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

Hooking the Southern Fish

by Geoff Chester, USNO Public Affairs
20 October 2020

The Moon waxes in the evening sky this the week, skirting the southern horizon as she scoots by Jupiter and Saturn before climbing northward through the dim autumnal stars.  First Quarter occurs on the 23rd at 9:23 am Eastern Daylight Time.  Luna forms an attractive triangle with the planets Jupiter and Saturn on the evening of the 22nd.  By the end of the week she’s closing in on the ruddy glow of Mars.

The annual Orionids meteor shower reaches its peak in the wee hours of the 21st.  These meteors are the product of none other than Halley’s Comet, which last graced our skies in 1986.  As the comet makes its 76-year journey around the Sun it sheds material along its orbital path, and every year in mid-October Earth intercepts this dusty trail.  The “radiant”, or the point in the sky that the meteors seem to originate from, is located near the stars of Orion’s “club” just northeast of the reddish-hued star Betelgeuse.  An observer at a dark-sky location can expect to see upwards of 20 meteors per hour, but occasional outbursts can sometimes double that total.  Orion rises at around 11:00 pm local time, so the best time to look for the shower is between 1:00 am and dawn.  Moonlight will not hamper this year’s display.

October evenings are mostly occupied by a number of dim obscure constellations.  Zodiacal groups such as Capricornus, Aquarius, and Pisces lack any stars that are brighter than third-magnitude.  At 10:00 pm the bright stars of the Summer Triangle are heeling over to the western horizon and the stars of the Winter Circle have yet to rise.  At this time, though, there is one lonely first-magnitude star close to the meridian in the southern part of the sky.  This star is Fomalhaut, brightest member of Pisces Austrinus, the Southern Fish.  Fomalhaut is the 17th brightest star in the sky, and its relative isolation from other bright stars makes it useful for spacecraft navigation.  It is just over 25 light-years away from the solar system.  It was the first star to have a putative planet discovered in a visible-light image taken by the Hubble Space Telescope in 2008.  The existence of this object is controversial, since more recent observations at other wavelengths indicate that it may actually be a loose concentration of material in one of the star’s many dust rings.  This may mean that we are seeing a planetary system in formation, making Fomalhaut the subject of intense observation.

The brightest stars among the autumnal constellations belong to a group of figures that are intertwined in mythology.  At 10:00 pm you’ll notice a large square-shaped asterism approaching the zenith.  Popularly called the Great Square, these stars form part of the constellation of Pegasus, the Flying Horse.  The northeastern star of the square is Alpheratz, which is shared with the constellation of Andromeda, the Chained Lady, which arcs northeastward from Alpheratz in two diverging chains of stars.  The brighter of these chains point to the wish-bone shaped Perseus, while the fainter chain angles northward to the W-shaped group of Cassiopeia.  All of these constellations are related in a great story from ancient Greek mythology that we’ll explore in the coming weeks.

It is gradually becoming more difficult to get quality observing time with Jupiter and Saturn.  Here in temperate northern climes we already have to deal with the low declination of these two vast worlds.  Even when they are on the meridian they are less than 30 degrees above the horizon.  This means that we have to look through more of our turbulent atmosphere to see them in detail.  At their best we are looking through almost twice the mass of air than we would if they were overhead.  They are best placed as evening twilight begins to darken the sky, but this is also when the ground radiates heat back up into the atmosphere.  Looking through a telescope magnifies this turbulence, blurring the view of the delicate cloud belts of Jupiter and the details in the rings of Saturn.  They are still worth a look, though, since moments of calm air can snap the views into momentary sharp focus.

Fortunately Mars is much higher in declination and is well above the horizon for most of the night.  The red planet is just past opposition and closest approach to Earth, so he is still near his peak brightness and largest angular diameter.  Visually he’s impossible to miss; his ruddy hue and bright glow are the brightest objects in the autumn sky.

Venus can still be seen in the pre-dawn sky, beaming down from the rising spring constellations.  This week she moves from the boundaries of Leo into the realm of Virgo.  Her eastward progress along the ecliptic is gradually bringing her closer to the Sun, but she will continue to be a fixture for early risers through the end of the year.  |  |  Navy FOIA  |  DoD Accessibility/Section 508  |  No Fear Act  |  Open Government  |  Plain Writing Act  |  Veterans Crisis Line  |  DoD Safe Helpline  |  Navy SAPR  |  NCIS Tips  |  Privacy Policy  |  Contact Webmaster
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