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The Sky This Week, 2018 September 18 - 24

The equinox and the Harvest Moon.
Mars and Phobos, 2018 August 24, 03:09 UT
imaged with the 12-inch f/15 Clark/Saegmüller refractor telescope
at the U.S. Naval Observatory, Washington, DC.

The Moon gradually climbs back to the northern hemisphere of the sky this week, passing near ruddy Mars on the evening of the 19th while waxing to the full phase, which will occur on the 24th at 10:52 pm Eastern Daylight Time.  Since this is the Full Moon that occurs closest to the autumnal equinox it is almost universally known (at least in the Northern Hemisphere) as the Harvest Moon.  Not only is this an appropriate name for the season, it also reflects an annual phenomenon that has been aiding farmers for millennia.  Growers in northern temperate zones have long known that each year, close to the time of the autumnal equinox, the Moon rises at shorter intervals on successive nights around the time of Full Moon.  Here in Washington the Moon rises half an hour later on successive nights around Full Moon; normally Luna’s successive risings occur about an hour later each night.  This effect becomes more pronounced as you move farther north.  Go to the Canadian prairie or the midlands of England and the interval shrinks to 20 minutes.  In Fairbanks, Alaska the difference is a mere five minutes.  North of the arctic circle moonrise actually occurs earlier on successive evenings around Full Moon!  The phenomenon occurs thanks to the inclination of the Moon’s orbit and its relationship to the ecliptic.  At this time of year Luna’s orbital plane intersects the eastern horizon at a shallow angle.  This means that her altitude relative to the horizon when she rises doesn’t change as much as it does at other times of the year.  The opposite case happens in the springtime; Luna’s orbit is nearly perpendicular to the horizon, and moonrise in Washington occurs well over later on successive evenings.

The autumnal equinox occurs on the 22nd at 9:54 pm EDT.  At this time the center of the Sun’s disc passes Ecliptic Longitude 180 degrees, and if viewed from the Earth/Moon barycenter crosses the Celestial Equator into the southern hemisphere of the sky.  If the Sun were a point source of light and if our fair planet had no atmosphere day and night would be of “equal” length on this date.  However, Old Sol subtends a significant disc, and Earth has a substantial atmosphere which can refract light when looking toward the horizon, so the actual date when night and day are exactly 12 hours long won’t occur until the 26th.  From then until March 17 of next year the nights will be longer than the days.  The times of the equinoxes are the times when the length of day and night change most rapidly, so we lose about three minutes of daylight each passing day.  These are some of my favorite nights of the year, as we hopefully get some clear fall weather to enjoy the last of the summer constellations.  Once the Moon leaves the scene the summer Milky Way is readily visible in the early hours of darkness.  By late night the sparse star fields of the autumn sky indicate a portal of sorts into the deeper reaches of space.  Looking past the few visible stars of the fall constellations, a modest to large telescope will reveal far-flung distant galaxies whose faint wisps of light have taken tens of million years to reach us.  Brighter stars return after midnight, when the first of winter’s constellations begin to rise.

Venus is making the big turn in her orbit that will soon lead to her passing inferior conjunction in late October.  She is rapidly catching up to the Earth, and closing the distance between the two planets.  She is now closer to Earth than Mars, and through the telescope her disc grows rapidly as she closes in.  This week she reaches her greatest brilliance of the current apparition, so she should be relatively easy to pick out low in the southwestern sky just after sunset.  You’ll need to act quickly, though, as she sets by 8:00 pm by the week’s end.

Jupiter still hangs in the southwest as dusk settles.  The giant planet sets by 9:30 pm this week, so if you’re going to take a gander at him in the telescope you’ll need to do so during twilight. 

Saturn can be found near the meridian as evening twilight ends.  He’s well-placed in the south near the top of the “teapot” asterism of Sagittarius.  His southerly declination means that we have to look through more of our turbulent atmosphere when we point the telescope his way, but a few minutes of patience will reward you with intervals of detailed viewing.  Fortunately, Saturn almost always puts on a good show, and you’ll never forget your first view of this distant world.

Mars rounds out the evening, shining with his distinct ruddy glow in the southeast as twilight ends.  The red planet is now receding from us, so his brightness is gradually fading and his disc is shrinking by about one arcsecond per week.  That said, a good four-inch or larger instrument should still show details on his dusty surface.  This week one of his most prominent dark features, Syrtis Major, is well-placed for evening viewing for observers in the U.S.

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