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The Sky This Week, 2020 September 29 - October 6

Shine on, Harvest Moon (and Mars, Jupiter, Saturn, and Venus!)
The Moon and Mars, imaged 2020 SEP 6 from Alexandria, Virginia.
The Moon and Mars, imaged 2020 SEP 6 from Alexandria, Virginia
with an Antares Sentinel 3-inch (80-mm) f/6 refractor and a Canon EOS Rebel SL2 DSLR

The Moon starts her long, lonely climb through the rising autumnal constellations this week.  Full Moon occurs on October 1st at 5:05 pm Eastern Daylight Time.  As the closest Full Moon to occur around the time of the autumnal equinox, this one is almost universally known (in the Northern Hemisphere, at least) as the Harvest Moon.  Her solitary journey through this barren part of the sky is interrupted by a close approach to ruddy Mars on the evening of the 2nd.

Long-time readers will recognize that every Full Moon throughout the year has a name associated with some aspect of sky lore.  These names reflect phenomena that people of different cultures noticed in their environment as Luna’s phases cycle through the year.  The Harvest Moon is one name that is widely recognized throughout northern temperate climes.  More than just a name, it indicates a specific phenomenon that only occurs at this time of year.  As the Moon orbits the Earth, the plane of her orbit mirrors the plane of Earth’s orbit around the Sun, known as the ecliptic.  Thus the Moon “mirrors” the Sun’s position in the sky, with Full Moon always occurring opposite the position of the Sun.  At the summer solstice the Sun is at its highest point along the ecliptic plane, so Full Moon occurs with the Moon at her lowest point above the southern horizon.  The opposite is true in winter, when we have a high-altitude Full Moon to light the longest nights of the year.  In between we have the equinoxes, so the Full Moon and the Sun share similar altitudes when crossing the meridian.  However, if you look at the angle that the ecliptic makes with the eastern horizon during the fall, it is much shallower than the angle that the ecliptic makes with the horizon in the spring.  Thus, even though Luna moves about 13 degrees eastward along the ecliptic every day, its distance below the horizon only changes by a few degrees, and times of successive moonrises around Full Moon are more closely spaced than at any other time of the year.  This effect becomes more pronounced as you move farther north.  Here in Washington the difference between moonrise times on the 1st and 2nd is 24 minutes, but in Scotland the difference is only 10 minutes.  Above the Arctic Circle the Moon actually rises earlier on several successive nights!

The net effect of all of this turned out to be a benefit for farmers back in the era before artificial lighting began to destroy the night.  That “extra” bit of moonlight around the time of Full Moon allowed farmers to have a bit more light for a few evenings to help them bring in their crops at harvest time.  A similar geometry occurs with the next Full Moon, which is widely known as the Hunter’s Moon.

While the light of the Harvest Moon washes out all but the brightest features of the night sky this week, we have a bevy of bright planets to keep us occupied each night.  Giant Jupiter is first to appear, popping out of the twilight glow in the southern sky shortly after sunset.  Old Jove crosses the meridian about an hour after sunset, and he remains well-placed for viewing for the next several hours.  Jupiter offers a good view in just about any telescope.  A 3-inch instrument will show the planet’s dark equatorial cloud belts and the ever-changing configurations of the four bright Galilean moons.  Double that aperture and a wealth of detail begins to show in the main cloud belts, and a night of steady seeing will show several other dark belts and bright zones.  The Great Red Spot, perhaps the planet’s most persistent feature, will be visible on the evenings of the 29th, 2nd, and 4th.  

Saturn becomes visible soon after Jupiter, just over seven degrees east of Old Jove.  The ringed planet reaches the second stationary point of this apparition on the 29th, gradually resuming direct eastward motion during October.  His motion is very sluggish, though, and Jupiter will overtake him in a spectacular close conjunction on December 21st.  Mark your calendars for this one!

Mars dominates the eastern sky in the late evening, his bright pink-hued glow staring down from an otherwise barren sky.  We are now in the middle of “prime time” for observing the red planet as he closes in on opposition on the 13th.  Due to Mars’ orbital eccentricity his closest approach to Earth will be in the wee hours of the 6th.  At around 3:00 am EDT that morning Mars will approach to within 62,066,500 kilometers (38,566,350 million miles), and he will outshine all of the planets except for Venus.  This will be the closest the red planet will approach us until 2033, but that opposition will take place far in the southern sky, so take advantage of the current one.  

Venus remains a fixture in the pre-dawn sky as she races eastward along the ecliptic.  This week she passes very close to the bright star Regulus in Leo.  In the eastern U.S. Venus will be half a degree west of Regulus before dawn on the 2nd, and half a degree east of the star on the morning of the 3rd.  Observers in the Eastern Hemisphere will be treated to a very close approach of Venus to the star.  They will need binoculars to separate them!

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