Skylights: September 2021

September is a month of transition. With the equinox approaching on the 22nd, the sky is getting darker earlier, and the summer haze and smoke from western wildfires are finally abating. Cooler nights mean fewer mosquitoes, the crickets and katydids start to quiet, and you may hear a distant owl or coyote during your skywatching sessions.

One of the most notable Full Moons of the year, the Harvest Moon, occurs on the 20th. This Harvest Moon lies near the incline of the ecliptic when it rises, opposite the Sun, which is on the decline, and therefore the interval between moonrises on successive nights is at a minimum, about 22 minutes between the 20th and the 21st. This extra light at dusk has aided farmers during the annual harvest season, hence the name Harvest Moon.

Watch the Moon throughout September as it pairs up with some familiar objects: The waning crescent is just one half degree from the star cluster M35 in Gemini on the morning of the 1st, passes 2.7° from the Beehive cluster (M44) in Cancer on the morning of the 4th, and is new on the 6th.  Early on the 7th, an opportunity to spot a one-day-young crescent occurs just after sunset, when it is 3.2° above Mars. After first quarter occurs on the 13th, the waxing gibbous passes below Jupiter and Saturn on the 16th and 17th, respectively. Finally, the waning gibbous passes 4.5° from Uranus on the morning of the 24th.. 

Mercury is in the evening sky all month, but due to the shallow angle of elongation, this remains an unfavorable apparition. Maximum elongation occurs on the 13th, with Mercury being 27° east of the Sun, and the elusive inner planet sets less than 50 minutes after sunset. During the latter part of September, Mercury approaches Spica, coming within 1.5° of the star on the 21st.

Venus begins the month near Spica, and moves nearly parallel to the southwestern horizon on successive nights, While Venus has appeared as a small gibbous for the past several months, you’ll notice its changing phase throughout September, as it morphs from a 72% gibbous at 15 arcseconds to a 62% gibbous at 19 arcseconds throughout the month. Venus sets about 90 minutes after the Sun.

After lingering around in the western twilight for the past few months, Mars has finally become hidden from view, as it reaches conjunction early next month.

Jupiter and Saturn rule the night in September. Remember how close they were just a year ago as they were approaching their historic conjunction? Now their spacing gives a good indication of Jupiter’s orbital velocity compared to Saturn, as the king of the planets leads Saturn by about 16 degrees, though due to Earth’s rotation, Jupiter appears to trail Saturn. The two gas giants also indicate the angle of the ecliptic in Capricornus, where both planets currently reside, as Jupiter is a bit higher in elevation than Saturn. At their present locations, together they are about halfway between the December solstice and vernal equinox points on the ecliptic.

Uranus rises around 10pm in mid-September in the constellation Aries. Its 6th magnitude teal glow can just be detected with the unaided eye from a clear and dark site, but it is more easily found with binoculars. It will be at opposition in early November.

While not typically thought of as an exciting destination during our planetary observation, Neptune shines at its brightest in September. At opposition on September 14th, our outermost planet is still a whopping 28.9 AU from Earth, and its blue-hued reflected sunlight takes four hours to reach us.

Although Neptune’s 2.4 arcsecond disk is barely discernible through a telescope on nights of average seeing, it is bright enough to be spotted through binoculars as a 7.9 magnitude point. Find Neptune by starting at the bottom edge of the Great Square of Pegasus, then move south to the Circlet, an asterism in Pisces consisting of 4th and 5th magnitude stars in a roughly circular pattern about 6°x5° in size. From here, travel south a bit more, about the same height as the Circlet, and look for a flattened diamond of 6th and 7th magnitude stars about 1.5° long. Neptune will be just a bit west of center in this diamond, and it progresses westward along its apparent retrograde path as the month progresses. On the 24th, it is located just 96 arcseconds below HD 221148, the westernmost and brightest star in the diamond.

Neptune doesn’t offer much visual interest in binoculars or small telescopes, but it’s fun and easy to visit our eighth planet at least once every year.

For those with a larger telescope, Neptune’s largest moon Triton can be observed, or even imaged with smaller scopes. Slightly larger than Pluto, Triton is about 13th magnitude, and is separated from Neptune by as much as 14 arcseconds, the same separation of Mizar and its companion in Ursa Major.

While it won’t be at opposition until late November, now is a great time to start watching dwarf planet Ceres, as it is traversing the Hyades cluster in Taurus over the next few months, and, through most of September, is within 2° of Aldebarn. The Hyades rises before midnight, and Ceres shines a bit brighter than 9th magnitude, so this will be a rewarding late-night binocular adventure. Located 2.6 AU from Earth at the beginning of the month, and 2.2 AU at the end, the 950-kilometer dwarf planet’s motion should be easily discernible from night to night as the Hyades provides an abundance of nearby stars to map its position against. As an additional challenge, turn your telescope towards it and see how small a time interval elapses before you detect its apparent movement. Can you see it move in an hour? How about a half hour? A well-calibrated tracking mount and reticle eyepiece will aid in this challenge.