Key Takeaways:
- On October 13, 2025, the Last Quarter Moon will be observed in conjunction with Jupiter starting around midnight, with Jupiter's Galilean moons visible through telescopic observation.
- Pluto reaches stationary apparent motion in Capricornus at 4 A.M. EDT on October 13, presenting a challenging target at magnitude 15.3, while the adjacent globular cluster M75 in Sagittarius (magnitude 8.5) offers another observational opportunity.
- Comet C/2025 A6 (Lemmon), noted at magnitude 5.7, will be visible in the predawn sky, located in Canes Venatici on the morning of October 15 and requiring optical aid for detection.
- The comet's coma may display a green luminescence attributed to excited diatomic carbon, with its projected peak brightness coinciding with its closest terrestrial approach on October 21.
Looking for a sky event this week? Check out our full Sky This Week column.
October 13: The Moon reaches Last Quarter
The Moon rises together with Jupiter around midnight on October 13 and are visible into the early-morning hours of October 14.
The Moon lies just to the lower left of bright Jupiter; the planet is now magnitude –2.2, the brightest point of light in that region of the sky, far outshining the stars. To Jupiter’s upper left are Castor and Pollux, the brightest stars in Gemini the Twins. Pollux is slightly brighter (magnitude 1.2) and glows a golden yellow. Castor (magnitude 1.6) is a blue-white star that easily splits into two through a telescope, revealing nearly twin stars some 5” apart.
Turning a telescope on Jupiter will show all four of its Galilean moons visible before dawn. Callisto is farthest east, with Io closest to the planet on that side and Europa between them. Ganymede lies alone to Jupiter’s west.
Pluto is stationary in Capricornus at 4 A.M. EDT. If you want to try your luck with the distant dwarf planet, Pluto is highest around 7:30 P.M. local daylight time, when it stands about 25° high in the south. It is located in southwestern Capricornus about 7° northwest of 4th-magnitude Psi (ψ) Capricorni. However, the tiny world glows an incredibly faint magnitude 15.3, so you’ll need a large scope and dark observing site to spot it. A go-to drive is your best bet, as it can take you right to the correct field. Pluto’s disk is just 0.1” wide; it will look like a very dim, “flat” star.
If you don’t have a large scope, there’s still plenty in this region of the sky to enjoy. Nearby is Sagittarius, a constellation rife with deep-sky wonders such as M75, a globular cluster right over the border from Capricornus. It is located just 3° northwest of Pluto’s position, and again a go-to scope will land you right on it. Glowing at magnitude 8.5, this ancient ball of stars covers about 6’ on the sky. It appears so faint and compact because it is quite distant — some 67,500 light-years from Earth.
Sunrise: 7:10 A.M.
Sunset: 6:21 P.M.
Moonrise: —
Moonset: 3:20 P.M.
Moon Phase: Waning crescent (36%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
Tomorrow morning, try catching Comet C/2025 A6 (Lemmon), recently observed at magnitude 5.7 and visible in the predawn sky about an hour and a half before the Sun rises.
On the morning of October 15, the comet is in Canes Venatici and reaches some 25° high an hour before sunrise. Step outside and look northeast to locate the Big Dipper, now standing on the end of its handle. To the right of the handle is Canes Venatici, and Lemmon is just under 1.5° southeast of the constellation’s beta star, 4th-magnitude Chara, this morning.
You’ll need at least binoculars to spot Lemmon. If you’re using a telescope, Astronomy Associate Editor Michael Bakich recommends starting with a low-power eyepiece and working your way up once you’ve found the comet. Taking a long sky exposure with a smartphone may also pull the comet out of the background, so give that a try if you’re familiar with the night photography settings on your phone. Deeper astrophotography using a camera attached to your telescope may show the coma’s green glow, generated by diatomic carbon, which glows green when excited by sunlight.
Comet Lemmon will be closest to Earth on the 21st and likely shine brightest at that time. You can find additional finder charts and more tips on how to observe it here.
