Friday, October 1 — Crescent moon passes the Beehive (predawn)
When the waning crescent moon rises over the east-northeastern horizon during the wee hours of Friday, Oct. 1, it will be positioned several finger widths above (or 4.5 degrees to the celestial northwest of) the large open star cluster in Cancer known as the Beehive Cluster and Messier 44. By the time the sky begins to brighten before dawn, the pair will be higher and the moon's orbital motion will have carried it slightly closer to the cluster. To better see the "bees", hide the moon beyond the edge of your binoculars' field of view (red circle).
Sunday, October 3 — Old moon is the Lion's Heart (predawn)
In the eastern predawn sky on Sunday, Oct. 2, the slim crescent of the old moon will resemble the smile of the Cheshire Cat. Fittingly perhaps, the moon will also temporarily become the heart of Leo, the lion since it will be positioned between the bright stars Regulus (to the moon's right) and Algieba (to its upper left). The stars that form the lion's neck and head are arranged in a fist-sized curve that extends upward from Algieba. The rest of the beast will extend downwards to the lower left (celestial east), ending at his tail star Denebola.
Monday, October 4 — Morning zodiacal light for mid-northern observers (predawn)
During autumn at mid-northern latitudes every year, the ecliptic extends nearly vertically upward from the eastern horizon before dawn. That geometry favors the appearance of the faint zodiacal light in the eastern sky for about half an hour before dawn on moonless mornings. Zodiacal light is sunlight scattered by interplanetary particles that are concentrated in the plane of the solar system — the same material that produces meteor showers. It is more readily seen in areas free of urban light pollution. Between now until the full moon on Oct. 20, look for a broad wedge of faint light extending upwards from the eastern horizon and centered on the ecliptic (the green line). It will be strongest in the lower third of the sky, around the bright star Regulus. Try taking a long exposure photograph to capture it. Don't confuse the zodiacal light with the Milky Way, which is positioned off to the south-southeast.
Wednesday, October 6 — New moon (1105 GMT)
The moon will reach its new phase on Wednesday, Oct. 6 at 7:05 a.m. EDT (1105 GMT). While new, the moon is travelling between Earth and the sun. Since sunlight can only reach the far side of the moon, and the moon is in the same region of the sky as the sun, the moon becomes unobservable from anywhere on Earth for about a day (except during a solar eclipse). On the evenings following the new moon phase, Earth's planetary partner will return to shine in the western sky after sunset.
Friday, October 8 — Dwarf planet Ceres changes direction (overnight)
On Friday, Oct. 8, the dwarf planet Ceres will cease its eastward motion across the stars of central Taurus. After tonight, Ceres will begin a westward retrograde loop (red path with dates:time) that will last until mid-January. In late evening, the magnitude 8.2 object will be located low in the eastern sky, several finger widths to the lower left (or 2.7 degrees to the celestial east) of the Bull's brightest star Aldebaran.
Saturday, October 9 — Young moon and Venus in Scorpius' claws (after sunset)
As the sky darkens after sunset on Saturday, Oct. 9, watch the southwestern sky for the pairing of the slim crescent moon shining just above (or 2 degrees to the celestial north of) very bright Venus — easily close enough for them to share your binoculars' field of view. Sharp-eyed skywatchers who can spot the moon in the late afternoon can also try to see Venus' bright speck below it in the daytime, even without binoculars! Once the sky darkens, after about 7:30 p.m. in your local time zone, the fainter claw stars of Scorpius will appear around the moon and Venus.
Monday, October 11 — Saturn stands still (evening)
On Monday, Oct. 11, Earth's faster orbit will cause Saturn to appear to stop moving with respect to the distant stars. The temporary pause in motion (red path with labeled dates:times) marks the end of a westward retrograde loop that began on May 23. After dusk, look for the yellowish, magnitude 0.5 planet in the lower part of the southern sky among the stars of western Capricornus — 15.5 degrees west of much brighter Jupiter.
Monday, October 11 — Ganymede's shadow and GRS cross Jupiter (evening)
Between 7 p.m. and 10:20 p.m. EDT on Monday, Oct. 11, observers in the Americas with telescopes can watch the large, black shadow of Jupiter's moon Ganymede cross Jupiter's disk, accompanied by the Great Red Spot. That start time corresponds to 8 p.m. CDT, 9 p.m. MDT, and 10 p.m. PDT. For observers in the western USA and Canada, only the later stages of the event will be occurring in a dark sky.
Tuesday, October 12 — First quarter moon (at 11:25 p.m. EDT)
When the moon completes the first quarter of its journey around Earth on Tuesday, Oct. 12 at 11:25 p.m. EDT (0325 GMT on Oct. 13), its 90-degree angle away from the sun will cause us to see the moon exactly half-illuminated — on its eastern side. At first quarter, the moon always rises around mid-day and sets around midnight, so it is also visible in the afternoon daytime sky. The evenings around first quarter are the best ones for seeing the lunar terrain when it is dramatically lit by low-angled sunlight, especially along the terminator, the pole-to-pole boundary between its lit and dark hemispheres.
Wednesday, October 13 — Half-moon below Saturn (evening)
The moon's monthly visit with the bright gas giant planets Saturn and Jupiter will kick off after dusk on Wednesday, Oct. 13. Before the sky has darkened, try using binoculars (red circle) to find the yellowish dot of Saturn positioned a generous palm's width to the upper left (or 7 degrees to the celestial northeast) of the half-illuminated moon. Or wait until Saturn is visible with your unaided eyes, after about 7 p.m. local time. Much brighter Jupiter will be shining off to their upper left all evening.
Thursday, October 14 — Moon between gas giant planets (evening)
After 24 hours of motion, the waxing gibbous moon will move east to sit below and between Jupiter and Saturn all evening on Thursday, Oct. 14. The trio, with much brighter and whiter Jupiter to the left (celestial east) of 16 times fainter, yellow-hued Saturn, will make a nice widefield photo when composed with some interesting foreground scenery.
Friday, October 15 — Bright moon below Jupiter (evening)
After sunset on Friday, Oct. 15, look low in the southeast for Jupiter shining a large palm's width to the upper right (or 8 degrees to the celestial northwest) of the bright, waxing gibbous moon. Somewhat fainter Saturn will become visible off to their right once the sky darkens more. The moon will bid adieu to the bright planets after tonight, until November 10-12.
Saturday, October 16 — Venus kisses Antares (evening)
In the southwestern sky on the evenings around Saturday, Oct. 16, the orbital motion of the bright planet Venus (red path with labeled date:time) will carry it closely above the bright, reddish star Antares, which marks the heart of Scorpius. The pair will be binoculars-close from Oct. 11 to 20 (red circle), but at closest approach on Saturday, they'll share the field of view in the eyepiece of a telescope at low magnification. If you have trouble seeing Antares beside 150 times brighter Venus, try hiding Venus just outside of your field of view.
Sunday, October 17 — Mare Imbrium's golden handle (all night)
On Sunday night, Oct. 17, the terminator on the waxing gibbous moon will fall west of Sinus Iridum, the Bay of Rainbows. The circular 155 mile (249 km) diameter feature is a large impact crater that was flooded by the same basalts that filled the much larger Mare Imbrium to its east — forming a rounded "handle" on the western edge of that mare. The "Golden Handle" effect is produced when sunlight strikes the prominent Montes Jura mountain range surrounding Sinus Iridum on the north and west. Sinus Iridum is almost craterless, but hosts a set of northeast-oriented wrinkle ridges that are revealed at this phase.
Monday, October 18 — Spotty Jupiter completes a retrograde loop (11:10 to 11:25 p.m. PDT)
On Monday, Oct. 18, Jupiter will appear to stop moving with respect to the distant stars of eastern Capricornus — marking the end of a westward retrograde loop that began in June. Meanwhile, starting at 11:10 p.m. PDT (0610 GMT on Tuesday, Oct. 19), observers with telescopes in western North America and the Pacific Ocean region can see the small, round, black shadows of the Galilean moons Ganymede and Io cast upon Jupiter's disk at the same time for approximately 15 minutes. Ganymede's shadow will begin to cross alone at 7:55 p.m. PDT. At the end of the double shadow transit event, Ganymede's shadow will lift off Jupiter, leaving Io's shadow to complete its solo transit at 1:25 a.m. PDT. By then, Jupiter will have set for observers in the continental USA and Canada.
Wednesday, October 20 — Full Hunter's Moon (1456 GMT)
The full moon of October, which occurs at 10:56 a.m. EDT (1456 GMT), on Wednesday, Oct. 20, is traditionally called the Hunter's Moon, Blood Moon, or Sanguine Moon. The Anishinaabe people of the Great Lakes region call this moon Binaakwe-giizis, the Falling Leaves Moon, or Mshkawji-giizis, the Freezing Moon. The Cree Nation of central Canada calls it Opimuhumowipesim, the Migrating Moon — the month when birds are migrating. The Haudenosaunee (Iroquois / Mohawk) of Eastern North America use Kentenha, the Time of Poverty Moon. Full moons in October always shine in or near the stars of Cetus and Pisces. Since it's opposite the sun on this day of the lunar month, the full moon will rise at sunset and set at sunrise.
Thursday, October 21 — Orionids meteor shower peak (predawn)
The annual Orionids meteor shower is produced when the Earth crosses through a cloud of small particles dropped by repeated passages of Comet Halley in its orbit. The shower runs from September 23 to November 27 and will peak between midnight and dawn on Thursday, Oct. 21. At that time the sky overhead will be moving directly into the densest region of the particle field, producing 10-20 fast meteors per hour. Orionids meteors can appear anywhere in the sky, but will seem to be streaking away from the constellation of Orion. Unfortunately, a very bright, waning gibbous moon will overwhelm many of the meteors on the peak night.
Thursday, October 21 — Bright moon below Uranus (all night)
In the eastern sky after dusk on Thursday night, Oct. 21, the very bright, waning gibbous moon will shine two finger-widths to the lower left (or 2.5 degrees to the celestial southeast) of magnitude 5.7 Uranus. By dawn on Friday morning, the moon's orbital motion will shift it farther from Uranus in the western sky, and the diurnal rotation of the sky will move it above the planet. While Uranus' blue-green dot can be seen in binoculars (red circle), I recommend noting its location between the stars of Aries and Cetus and seeking it out a few nights later, when the bright moon will have moved away.
Friday, October 22 — Crater Copernicus (all night)
The prominent crater Copernicus is located in eastern Oceanus Procellarum — due south of Mare Imbrium and slightly northwest of the moon's center. This 800 million-year-old impact scar is visible with unaided eyes and binoculars — but telescope views will reveal many more interesting aspects of lunar geology. Several nights before the moon reaches its full phase, Copernicus exhibits heavily terraced edges (due to slumping), an extensive ejecta blanket outside the crater rim, a complex central peak, and both smooth and rough terrain on the crater's floor. Around full moon, Copernicus' ray system, extending 500 miles (800 km) in all directions, becomes prominent. Use high magnification to look around Copernicus for small craters with bright floors and black haloes — impacts through Copernicus' white ejecta that excavated dark Oceanus Procellarum basalt and even deeper highlands anorthosite.
Monday, October 25 — Mercury at greatest western elongation (predawn)
On Monday, Oct. 25, the planet Mercury will reach a maximum angle of 18 degrees from the sun, and peak visibility for the current morning apparition. Look for the innermost planet shining brightly, very low in the east-southeastern sky between about 6:15 and 7 a.m. in your local time zone. In a telescope (inset) Mercury will exhibit a 57%-illuminated, waxing gibbous phase. Mercury's position above the nearly upright morning ecliptic (green line) will make this an excellent apparition for Northern Hemisphere observers, but a poor one for those located near the Equator, and farther south.
Monday, October 25 — Waning moon near Messier 35 (all night)
When the waning gibbous moon rises among the stars of Gemini after 9 p.m. local time on Monday, Oct. 25, it will be positioned a finger's width to the left (or 1 degree to the celestial northeast) of the large open star cluster named the Shoe-Buckle Cluster or Messier 35. The two objects will share the view in binoculars (red circle) all night long, although the moon will move farther from the cluster hour by hour. To better see the cluster, which is nearly as wide as the moon, try hiding the moon just outside the left-hand edge of your binoculars' field of view.
Monday, October 25 — Io and Ganymede Shadows Cross Jupiter (from 08:08 to 10:22 UT)
Starting late on Monday night, Oct. 25, observers with telescopes from Alaska to Hawaii and across the Pacific Ocean can watch the small black shadows of two of Jupiter's moons cross the planet's disk at the same time. At 10:08 p.m. Hawaii Standard Time (or 08:08 UT on Oct. 26), Io's small shadow will join Ganymede's larger shadow while it is already crossing. The two shadows will appear together for 2.5 hours. Meanwhile, Io's more rapidly moving shadow will catch up to and overtake Ganymede's shadow just before they both move off Jupiter at 12:22 a.m. HST, or 10:22 UT.
Thursday, October 28 — Third Quarter Moon (2005 GMT)
The moon will officially reach its third quarter phase at 4:05 p.m. EDT (2005 GMT), on Thursday, Oct. 28. At third quarter our natural satellite always appears half-illuminated, on its western side — towards the predawn sun. It rises in the middle of the night and remains visible in the southern sky all morning. The name for this phase reflects the fact that the moon has completed three-quarters of its orbit around Earth, measuring from the previous new moon. The ensuing week of moonless evening skies will be ideal for observing deep sky targets.
Friday, October 29 — Venus at greatest eastern elongation (after sunset)
On Friday, Oct. 29, Venus will officially reach its widest separation of 47 degrees east of the sun. Viewed in a telescope (inset), the planet will exhibit a half-illuminated phase. For best results, view the planet during evening twilight when the contrast between the bright planet and the surrounding sky is reduced. After today, our sister planet will continue to brighten and increase in apparent disk diameter as it swings sunward ahead of inferior conjunction in early January.
Saturday, October 30 — The Andromeda Galaxy (all night)
In October, the Andromeda Galaxy is climbing the northeastern sky during the evening. This large spiral galaxy, also designated Messier 31 and NGC 224, is 2.5 million light-years from us and covers an area of sky measuring 3 by 1 degree (or six by two full moon diameters)! Under dark skies, M31 can be seen with unaided eyes as a faint smudge located 1.4 fist diameters to the left (or 14 degrees to the celestial northeast) of Alpheratz, the star that forms the left-hand (northwestern) corner of the square of Pegasus. The three westernmost stars of Cassiopeia, Caph, Shedar and Navi (Gamma Cas), also conveniently form an arrow that points towards M31. Binoculars (red circle) will reveal the galaxy better. In a telescope, use low magnification and look for M31's two smaller companion galaxies, the foreground Messier 32 and more distant Messier 110 (inset).
Sunday, October 31 — Medusa's eye brightens (at 6:44 p.m. EDT)
Algol, also designated Beta Persei, marks the glowing eye of Medusa from Greek mythology, and is among the most accessible variable stars for skywatchers. During a ten-hour period that repeats every 2 days, 20 hours, and 49 minutes, Algol dims and re-brightens noticeably when a companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star, reducing the total light output we perceive. Algol normally shines at magnitude 2.1, similar to the nearby star Almach in Andromeda. But when fully dimmed, Algol's magnitude 3.4 is almost the same as Rho Persei (ρ Per), the star sitting just two finger widths to Algol's lower right (or 2.25 degrees to the celestial south). On Sunday, Oct. 31 at 6:44 p.m. EDT (2244 GMT), Algol will sit low in the northeastern sky and shine at its minimum brightness. Five hours later the star will return to full intensity from a perch nearly overhead in the eastern sky.