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The Sky This Week, 2020 August 4 - 11

Mid-summer treats, the Perseids and the Milky Way
The Summer Milky Way, imaged 2020 July 26 from Mollusk, Virginia.
The Summer Milky Way, imaged 2020 July 26 from Mollusk, Virginia
with a Canon EOS Rebel SL2 DSLR, 18mm @ f/5.6, composite of 12 90-second exposures at ISO 3200.

The Moon climbs northward along the ecliptic in the morning skies this week, passing through the rising autumnal constellations.  Last Quarter occurs on the 11th at 12:45 pm Eastern Daylight Time.  Early risers on the morning of the 9th get a nice treat as Luna poses just one degree south of the brightening planet Mars. 

August is the month to look for meteors.  In addition to an increase in the “background” count of random shooting stars, the annual Perseids meteor shower peaks at the end of the week.  This display is one of the most consistent from year to year, and shower members can be seen from mid-July through August.  This year the peak activity occurs on the nights of the 11th-12th and 12th-13th.  A single observer at a dark location may be able to see around 60 shooting stars per hour starting at around 11:00 pm local time when the radiant, in the constellation of Perseus, climbs into the northeastern sky.  The Perseids are particles that have streamed off of Periodic Comet 109/P Swift-Tuttle, which circles the Sun once every 133 years.  The comet was co-discovered in 1862 by Horace P. Tuttle, one of the most prolific American comet seekers, at Harvard College Observatory.  Tuttle had an interesting life and career, spending time at the U.S. Naval Observatory.  Each year in August Earth intersects the orbit of the comet, running headlong into the debris sputtered off its surface each time it rounds the Sun, causing the annual display.  Historical records indicate that the shower has been an ongoing event since at least the year 36 CE.  You’re best time to catch them will be between 11:00 pm and 1:00 am, when the waning Moon rises to brighten the sky.  The meteors are fast, flashing across the sky in mere seconds; the brightest ones leave persistent trains.

The waning Moon also means that it’s time once again to explore our home galaxy, the Milky Way.  From dark skies the luminous band becomes apparent by around 10:00 pm local time.  The densest part of the Milky Way is due south at this time, sandwiched between the stars of Scorpius and Sagittarius.  Unfortunately, many people have never seen this subtle sky glow, and on first encountering it they mistake it for a terrestrial cloud.  However, this cloud doesn’t move with respect to the stars, and it doesn’t take much optical aid to realize that it is comprised of uncountable numbers of stars.  A steadily held pair of binoculars is enough to start to resolve the brighter patches and dark lanes that snake through them.  Binoculars will also reveal that some of the brighter knots scattered among the star clouds are individual clusters of stars and glowing clouds of interstellar gas and dust.  One of my favorite sights of summer is a prominent star cluster located between the two stars that form the “stinger” of the Scorpion and the “spout” of the Teapot asterism of Sagittarius.  Known as Ptolemy’s Cluster or Messier 7, it is easily resolved in binoculars, and a small-aperture low-power telescope reveals a beautiful scattering of pale blue stars set against a background of millions of fainter ones.  Sweep up the Milky Way from this cluster and you’ll encounter a dozen similar treats!

Jupiter is prominent in the southeastern sky as twilight fades.  The giant planet crosses the meridian at around 11:30 pm, so you have plenty of time to get him focused in the telescope eyepiece.  Jupiter offers the largest apparent disc of any of the planets, and you should be able to see his dark equatorial cloud belts in telescopes of 3-inch aperture.  Almost any optical aid will reveal the planet’s four large Galilean moons, but a modest telescope will allow you to see their shadows cross the planet’s face.  On the evening of the 7th we get a shadow “two-fer”, with Ganymedes’ shadow on the disc until 9:53 pm EDT and Io’s umbra entering transit at 10:13 pm. 

Saturn follows Jupiter by about 10 degrees as they parade across the southern horizon.  While they appear close in the sky, they are actually very far apart.  Currently Old Jove is about 631 million kilometers (392 million miles) from Earth; Saturn is well over twice a far distant!  In fact, we don’t really see either planet in “real time” since the speed of light is finite.  Jupiter’s light takes 35 minutes to reach us, while Saturn’s glow takes just over 75 minutes!

Mars continues to brighten as he drifts through the stars of Pisces.  His eastward progress is beginning to slow as he approaches opposition in the fall.  He will spend the next few months in this general area of the sky, becoming very prominent by early October.  He will get a close visit from the Moon before dawn on the 9th.

Venus continues to drift eastward among the rising winter stars in the gathering morning twilight.  You should have no trouble spotting her in the east as she passes north of the prominent stars of Orion.