Friday, September 15
Eridanus is among the largest and oldest of our constellations, dating originally to the Babylonians, and included in 2nd Century CE astronomer Ptolemy's 48 constellations in his Almagest. It is dim and wandering north to south, terminating at its southern end with its Alpha star magnitude 0.5 Achernar. With a notable planetary nebula, galaxies and fine double stars, there is plenty to see. Bordering constellations include (1) Taurus, (2) Orion, (3) Lepus, (4) Caelum, (5) Horologium, (not shown: Hydrus, Tucana,) (6) Phoenix, (7) Fornax and (8) Cetus.

Skill Level: Beginner

Suggested Gear: Lawnchair and naked eyes, no equipment needed

Tomorrow Morning: Cleopatra's Eye Planetary

Saturday, September 16
NGC 1535 in Eridanus is an excellent planetary nebula, easy to find using (0) Omicron and (G) Gamma Eradini, discovered in 1785 by William Herschel. At an apparent magnitude of 10.5 and only 0.6' magnitude, you should easily note its blue-green color (similar to the Eskimo Nebula in Gemini) and under high power, its annular inner shell. This section of Randolph Jay's sketch is what to expect.

Skill Level: Beginner

Suggested Gear: Orion SkyQuest XT8 Classic Dobsonian Telescope, Orion AstroView 6 Equatorial Reflector Telescope

Tomorrow Morning: Double Star 32 Eradini

Sunday, September 17
If you visited NGC 1535 yesterday, finding double star 32 Eradini will an easy hop again using (0) Omicron and (G) Gamma Eradini angling west to the apparent magnitude 4.4 target. The primary shines at magnitude 4.8 an easy 6.9 arc-seconds apparent separation from its magnitude 5.89 secondary at PA 349. Their first measured separation was 4.3 arc-seconds in 1781, so it has increased but at 313 light years it is still uncertain if this is a line-of-sight pair or binary.

Skill Level: Beginner

Suggested Gear: Orion Observer 134mm Equatorial Reflector Telescope, Orion SpaceProbe 130ST Equatorial Reflector Telescope

Tomorrow Morning: Venus Shines!

Monday, September 18
If you're up before sunrise this morning don't miss out on Venus at its brightest! At 26% illumination, you'll see enjoy its distinct large 38 arc-second crescent phase viewing in a telescope, and a searing magnitude of -4.54. Compare its brilliance with Leo's brightest star Regulus, at magnitude -0.09 and if you're lucky, Mercury low over the horizon shining at -1.42. Watch for Mercury in five days, when it too will be at its highest.

Skill Level: Beginner

Suggested Gear: Orion 2x54 Ultra Wide Angle Binoculars, Orion SkyLine Deluxe Green Laser Pointer

Tomorrow Evening: Io's Shadow, GRS

Tuesday, September 19
Io shadow and the Great Red Spot (GRS) are the show tonight on Jupiter. The event will be underway as the planet rises over the east-northeastern horizon, with Io ready to egress the disk by 21:30 while its shadow encroaches on the GRS, both now past the meridian. If you can watch for another hour, Io will be actually within the GRS in the middle of the southern equatorial belt! Give it another hour and watch the GRS spin out of view just as Io pops off the leading limb.

Skill Level: Beginner

Suggested Gear: Orion SkyQuest XT8 Classic Dobsonian Telescope, Orion AstroView 6 Equatorial Reflector Telescope

Tomorrow Evening: Moon Targets

Wednesday, September 20
At the September Solstice, the Moon sits low in the south. Tonight it is a 32% illuminated waxing crescent at 6 days. Your 10X binoculars will show the walled plain (1) Maurolycus on the Terminator in the south, 69 miles diameter with very high walls and off-center mountain. (2) Aristoteles in the north with 50mm shows the crater's very high terraced walls and flat floor in a 53 mile diameter formation. Use 100mm to observe (3) Rima Ariadaeus' large 133 mile east-west rille. And with 200mm view (4) Rimae Daniell's four large 121 mile long rilles.

Skill Level: Beginner

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

Tomorrow Morning: Galaxy NGC 1300

Thursday, September 21
Eridanus contains the second largest documented "supervoid" in our skies, lacking galaxies. But NGC 1300 near the star pair (T) Tau and 15 Eradini is a spectacular exception. A barred spiral with a bright core, as shown here in Peter Kiss' fine sketch, it measures a large 6.2x4.1 arc-minutes apparent size while having an apparent magnitude of 11.4. But note its dimmer surface brightness of 13.8. Hop to it with help from (R) Rigel in Orion, (M) Menkar in Cetus, (A) Acamar, low in Eridanus.

Skill Level: Beginner

Suggested Gear: Orion SkyLine 12" Dobsonian Reflector Telescope, Orion SkyQuest XT10 Classic Dobsonian Telescope

Tomorrow Evening: Moon Targets

Friday, September 22
The First Quarter Moon is up tonight, at 8.6 days and 53% illumination. Use your 10X binoculars for walled plain (1) Ptolemaeus' 93 mile diameter, featuring a large flat floor and high walls. With 50mm try crater (2) Archimedes, west of Montes Apennines. At 50 miles diameter enjoy the large flat floor and terraces high walls. Next to Archimedes in 100mm is (3) Spurr, an 8 miles diameter ghost crater completely filled with lava and destroyed to the north. 300mm gets views of diminutive (4) Rima Birt, north-south oriented with a sharp small central S curve!

Skill Level: Beginner

Suggested Gear: Orion 10x50 WA Binoculars, Orion SkyLine 12" Dobsonian Reflector Telescope

Tomorrow Evening: Mercury Highest At Equinox!

Saturday, September 23
Mercury is at its highest this morning, showing us the Ecliptic is very close to East, as we would expect on an equinox. And today is later equinox of the year, autumn to some, to others spring! What you call it depends on which side of the equator you are on at the time! What doesn't depend on your location is the fine view of the inner planets bright and high, in the brightening morning sky. Next such demarcation, end of year Solstice one quarter orbit away.

Skill Level: Beginner

Suggested Gear: Orion 2x54 Ultra Wide Angle Binoculars, Orion SkyLine Deluxe Green Laser Pointer

Tomorrow Evening: More Mooning!

Sunday, September 24
At 75% illumination, tonight's Moon is waxing gibbous and 10 days old. A 10X binocular shows beautiful (1) Sinus Iridum, with waves of crest breaking over dark lava. 50mm showing (2) Crater Hippalus' 35 mile damaged diameter on the southeast shore of Mare Humorum. With 100mm volcanic (3) Dome Milichius is isolated toward the Terminator, and 200mm shows the "claw marks" of arching (4) Rimae Hippalus.

Skill Level: Beginner

Suggested Gear: Orion 10x50 WA Binoculars, Orion SkyQuest XT8 Classic Dobsonian Telescope

Tomorrow Morning: Io Shadow and GRS

Monday, September 25
Is your telescope outside early this morning? Try for this great view of Jupiter sporting a shadow transit by fleet moon Io, along with the Great Red Spot (GRS), and an ingress on the disk by the moon all in short order! The fun actually starts around 04:00 PDT with Io's shadow appearing simultaneously with the front edge of the GRS, then rapidly beginning to separate with the spot preceding. Io will become a dimple on the trailing limb by 04:54, disappearing into the disk and sitting in front of the GRS by 05:17 as daylight encroaches.

Skill Level: Beginner

Suggested Gear: Orion Observer 90mm Equatorial Refractor Telescope, Orion SkyLine 12" Dobsonian Reflector Telescope

Tomorrow: Carbon Star SY Eri

Tuesday, September 26
Eridanus contains an easy to find carbon star close to two bright naked-eye stars; (R) Rigel in Orion and (B) Beta Eradini. The two insets show 10 and 5 degree fields of view, the lower inset containing a pointer to SY Eradini. Some sources put SY at a maximum magnitude 8.3 with a magnitude 10 midpoint. Others show 10.4 to 11.4. As it is such an easy location, it is worth a quick hop to see how red it is, and compare its apparent brightness to nearby stars and determine which data is correct.

Skill Level: Beginner

Suggested Gear: Orion Observer 134mm Equatorial Reflector Telescope, Orion SpaceProbe 130ST Equatorial Reflector Telescope

Tomorrow Evening: Big Moon Targets

Wednesday, September 27
With the Moon at 97.6% illumination let's look at some limb targets. 10X binoculars show the round feature (1) Grimaldi, 134 miles diameter and filled with dark lava. 50mm instruments show 35 mile diameter crater (2) Cavalarius with high terraced walls and central mountain. (3) Rimae Sirsalis in 200mm is a stunning 182 mile long north-south system of rilles. With 300mm check Rimae Grimaldi's system of rilles to the dark features southeast.

Skill Level: Beginner

Suggested Gear: Orion 10x50 WA Binoculars, Orion SkyLine 12" Dobsonian Reflector Telescope

Tomorrow Morning: Double Star 55 Eradini

Thursday, September 28
55 Eradini is a fine double star for small telescopes, with a nice 9.3 arc-seconds separation of near equal magnitude stars, at 6.74 and 6.78. These two are thought to be distant at over 2100 light years, and there has been very little apparent change in PA or separation, leaving them likely a line of sight pair.

Skill Level: Beginner

Suggested Gear: Orion SkyScanner BL102mm TableTop Reflector Telescope, Orion StarBlast 6 Astro Reflector Telescope

Tomorrow Evening: September's Full Moon

Charts from Starry Night Pro. NGC 1535 sketch courtesy Randolph Jay. 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.