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The Night Sky Tonight: September 17 – September 29

By Mark Wagner

Mark Wagner brings us highlights of what's happening in the sky each night this week. Click on each image to enlarge the view. Happy gazing!

Friday, September 17
The Moon is waxing gibbous phase at 91% illumination. Use 10X Binoculars to find (1) Gassendi, the 67 mile diameter walled plain on the north shore of Mare Humorus. A 50mm shows (2) Crater Aristarchus, 24 miles diameter and 9,100 feet in height, notably visible in Earthshine. (3) Dome Marian T requires 100mm instruments. It is a small isolated volcanic dome with summit crater, 6 miles in diameter. Return to Gassendi using 200mm to view (4) Rimae Gassendi, a 91x2 mile system of rilles in the walled plain's arena.

Skill Level: Beginner

Suggested Gear: Orion Scenix 10x50 Wide-Angle Binoculars, Orion SkyQuest XT8 Classic Dobsonian Telescope

Tomorrow Morning: Orion's M78.

Saturday, September 18
This morning before sunrise, if your skies are dark enough try finding Messier 78 (M78), an oval shaped reflection nebula just north of Orion's Belt on the way toward (A) Betelgeuse. Also known as NGC 2068, it is one of several nebulae in the immediate region. You'll note the clipped oval shape, and pair of stars involved. There is even a periodic nebula in the region, found seemingly "discovered" (again) by Jay McNeil most recently and dubbed McNeil's Nebula. M78 was discovered by Pierre Mechain in 1780.

Skill Level: Beginner

Suggested Gear: Orion SkyQuest XT8 PLUS Dobsonian Reflector Telescope, Orion SkyQuest XT10 PLUS Dobsonian Reflector Telescope

Tomorrow Evening: Jupiter Tonight.

Sunday, September 19
Tonight offers nice views of the interplay between Jupiter's moons Io and Ganymede, along with an ongoing shadow transit by Callisto. When the shadow leaves the planet's disk, watch opposite as the Great Red Spot will spin into view, with Ganymede pairing nicely by 01:00. Its great fun to watch shadow transits and seeing the moons pop onto and off of the disk of the planet. Hope for good seeing and great detail!

Skill Level: Intermediate

Suggested Gear: Orion CC8 8" f/12 Classical Cassegrain Telescope, Orion 180mm Maksutov-Cassegrain Telescope Optical Tube

Tomorrow Evening: Last Full Moon Of Summer 2021.

Monday, September 20
Tonight is the last Full Moon of Summer for 2021. Nights are growing longer for northerners, shorter for those in the south. This Full Moon is close to the autumnal equinox, making it the Full Harvest Moon. Harvest Moon is culturally significant in multiple cultures. Farmers and Native American tribes saw it a celebrated natural calendar date. In Japan there is Tsukimi tradition, having Moon viewing parties. Step outside, watch, and consider the generation of human who watched, and celebrated.

Skill Level: Beginner

Suggested Gear: Orion StarBlast II 4.5 Equatorial Reflector Telescope, Orion SpaceProbe 130ST Equatorial Reflector Telescope

Tomorrow Evening: Constellation Aquila

Tuesday, September 21
Aquila is nicely placed to cross the meridian in early evening tonight. Its (A) star Altair famous as a member of the Summer Triangle, with tonight being the last evening of summer. This constellation dates to the 2nd or 3rd century B.C., and represents the bird that carries Zeus' thunderbolts in Greco-Roman mythology. Bordering it are constellation (1) Sagitta, (2) Delphinus, (3) Aquarius, (4) Capricornus, (5) Sagittarius, (6) Scutum, (7) Serpens Cauda and (8) Hercules. Known as the graveyard of stars, Aquila contains a great number of planetary nebulae.

Skill Level: Beginner

Suggested Gear: Orion 2x54 Ultra Wide Angle Binoculars, Orion DeepMap 600 Folding Star Chart

Tomorrow Morning: Autumnal Equinox.

Wednesday, September 22
Every year we mark time with cardinal points in our planets orbit of our home star, the Sun. Today is an equinox, when day and night are of equal length. Tomorrow night grows longer in the northern hemisphere, and shorter in the south. Other cardinal points are the two solstices, and yet more exist as lesser known or what are termed cross-quarter days, between the four cardinal points. Can you name the cross-quarters, and their significance to societies?

Skill Level: Beginner

Suggested Gear: Orion SkyLine Deluxe Green Laser Pointer, Orion StarShoot Compact Astro Tracker, Deluxe Kit

Tomorrow Evening: Variable Star R Aquilae.

Thursday, September 23
Variable stars constitute non-solar system objects that we can actually witness changing. R Aquila is a prime example, a Mira type variable ranging from naked-eye at magnitude 5.5 to a dim speck with a nadir of magnitude 12.0, over a 270 day period. When first observed, its period was over 300 days! Where is it now? You can check the AAVSO (American Association of Variable Star Observers) web-page to search the data for it current brightness. Some variable are short amplitude. Nearby U Sagitta brightens and dims in only 3.38 days!

Skill Level: Beginner

Suggested Gear: Orion StarSeeker IV 80mm GoTo Refractor without Controller, Orion StarSeeker IV 130mm GoTo Reflector without Controller

Tomorrow Morning: Moon Targets.

Friday, September 24
Today's waning gibbous Moon is 88.1% illuminated at 17.5 days, close to apogee. 10X binoculars show (1) Crater Janssen in the south, damaged and circular at 115 miles diameter. With a 50mm for (2) Crater Atlas, paired with Hercules, Atlas is 53 miles diameter offering features for all instruments. Unusual (3 Crater Messier in Mare Fecunditatis is great with 100m along with Messier A. Messier is oval, Messier has a fascinating ray. Return to Crater Atlas with 200mm and view (4) Rimae Atlas, a system of rilles in a Y shape with the crater's arena.

Skill Level: Beginner

Suggested Gear: Orion Resolux 10x50 Waterproof Astronomy Binoculars, Orion StarBlast II 4.5 Equatorial Reflector Telescope

Tomorrow Evening: Action On Jupiter.

Saturday, September 25
Start watching Jupiter early in the evening to see the (GRS) Great Red Spot already on the planet's disk. By 22:00 Io will ingress with Callisto emerging from behind the planet 20 minutes later. Keep watching as just before 23:00 Io's shadow will start its transit, as the GRS is about to spin out of sight. Do you see Io at the meridian against the light colored equatorial belt? Use high magnification and hope for steady skies to view all these events in detail. Sharp views of Jupiter at high power can be unforgettable!

Skill Level: Intermediate

Suggested Gear: Orion StarSeeker IV 127mm GoTo Mak-Cass without Controller, Orion StarSeeker IV 150mm Mak-Cass GoTo without Controller

Tomorrow Morning: More Moon Targets.

Sunday, September 26
This morning's waning gibbous Moon is at 19.51 days with 72.3% illumination. Use 10X binoculars to view (1) Rupes Altai, a 291 mile long southwest to northeast 3000 foot high cliff. With 50mm (2) Crater Aristoteles shows 11,000 foot high circular walls defining its 53 mile diameter. (3) Catena Albufeda is an interesting chain of craterlets west of Rupes Altai, visible in 100mm instruments. With 200mm try (4) Rimae Hypatia, the 109 mile long rille on the shores of Mare Tranquilitatis.

Skill Level: Beginner

Suggested Gear: Orion UltraView 10x50 Wide-Angle Binoculars, Orion SpaceProbe 130ST Equatorial Reflector Telescope

Tomorrow Early Morning: Europa Shadow Transit.

Monday, September 27
As midnight passed this morning a terrific shadow transit crossed the face of Jupiter followed by the (GRS) Great Red Spot spinning into view. The transit began just before midnight, with Europa leaving the planet's disk at approximately 00:52. As Europa was leaving, the GRS was entering at 00:41. As all this was starting, two moon closely passed each other, Io and Ganymede, at 00:10. While watching for such events in the eyepiece, you can scan for festoons and barges in the belts and bands. So much to see!

Skill Level: Intermediate

Suggested Gear: Orion 180mm Maksutov-Cassegrain Telescope Optical Tube, Orion StarSeeker IV 150mm Mak-Cass GoTo without Controller

Tomorrow Evening: A Ring Nebula In Aquila.

Tuesday, September 28
NGC 6751 is an easy to find relatively bright ring shaped planetary nebula at the end of the eagle's tail in Aquila. Easy to locate near (L) Lambda Aquilae, it is known as "The Glowing Eye" and "Dandelion Puffball Nebula". Hubble Space Telescope images of this object are spectacular. It sits 6500 light years distant shining at magnitude 11.9, so you'll need darker skies to observe it. Use a narrow band filter such as the Orion Ultrablock to enhance contrast. This planetary is powered by a Wolf-Rayet central star.

Skill Level: Beginner

Suggested Gear: 2" Orion UltraBlock Narrowband Eyepiece Filter, Orion SkyQuest XT10 Classic Dobsonian Telescope

Tomorrow Morning: Moon Targets.

Wednesday, September 29
Today's waning crescent Moon is a day past last quarter, 45% illuminated at 22.5 days. Use your 10X binoculars on spectacular (1) Clavius near the south pole, enjoying the internal arced chain of craters. (2) Copernicus in 50mm instruments is tremendous showing terraces on its walls and three central mountains. Nearby (3) Stadius T is an amazing north-south chain of overlapping craterlets best in 100mm or larger. (4) Rima Birth in 200mm or larger is always great, next to the Straight Wall.

Skill Level: Beginner

Suggested Gear: Orion StarSeeker IV 80mm GoTo Refractor without Controller, Orion StarSeeker IV 114mm GoTo Reflector without Controller

Tomorrow Evening: Tight Double Star Pi Aquilae.

Thursday, September 30
Pi Aquilae is an easy to located tight double star near (A) Altair in the constellation Aquila. You will need to get to over 200X to split it, but it is a terrific near equal pair at magnitudes 6.34 and 6.75 with a separation of 1.4 arc-seconds. Under dark skies you can see this as a single star without optical aid. The pair sit 515 light years distant. In the sketch, a third member at magnitude 12.9 is shown.

Skill Level: Beginner

Suggested Gear: Orion StarSeeker IV 102mm GoTo Mak-Cass without Controller, Orion 8" f/4 Newtonian Reflector Astrograph

Tomorrow Morning: Moon Targets.




Charts from Starry Night Pro. Equinox photo courtesy NASA. Pi Aquilae sketch from Voyager 1 on Cloudy Nights. Lunar images from NASA LRO, Other images from Virtual Moon Atlas and Starry Night Pro.

Mark Wagner is a lifelong astronomy enthusiast and deep sky observer in the San Francisco bay area. Visit our Facebook Page if you'd like to post comments, questions, sketches or images you've taken to our Night Sky Tonight post.