Skylights: September 2025

Sun

The length of daylight is now drastically less than it was just a few weeks ago, and it is decreasing noticeably each week. September 3 is our last day with 13 hours of sunlight. Days will be shorter than 13 hours until April 8.

The last sunset in the 7:00pm hour is at 7:01pm on the 11th. The next 7:00pm sunset will be March 22nd.

After traversing Leo for the past 37 days, the Sun enters Virgo on the 16th, where it will spend the longest duration in any constellation, 44.5 days.

The equinox occurs at 2:19pm on the 22nd. At that moment, the Sun lies directly overhead at Earth’s equator, and from a geocentric reference point, the declination of the Sun is 0°. It then moves south of the equator, and remains there until March 20th.

The date of true equinox, that is, equal parts of day and night, comes three days later, on the 25th, when we experience 12 hours of daylight. The difference is due largely to atmospheric refraction of light, and the Sun’s apparent diameter of about one-half degree.

Moon

Watch the 96.9% waxing gibbous Moon occult magnitude 2.9 Deneb Algedi (delta Capricorni) for 64 minutes, beginning at 2:06am on the 6th.

September 7 sees the Full Corn Moon, at 2:09pm. September usually gives us the Harvest Moon, perhaps the most familiar of the traditional full Moon names. But why, in 2025, is September’s full Moon the Corn Moon?

The reason lies in the definition of the Harvest Moon being the full Moon that is closest to the September equinox, with no distinction of whether it falls before or after. This month’s full Moon occurs 15 days and 10 minutes before equinox, and October’s lands 14 days, 9 hours and 39 minutes after, giving October the Harvest Moon in 2025.

For this reason, we have an additional named full Moon in September, the Corn Moon, so named, much like the Harvest Moon, to coincide with the time to harvest corn in the northern United States. 

Watch the Corn Moon rise at 7:11pm on the 7th, just three minutes past sunset. It sets 12 hours and one minute later, on the morning of the 8th, 53 minutes past sunrise.

You may notice a pale yellow “star” just to the east of the Moon on this night. This is our best ringed planet Saturn. Its proximity to the full Moon tells us that it is near its opposition, when it is at its closest and brightest for the year.

This Corn Moon also produces a total lunar eclipse, which, unfortunately for us, is visible just about everywhere on Earth except in the Americas. The next total lunar eclipse we will experience is on March 3, 2026.

The Moon, as it enters its waning gibbous phase, remains close to Neptune and Saturn as it rises on the 8th.

Late evening on the 12th, the waning gibbous Moon can be seen 2.5° east-northeast of the Pleiades cluster in Taurus. A spectacular occultation of the Pleiades occurs for observers in the eastern hemisphere, but for us, the Moon rises three hours after it concludes. With binoculars, you can find Uranus in the same field of view, 4.9° to Moon’s the south-southwest.

The Moon is last quarter in Aries, at 6:33am on the 14th. Before the onset of morning twilight, the Moon can be seen near Elnath (beta Tauri).

Just after midnight on the 15th, the Moon is 3.3° north-northeast of M35, an open cluster in Gemini. The next day, the waning crescent is 4.1° north of Jupiter.

The waning crescent Moon joins Jupiter and the twins Castor and Pollux on the 16th. 

On the 19th, don’t miss the spectacular arrangement of the 5.7% illuminated crescent in a line with Venus and Regulus, all within a 1.3° low power telescope field of view. The Moon rises at 4:02am, and the entire arrangement can be seen through twilight, with the best linear arrangement occurring around 5:30am, and the Moon’s southern limb closest to Venus, 0.3°, at 6:30am.

The next morning, the 20th, watch a very old (just 35 hours from new), 1.9% illuminated Moon rise at 5:02am. The remarkably thin crescent rises just before the start of astronomical twilight from an azimuth of 79.6°, about 10.3° to the 6:30 position of Jupiter.

The Moon is new at 3:54pm on the 21st, beginning Lunation 1270.

The 9.0% waxing crescent Moon makes an appearance near Mars after sunset on the 24th. Find the Red Planet 6.4° to the 2:30 position from the Moon. Also take note of the angle of the illuminated portion of the Moon with respect to the horizon. The cusps of the crescent are close to vertical. This is due to the low angle of the ecliptic on our southwestern horizon during autumn evenings, combined with the Moon’s own offset angle, being near its most southerly position.

The waxing crescent Moon passes 2.7° southeast of Antares, in Scorpius, on the 27th. First  quarter occurs at 7:54pm on the 29th, in Sagittarius, the most southerly first quarter Moon of the year.

Mercury

September is not a particularly good month for viewing Mercury.

You may be able to spot it just 1.2° north of Regulus on the 2nd, when it rises just under an hour before sunrise. 

Mercury reaches superior conjunction on the 13th, and then it returns to the evening sky for its most unfavorable apparition of the year. Later in the month, its nightly motion has it moving towards the south, parallel to the horizon, yet attaining no significant elevation, making observing the innermost planet difficult as it sets within a half-hour of sunset over the next few weeks.

Venus

Venus spends the first few days of the month within a binocular field of view of the Beehive cluster, M44, in Cancer. It is closest on the 1st, when it is just 1.2° south of the open cluster.

Look at Venus with binoculars or a telescope on the morning of the 2nd, when it will be just 0.2° northeast of Asellus Australis (delta Cancri), a 4.0 magnitude type K giant star that lies 131 light years distant. The eight step difference in apparent magnitude between Venus and the star represents a difference of nearly 1500 in intrinsic brightness.

The brilliant planet crosses into Leo on the 9th, and appears 1.2° closer to Regulus each morning until the 21st, when it is joined by the 5.7% crescent Moon 0.3° to its north, while simultaneously passing just 0.6° north of Leo’s brightest star.

Later in the month, Venus rises 2½ minutes later each morning, and is appearing noticeably lower in the sky before sunrise than it was just a few short weeks ago.

Through a telescope, Venus shows a diminishing size as it moves towards the opposite side of its orbit relative to Earth, and its illuminated fraction gradually increases from 84% to 91% through September.

Mars

Mars is low in the west after sunset, in Virgo. It becomes increasingly difficult to observe, though not completely out of view, as the month progresses.

Mars passes Spica on the 13th, coming to within 2.2° to the north-northeast of the brightest star in Virgo, but both objects will be difficult to observe without binoculars and a clear western horizon after sunset.

The Red Planet remains above the horizon for just an hour after sunset during the latter half of the month, setting just outside of nautical twilight. The waxing crescent Moon points the way to the Red Planet, just 6.4° to its southeast, on the 24th.

Jupiter

Jupiter is in Gemini, and rises during the early morning hours. On the 6th and 7th, it passes within 0.2° of magnitude 3.5 Wasat (delta Geminorum).

The waning crescent Moon passes 4.1° north of Jupiter on the morning of the 16th.

Some interesting Galilean moon events to observe: 

The Galilean moons are arranged in order of their orbital distance from the planet, and spaced approximately equally, to the west of the planet on the 17th. On the following morning, Jupiter appears to have just two moons, as both Europa and Ganymede are behind the planet and in its shadow.

On the 25th, watch Ganymede emerge from eclipse at 3:24am. Europa then goes into eclipse at 4:22am, as Ganymede goes into occultation behind Jupiter at 5:14am.

Io goes into eclipse at 3:38am on the 26th. 

Both Io and Europa, as well as their shadows, transit Jupiter’s globe on the 27th. Europa’s shadow is already on the cloudtops as Jupiter rises at 12:28am, so it will be difficult to see. Io’s shadow begins transiting at 12:54am. Europa begins transiting at 1:26am, and its shadow ends its transit at 1:40am. At 2:10am, Io begins transiting, just as its shadow is about at the central meridian of Jupiter. Io’s shadow begins to depart the eastern limb of the planet at 3:10am, and Europa and Io conclude their transits at 4:14am and 4:23am, respectively. At 4:59am, Io begins to overtake Europa, while being partially eclipsed by it. This partial eclipse ends about 20 minutes later.

Jupiter is 5.801 au from Earth and is located within Gemini. Its 99.5% illuminated disk is 34.0 arcseconds across, and shines at magnitude -2.0. Jupiter rises at 2:07am.

Saturn

Saturn is at its best in September. Located in Pisces, the ringed planet is visible all night. 

Saturn’s opposition is on the 21st, at a distance of 8.547 au. Observing Saturn on this night, and for several nights before and after, will show the planet and rings devoid of any shadows. This will look especially interesting this year, as the ring plane angle is just 2.2° relative to our line of sight.The relative tilt continues to become more narrow until November 24, when it will be just 0.5°.

As it is moving retrograde, Saturn retreats from Pisces and back into Aquarius on the 29th. It remains within the celestial water bearer until mid-January..

September continues to provide some notable events involving Saturn’s largest moon, Titan.

A shadow transit of Titan begins just after 1:00am on the 4th. A week later, the moon goes into eclipse beginning at 11:14pm on the 11th. It begins to emerge from the southwestern limb of the planet at 3:31am.

Following another week, just after 1:30am on the 19th, Titan’s shadow transits Saturn’s northern hemisphere, and finally, the moon is occulted by Saturn on the 27th, beginning at 9:38pm. It reemerges from the thin shadow just beyond the planet’s southwestern limb beginning at 2:05am.

With Saturn’s opposition, combined with having a bit less light coming from the planet’s rings, now is a good time to observe some of the planet’s other moons.

Tethys, Dione, and Rhea can be seen relatively easily on a dark, transparent night, using a telescope as small as 6 inches, but Mimas and Enceladus can be a bit more of a challenge, due to their smaller size, making them dimmer, plus their proximity to the planet.

We’ll start with Enceladus, which at 500 kilometers is comparatively small, but also one of the brightest objects in the solar system, as its surface is covered with ice. At its closest, over the next few weeks, it shines at magnitude 11.8, and extends to a maximum elongation of 39 arcseconds on either side of Saturn, which occurs twice during its 33-hour orbit.

Mimas, with an orbital period of just under 23 hours, shines at magnitude 12.9, and is elongated as much as 30 arcseconds from Saturn. A 10-inch telescope with high magnification on a steady night should reveal the 400-kilometer moon, or use a well-calibrated planetary imaging setup.

Uranus

Uranus, at magnitude 5.7, is an easy-to-locate object within the same binocular field of view as the Pleiades cluster in Taurus. From Atlas, the easternmost of the brightest member stars, look 4.2° to the south-southeast to see the pale blue-green glow of Uranus, about 19 au distant.

Uranus becomes stationary in its apparent motion on the 17th, and will continue moving westward through February 4.

On the 23rd, the 64.5% waning gibbous Moon lies 2.6° to the east of the Pleiades and 4.9° north of Uranus, placing the three objects within the same binocular field of view.

Neptune

Neptune, like Saturn, is moving retrograde through Pisces, and reaches opposition on the 23rd, just two days after Saturn. The two planets can be seen within the same wide field of view of small telescopes throughout the month, ranging in apparent separation from 1.8° on the 1st to 3.0° on the 30th, as Saturn moves westerly at a faster rate than Neptune, due to its closer position relative to Earth.

Neptune’s opposition on the 23rd brings it to its closest distance at 28.884 au. It shines at  magnitude 7.7, and shows a tiny globe just 2.4 arcseconds across through large telescopes. It can be found in Pisces, 2.4° northeast of Saturn.

Neptune being closest to Earth is also a good time to view its largest moon, Triton. At ¾ the size of Earth’s Moon, Triton orbits Neptune at a distance of 355,000 kilometers, just slightly less than the Earth-Moon distance. This puts it at a maximum separation from Neptune, near opposition, at 17 arcseconds. Its orbital period of 5.8 days puts it in favorable viewing position approximately every three nights, although its orbital inclination around Neptune, and the Neptune system’s orientation with respect to Earth, currently allows continuous visibility, with the large moon not getting closer than 9 arcseconds from the planet. GIven that its magnitude of 13.4 is approximately 1/200th the brightness of Neptune, it is still best to look for it at its maximum elongations, which occur on the nights of the 14th-15th, 17th-18th, 20th-21st, 23rd-24th, 26th-27th, and the 29th-30th. After that, bright moonlight begins to interfere with observations.

Minor Planets

As the bright Moon moves out of the late evening sky past mid-month, an opportunity opens to observe the two closest dwarf planets.

Although not the closest, we’ll start at Pluto since it is closer to the meridian after dusk. Starting at Dabih, and moving southward 8.9°, the magnitude 14.5 object should be detectable in a large telescope, or with an astroimaging setup.

Pluto is 34.733 au from Earth and is located within Capricornus. Its 100.0% illuminated disk is 0.1 arcseconds across, and shines at magnitude 14.5. Pluto sets at 01:51am.

Ceres is located in Cetus, about 18.0° east-southeast of Saturn. It is fairly close to eta Ceti, coming within 0.7° to the north-northwest of the magnitude 3.5 star on the 21st. Ceres continues to move west-southwestward at about 0.2° per day.

Asteroid 4 Vesta, the brightest of the asteroids, crosses into Scorpius on the 20th. Shining at magnitude 7.7, Vesta is within a few arcminutes of magnitude 4.0 Jabbah (nu Scorpii) on the 24th. Jabbah itself deserves attention as it is not only listed on the Astronomical League’s Double Star Observing Program, but it is a multiple star system, with four components resolvable in amateur telescopes, not unlike the more familiar Double Double in Lyra. Jabbah’s pairs are separated by 41 arcseconds, with the brighter pair separated by a challenging 1.3 arcseconds, and the dimmer one at 2.4 arcseconds. Jabbah’s low elevation in our sky means that splitting them requires the steadiest of skies. 

Asteroid 2 Pallas, which was at opposition in early August, can be seen moving southward through Dephinus in early September, crossing into eastern Aquila on the 7th.. At the start of September, it is about midway between Altair, to its west, and the globular cluster Caldwell 47 (NGC 6934), to its east in Delphinus. The asteroid is 2.57 au away and shines at magnitude 9.5.

Asteroid 6 Hebe, which reached opposition in late August, is magnitude 7.7, in Pisces. On the 5th and 6th, it passes 1.2° to the west-northwest of Caldwell 63 (NGC 7293), the Helix planetary nebula. The asteroid continues to move south-southwestward at about 0.3° per day.

Stars

September’s night skies offer us a near perfect combination of conditions to make this one of the most ideal times of year for skywatching. Darkness sets in early enough to get several hours of observing before midnight. The summer Milky Way is in prime viewing position immediately following twilight, arcing high overhead, offering some of the best sights during those early evening hours, and the lower humidity and cooler nights give way to more transparent skies.

Looking to the west, we see the beacon star of spring, Arcturus, getting lower each passing evening, and its brilliant yellow-orange glow gives way to colorful scintillation as its 37-year-old starlight has to pass through increasingly dense and turbulent layers of our atmosphere before reaching us. To its right, in the northwest, we see the familiar Big Dipper asterism begin to attain its “cup upright” position of early autumn.

To the east of Arcturus, we find that Corona Borealis, host constellation of the still-anticipated Blaze Star, is no longer with us long into the nights, as it also sets around midnight.

To the south, the center of the Milky Way has now crossed to the west of the meridian, which brings the Cygnus portion and the Great Rift into the best viewing position. It is here that we can take a journey to a peculiar group of stars that is easy to locate and see with any optics, even on the brightest nights.

Amidst the Great Rift, a section of the Milky Way obscured by dust that follows the galactic equator through the familiar Summer Triangle, we turn to a point that lies on the line between Vega and Altair, the western segment of the triangle. About ⅘ of the distance from Altair to Vega, binoculars or a small telescope reveals a distinct pattern of ten stars between magnitude 5 and 7 that was once thought to be an open cluster.

Backyard astronomers know it by its familiar name, the Coathanger, as it is marked by a nearly straight line of six stars in an east-to-west orientation, with a hook extending southward from the midpoint of the bar. The formation is about 1.5° in length by 0.5° in height, making it easy to resolve in binoculars, and filling the field of a low-power telescope, which also reveals a myriad of fainter 8th-10th magnitude stars within and around the brighter pattern.

From a dark site, it is visible to the naked eye as an unresolved patch of light. The first recording of its existence was by Persian astronomer Abd al-Rahman al-Sufi in his catalog The Book of Fixed Stars, which was compiled in 964, over six centuries before the advent of the telescope.

Some star atlases identify it as Brocchi’s Cluster, named for Dalmiro F. Brocchi, an amateur astronomer who compiled charts for the American Association of Variable Star Observers (AAVSO) in the early 20th century.

Formally known as Collinder 399 (or Cr 399 for short), it is one of the more notable entries in Swedish astronomer Per Collinder’s catalog of 471 star clusters without a corresponding NGC designation. The Collinder catalog, compiled in 1931, also contains objects such as the Hyades and Pleiades clusters in Taurus, Orion’s Belt, and the Mirfak Cluster in Perseus.

Parallax data from ESA’s Hipparcos mission revealed that the cluster’s members are spread out much too far to have been gravitationally bound at any point. More accurate positions and parallax from the Gaia mission show that the stars range in distance from 235 to 1.734 light years, and that this “cluster” is just a chance alignment of unrelated stars, rather than an astrophysical object, something we now refer to as an asterism.

The Coathanger is a neat little object to visit whenever the Summer Triangle is visible, even when it is low in the sky. Bright and easily recognizable, it is an ideal target for public astronomy events and open nights, and be sure to show it to Francine when she visits your telescope.