, ranging from the icy blue of Sirius, the brightest star in the night, to the golden yellow of Capella, northernmost of the circle’s stars, to the red-tinted Aldebaran, the “eye” of Taurus, the Bull, and Betelgeuse, the left-hand “shoulder” of the Hunter himself.  Now sight in on Castor, the fainter of the two Gemini “twin stars” to the northeast of Orion.  Your telescope will reveal two stars tucked close together.  Each of these stars is itself a binary star, as is a fainter companion nearby, thus making Castor a six-star system!

As mentioned earlier, the planet Mercury is now putting in an appearance in the evening twilight sky.  Mercury never strays very far from the Sun, so he’s almost never visible against a dark sky.  This week, though, he advances eastward from the Sun’s glare for one of his better evening apparitions for the year.  The Moon will help you find him at dusk on the 14th, and each night for the next week or so you should look in the same general part of the sky for his glow.  

Mars continues his eastward trek through the late autumn stars.  He is still easy to spot despite his fading light.  He is now some 10 times fainter than he was at opposition, but his red hue and lack of nearby bright stars should make him easy to pick out.  Through the telescope you’ll see a tiny gibbous pink disc.

Venus is now becoming a challenge to early-rising skywatchers.  She now rises in gathering twilight an hour before the Sun.  You will need a flat eastern horizon to catch her before she is overwhelmed by the pending sunrise.
 
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