Skylights: October 2023

The most significant event occurring in the sky during October is a solar eclipse on the 14th. This is an annular eclipse that appears over the western states, and Rhode Island is far from the path of annularity. An annular eclipse is a special category of a partial eclipse where even if you are on the centerline of the eclipse, the Moon is too far away to block the entire disk of the Sun, thereby leaving a ring of the Sun’s photosphere visible during the midpoint of the eclipse. As it is a partial eclipse, there is no time when it is safe to view it directly without proper filters.

Although Rhode Island is quite distant from the path of annularity, we will still experience a partial eclipse during which the Moon covers about 19% of the Sun’s diameter. For us, the event begins with first contact, at 12:16pm, continues to maximum eclipse at 1:25pm, and ends at 2:34pm.

The last 6pm sunset occurs on the 17th. The next day the Sun sets after 6:00pm will be March 10.

After spending the past 45 days in Virgo, the Sun enters Libra on October 31.

Just past midnight on the 3rd, the waning gibbous Moon is just 1° south of the Pleiades. This is a spectacular view in a small telescope at low magnification.

Last Quarter Moon is on the 6th, in Gemini.

On the morning of the 7th, the waning crescent Moon is just 1.5° south of Pollux. It is then 3.5° northwest of M44 on the following morning.

The Moon is new on the 14th, bringing us the partial solar eclipse detailed above..

The Full Hunter's Moon occurs at 4:24pm EDT on the 28th. It rises at 5:38pm, which is seven minutes before sunset. The Moon is joined by Jupiter, which rises about 20 minutes later. The two are just 2.3° apart early the following morning.

You may notice, right at moonrise, that the southeastern limb of the Moon appears a bit dim. It  is because there is a partial eclipse occurring during this month’s Full Moon. While we won’t see the partial phases, it is still in the outer shadow, the penumbra, at moonrise, and continues to move out until the penumbral eclipse ends at 6:26pm EDT.

The maximum 12% partial eclipse is visible over Africa, Europe and most of Asia.  

The next partial lunar eclipse visible from Rhode Island is an 8% on September 17, 2024, and the next total lunar eclipse occurs on March 14, 2025.

The crescent Moon appears 4° east of Antares on the 18th.

Another noteworthy event involving the Moon in October occurs on International Observe the Moon Night, October 21, when the First Quarter Moon occults magnitude 4.5 59 Sagitarii at 8:13pm EDT. The star reemerges from the bright limb of the Moon, about 3 arcminutes from the southern limit of the terminator, 29 minutes later.

 The waning gibbous Moon is just 0.9° south of Alnath (beta Tauri) early in the morning of November 1.

Early October is the best time to see Mercury, as the innermost planet rises an hour before sunrise on the 1st, and is positioned nearly vertically above the horizon with respect to the Sun.

Superior conjunction occurs on the 20th, followed by a lackluster evening apparition.

Venus continues to put on its best performance in the morning sky in October. Moving eastward through Leo, the brilliant planet passes 2.3° south of Regulus on the 9th, and on the 10th, the 25.6-day crescent Moon is 5.8° north of Venus, and 3.6° north of Regulus, forming a near-perfect line in the predawn sky.

Venus is at greatest elongation on the 23rd, extending 46° west of the Sun. It is at its point of greatest elongation that the Sun-Earth-Venus geometry presents Venus in a 50% illuminated “quarter Moon” phase. From this point on, Venus will steadily decrease in angular size and become more gibbous in phase, until it passes superior conjunction in June 2024..

Earliest Venusrise occurs at 3:03am EDT on October 7.

On October 6, Earth crosses 1 AU distance from the Sun, moving towards perihelion on January 2.

Although still in the evening sky, Mars has become difficult to observe as it remains low on the horizon, and sets within 30 minutes of the Sun. The Red Planet is now about as far as it gets from Earth, at over 2.5 AU away, and its diminutive 3.7 arcsecond disc is smaller than that of Uranus.

The 1.3% illuminated, 1.1-day crescent Moon is 3.2° east-southeast of Mars on the 15th. 

Mars passes into Libra on the 24th.

Reaching opposition early next month, Jupiter is visible in the east soon after dark, and shines brightly throughout the night as it moves westward (retrograde) through Aries.

October begins with the waning gibbous Moon 2.0° to the north-northwest of Jupiter, making a striking pair as the two rise into the evening sky together.

As Earth draws towards its closest point to Jupiter, just 3.98 AU away in late October, the giant planet attains a sizable diameter of 49.5 arcseconds. This is about the largest that any planet can appear from Earth except for Venus, which, near its inferior conjunction, can appear as large as 60 arcseconds.

Jupiter is joined by the full Moon on the 29th, when at about 3:00am, the two are just 2.3° apart. With a wide-field telescope, this is a good opportunity to compare their apparent sizes. Looking at them separately, you may not have realized that Jupiter is greater than 1/40th the size of the Moon in our sky. 

There are some interesting Galilean moon events to watch this month. A transit of Ganymede occurs between 9:05pm and 10:15pm EDT on the 5th, when the moon is never fully “inside” of Jupiter’s disk. At the same time, Io’s shadow will be transiting Jupiter, followed by the moon itself a few minutes after Ganymede’s transit ends.

An almost identical series of events occurs just after midnight on the 13th, with Io and Ganymede both transiting Jupiter’s disk simultaneously. Look for the shadows of each moon before the transit, with Ganymede’s shadow crossing Jupiter’s southern polar region beginning around 10:00pm EDT on the 12th.

Finally, another double transit of Io and Ganymede takes place on the 20th, this time with Io somewhat ahead of Ganymede. These three double transits demonstrate the orbital resonances of the Galilean moons, with Ganymede making one circuit for each four of Io. 

Visible in the southeast after sunset, Saturn is at its best position for early evening observers, slowly moving westward (retrograde) through Aquarius. Now that we’re a few weeks removed from Saturn’s opposition in August, the planet-Earth-Sun angle is wide enough to once again give the ringed planet its majestic three-dimensional appearance in a telescope, as the shadow of its globe can be seen on the far side of the rings.

On the morning of the 24th, the waxing gibbous Moon passes 4.3° southwest of Saturn

Uranus, our seventh planet, is quite easy to locate in Aries. Once the Pleiades have risen, look at them with binoculars, then slowly move along a line towards Jupiter. About halfway along this line you will find a magnitude 4.4 star. This is Botein (delta Arietis), a class K2 giant star that lies 170 light years away that is about twice the mass of the Sun, and shines with 45 times its luminosity. From this star, move southward, perpendicular to the line connecting the Pleiades and Jupiter. You’ll only need to move about five Moon diameters (2.4°) until you find the magnitude 5.7 blue-green glow of Uranus, 155 light minutes (18.7 AU) away.

The Moon is 1.6° north-northwest of Uranus on the 29th, shortly after Moonrise.

Neptune is in Pisces, and is fairly easy to find, lying almost directly on a line extended from magnitude 4.1 iota Piscium through magnitude 4.5 lambda Piscium, the two stars making up the eastern segment of the Circlet asterism, and continuing southward about the same distance. You’ll see a magnitude 5.5 star, 20 Piscium. Locate Neptune within 0.5° to the west-southwest of this star.

On the 25th, the waxing gibbous Moon passes within 2.0° to the south of Neptune.

Dwarf planets & asteroids

Ceres is now in Libra, and sets soon after the Sun. It will become visible again in December.

Pluto is still in eastern Sagittarius, about 1.5° southwest of globular cluster M75. It is best viewed in early evening, when it is highest in the sky.

Vesta moves into Gemini on October 7, and begins its retrograde loop late in the month, when it brightens to magnitude 7.5.

An interesting bit of celestial coincidence occurs during early October. Because the sky is divided into 24 hours of right ascension, it can be thought of as a clock which completes one cycle for each sidereal rotation of Earth, which is approximately four minutes faster than the 24-hour mean solar day. This is the point at which stars on a given line of right ascension cross the meridian. The right ascension crossing the meridian at a particular location on Earth is known as sidereal time, although it indicates a position (pertaining to the sky) more than an actual time. 

Since sidereal time is indicated in hours, minutes, and seconds, there is one point during the year when sidereal time lines up with local time. This “sidereal synchronization” occurs on October 2 at 6:56pm (18h 56m 30s) EDT for most of Rhode Island. Your time (and date) of sidereal synchronization will vary by time zone, and to an even greater extent, where you are within your time zone. Generally, the farther east you are, the earlier the synchronization occurs, and later for observers farther west.

The Summer Triangle remains a prominent feature of the early evening sky in October, as the large and bright asterism is high overhead after evening twilight fades. But you will notice that in the south, the familiar summer star patterns of Scorpius and Sagittarius are slowly sinking towards the southwestern horizon.

The springtime beacon of Arcturus departs the western sky during the early evening hours, and the Big Dipper assumes its autumnal horizontal orientation, with the Little Dipper rotated into position that appears to be spilling its contents into the Big Dipper.

The King Cepheus lies high in the north, welcoming your exploration of its deep sky treasures. Cassiopeia and Andromeda are high in the north and east, bringing even more spectacular objects within view, including the Andromeda Galaxy.

To the southeast, Fomalhaut (alpha Piscis Austrinus) is usually a lone, bright beacon in an apparent void of stars, but it currently appears as if it’s a pillar upon which Saturn is standing, about 20° above it. 

In early October, you may notice a few meteors originating from an area near the head of Draco, about 15° northwest of Vega, as the annual Draconid shower peaks on the 8th-9th.

By mid-month, Orion will appear low in the east by 11:00pm, and don’t miss his annual meteor shower, peaking on the 20th-21st.

If you’re an early morning observer, the Winter Hexagon, consisting of the constellations Auriga, Taurus, Orion, Canis Major, Canis Minor, and Gemini, appears high in the south before twilight.