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The Sky This Week, 2019 March 12 - 19

Reach for the Moon!
Reaching for the Moon, imaged from Mollusk, Virginia, 2019 February 16
Reaching for the Moon, imaged from Mollusk, Virginia, 2019 February 16
Imaged with a Canon EOS Rebel SL2 DSLR

The Moon continues to wax in the evening sky this week, cruising high along the ecliptic as she passes through the heart of the Great Winter Circle.  By the week’s end she passes into the springtime constellations as she fattens up to Full Moon, which will occur on the 20th at 9:43 pm Eastern Daylight Time.  Look for the Moon to the west of the bright star Aldebaran on the evening of the 12th.  On the 18th she may be found gliding just two degrees north of the star Regulus, brightest star in the constellation of Leo, the Lion. 

We’re rapidly closing in on the vernal equinox, which defines the beginning of the astronomical season of spring.  This is the time of year when the length of daylight increases at its fastest rate of the year, increasing by an average of 2.5 minutes per day.  Although the term “equinox” means “equal night”, the date when we here in Washington have 12 hours between sunrise and sunset will fall on the 17th.  This is due to a number of factors, the first of which is that the Sun isn’t a point source of light.  Sunrise and sunset are defined by the first and last visibility of the Sun’s apparent upper limb respectively, and there is a slight effect of refraction in the Earth’s atmosphere that displaces the apparent Sun slightly above its true position on the horizon.  If the Sun were a star-like point of light and there was no atmosphere around our planet the date of “equal night” would indeed coincide with the date of the equinox.  That said, the length of day will exceed that of night from the 17th until September 26th, so enjoy your daylight hours.

Astronomers don’t necessarily welcome the longer days, and now with Daylight time in effect the Washington sky doesn’t get fully dark until 8:30 pm EDT.  On the one hand this gives those of us who must travel to dark sites a chance to do so after rush hour, but on the other it means that we have to stay up later to enjoy the treasures of the night sky.  Fortunately this week we have the waxing Moon to enjoy, and she can be observed easily from the moment of sunset.  This week she moves high along the plane of the ecliptic, minimizing the thickness of our atmosphere that can add turbulence to Earth-based observers.  Observing the Moon is quite easy, requiring little more than a steadily-held pair of binoculars to reveal details of mountains and craters along the terminator, the line dividing the sunlit hemisphere of the Moon from the dark.  Luna really begins to blossom in a telescope, and you don’t need a large one to see fine details on her battered surface.  Each passing night brings a new set of features into view, and careful study will show an incredible variety of landforms frozen in the rocky lunar crust.  The craters that pepper the surface are the stark reminders of the earliest history of our solar system, when millions of rocky fragments called “planetessimals” whizzed between the forming planets.  Over the course of the first billion or so years of the solar system these small bodies smashed willy-nilly into larger ones, and those objects that weren’t big enough to form atmospheres and oceans bore the countless impact scars.  A typical lunar crater is several tens of kilometers across and over 3.5 billion years old.  By contrast, the famous Meteor Crater east of Flagstaff, Arizona is about one kilometer across and around 50,000 years old; it would be invisible in most amateur telescopes if it were located on the Moon.

Ruddy Mars is steadily trekking eastward along the ecliptic, drawing a bead on the star Aldebaran as he moves toward the stars of Taurus, the Bull.  You should be able to watch his motion over the next few weeks as he nears the Pleiades star cluster. 

Jupiter may be found shining above the southern horizon as morning twilight gathers.  Daylight Time gives us an extra hour to view him through the telescope.  Even though he is near his lowest declination in his 12-year journey around the Sun he’s still worth the effort to look at.  On mornings with steady air you can see a wealth of detail in his cloudy atmosphere, and the four bright Galilean moons provide endless configurations to enjoy.

Jupiter is followed into the sky by Saturn.  He, too is at a far southerly declination, but his rings are still tipped at their widest presentation to our line of sight.  Both planets will be best placed for viewing in the summer sky, but early mornings usually offer the steadiest air to study them.

Venus shines brightly in the southeast as morning twilight brightens the sky.  You shouldn’t have any trouble finding the dazzling planet in the brightening sky, and if you follow her as she rises you can probably follow her until well after the Sun comes up.

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