The Sky This Week from September 20 to 29

As summer turns to fall, the giant planets of the outer solar system take center stage.
By | Published: September 20, 2019 | Last updated on May 18, 2023
The zodiacal light partners with the Milky Way

The arc of the Milky Way stretches from Sagittarius (at left) to Auriga (at right), where it merges with the cone-shaped glow of the zodiacal light. The celestial scene appears above Lake Namtso in Tibet, China. Considered one of Tibet’s three holy lakes, Namtso is famous for its high altitude: 15,486 feet (4,720 meters).

Jeff Dai
Friday, September 20
Any clear evening this week is a good time to explore the constellation Sagittarius the Archer. This star group lies due south and at peak altitude near the end of twilight and remains conspicuous until after 10 p.m. local daylight time. The brightest stars within the constellation form a distinctive asterism in the shape of a teapot. The central regions of the Milky Way pass through Sagittarius, so it’s always worth exploring this region through binoculars or a telescope.

Last Quarter Moon
The photographer captured the waning Moon just 1 hour and 6 minutes short of being perfectly half-lit. Observers have a chance to see their own Last Quarter Moon the night of September 21/22.
Victor C. Rogus
Saturday, September 21
Last Quarter Moon arrives at 10:41 p.m. EDT. It rises about 20 minutes before midnight local daylight time and will climb high in the southeast as dawn approaches. During this period, our half-lit satellite slides eastward along the border that separates the constellations Orion the Hunter and Gemini the Twins.

Sunday, September 22
Although autumn arrives with the equinox tomorrow, the Summer Triangle remains prominent in the evening sky. Look high in the west after darkness falls and your eyes will fall on the brilliant star Vega in the constellation Lyra the Harp. At magnitude 0.0, Vega is the brightest member of the triangle. The second-brightest star, magnitude 0.8 Altair in Aquila the Eagle, lies some 35° southeast of Vega. The asterism’s dimmest member, magnitude 1.3 Deneb in Cygnus the Swan, stands about 25° east-northeast of Vega. For observers at mid-northern latitudes, Deneb passes through the zenith at approximately 9:30 p.m. local daylight time, about an hour after twilight ends.

Monday, September 23
With the days growing shorter and kids back in school, it shouldn’t be surprising that summer’s reign is just about over. The warmest season comes to an official close at 3:50 a.m. EDT, when Earth reaches the autumnal equinox. This marks the moment when the Sun crosses the celestial equator traveling south. Our star rises due east and sets due west today. If the Sun were a point of light and Earth had no atmosphere, everyone would get 12 hours of sunlight and 12 hours of darkness. But the air and finite size of our star make today a few minutes longer than 12 hours.

Tuesday, September 24
Jupiter continues to dominate the evening sky from its perch in southern Ophiuchus the Serpent-bearer. The giant planet shines at magnitude –2.1 and stands some 20° above the southwestern horizon as twilight fades to darkness. When viewed through a telescope, the planet shows a 36″-diameter disk with striking details in its dynamic atmosphere. You also should see four bright points of light arrayed around the planet: the Galilean moons Io, Europa, Ganymede, and Callisto.

Wednesday, September 25
Observers of the outer solar system can get good views of Uranus during the late evening hours. The ice giant planet rises around 8:30 p.m. local daylight time and climbs nearly halfway to the zenith in the southeastern sky by midnight. It reaches its peak some 60° above the southern horizon around 3 a.m. Uranus glows at magnitude 5.7 against the backdrop of southern Aries the Ram. Use binoculars to find the planet 2.3° south of the similarly bright star 19 Arietis. A telescope reveals Uranus’ blue-green disk, which spans 3.7″.

The ringed planet shines brightly
The Hubble Space Telescope captured this view of Saturn when it was close to Earth this past summer. Observers can still get marvelous views of the ringed world through backyard scopes.
NASA/ESA/A. Simon (GSFC)/M.H. Wong (UC, Berkeley)
Thursday, September 26
Although magnificent Saturn reached its peak more than two months ago, our view of the ringed planet remains spectacular. It resides among the background stars of Sagittarius the Archer, a region that appears highest in the south during the latter stages of twilight and doesn’t set until after midnight local daylight time. Saturn continues to shine brightly, too, at magnitude 0.5. When viewed through a telescope, the planet’s disk measures 17″ across while the dramatic ring system spans 38″ and tilts 25° to our line of sight.

Friday, September 27
Neptune appeared at its best at opposition earlier this month, but its visibility hardly suffers this week. The outermost major planet lies in the southeastern sky once darkness falls and climbs highest in the south around midnight local daylight time. Neptune glows at magnitude 7.8, which is bright enough to spot through binoculars if you know where to look. The trick is to find the 4th-magnitude star Phi (φ) Aquarii, which lies about 15° (two binocular fields) east-southeast of Aquarius’ distinctive Water Jar asterism. Tonight, Neptune appears just 0.6° west-southwest of Phi. When viewed through a telescope, the ice giant planet shows a blue-gray disk measuring 2.4″ across.

The Moon reaches perigee, the closest point in its orbit around Earth, at 10:24 p.m. EDT. It then lies 222,328 miles (357,802 kilometers) away from us.

Saturday, September 28
New Moon occurs at 2:26 p.m. EDT. At its New phase, the Moon crosses the sky with the Sun and so remains hidden by our star. Because the Moon reached its closest point to Earth yesterday evening, residents in coastal areas can expect higher than normal tides for the next few days.

The Moon’s absence from the morning sky these next two weeks provides observers with an excellent opportunity to view the zodiacal light. From the Northern Hemisphere, the time around the autumnal equinox is the best for viewing the elusive glow before sunrise. It appears slightly fainter than the Milky Way, so you’ll need a clear moonless sky and an observing site located far from the city. Look for a cone-shaped glow that points nearly straight up from the eastern horizon shortly before morning twilight begins (around 5:30 a.m. local daylight time at mid-northern latitudes). The Moon remains out of the morning sky until October 12, when the waxing gibbous returns and overwhelms the much fainter zodiacal light.

Sunday, September 29
The variable star Algol in Perseus reaches minimum brightness at 12:34 a.m. EDT tomorrow morning. Observers on the East Coast who start watching around midevening can see the star’s brightness diminish by 70 percent over the course of about five hours. Those in western North America will see Algol brighten noticeably from late evening until dawn starts to paint the sky, when the star stands high in the west. This eclipsing binary system runs through a cycle from minimum (magnitude 3.4) to maximum (magnitude 2.1) and back every 2.87 days.