Wednesday, September 1 - Waning Moon close to Messier 35 (predawn)
When the waning crescent moon rises among the stars of Gemini at about 1 a.m. local time on Wednesday, Sept. 1, it will be positioned a finger's width to the upper left (or 1 degree to the celestial north) of the large open star cluster named Messier 35. The two objects will share the view in binoculars and telescopes (red circle) until the dawn twilight overwhelms the stars. To better see the cluster, which is nearly as wide as the moon, try hiding the moon just outside the upper left edge of your binoculars' field.
Thursday, September 2 - Io's Shadow on Jupiter (from 10:45 p.m. to 12:55 a.m. PDT)
From time to time, the small, round black shadows cast by Jupiter's four Galilean moons become visible in amateur telescopes as they cross (or transit) the planet's disk. On Thursday night, Sept. 2, observers in the Americas can watch Io's small shadow on Jupiter from approximately 10:40 p.m. PDT (or 1:45 a.m. EDT and 05:45 GMT) until 12:55 a.m. PDT (or 3:55 a.m. EDT and 7:55 GMT). Io itself will be visible just before it moves onto Jupiter at 10:17 p.m. PDT and again after it moves off Jupiter at 12:38 a.m. PDT.
Saturday, September 4 - Crescent Moon Buzzes the Beehive (predawn)
For about an hour before dawn on Saturday morning, Sept. 4, look in the eastern sky for the slim crescent of the waning moon shining several finger-widths to the left (or 3 degrees to the celestial northeast) of the huge open star cluster known as the Beehive, Praesepe, and Messier 44. The moon and cluster will be close enough to share the field of binoculars (red circle), but you'll see more of the "bees" if you tuck the moon just out of sight on the left. Hours earlier, observers in Europe will see the pair somewhat closer together.
Sunday, September 5 - Venus Gleams Above Spica (after sunset)
Above the west-southwestern horizon on the evenings surrounding Sunday, Sept. 5, the orbital motion of the very bright planet Venus will carry it closely past Virgo's brightest star, Spica. At closest approach on Sunday evening Venus will shine only a thumb's width above (or 1.5 degrees to the celestial north of) Spica, allowing them to appear together in binoculars and low power telescopes (red circle). Venus will pop into view first after sunset - but you'll need to let the sky darken more to see 100 times fainter Spica with your unaided eyes. Start looking at about 8 p.m. local time. The pair will be binoculars-close from Sept. 3-8, but ensure that the sun has fully set before using optical aids to view them.
Monday, September 6 - New Moon (at 8:51 p.m. EDT)
The moon will reach its new phase on Monday, Sept. 6 at 8:51 p.m. EDT. (That corresponds to 0:51 GMT on Tuesday, Sept. 7.) While new, the moon is traveling 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). After the new moon phase Earth's planetary partner will return to shine in the western sky after sunset.
Tuesday, September 7 - Morning Zodiacal Light for Mid-Northern Observers (predawn)
For about half an hour before dawn during moonless periods in September and October annually, the steep morning ecliptic favors the appearance of the zodiacal light in the eastern sky for observers in mid-northern latitudes around the world. Zodiacal light is sunlight scattered by interplanetary particles concentrated in the plane of the solar system. It is more readily seen in areas free of urban light pollution. During the two-week period from now until the full moon on Sept. 20, look just above the eastern horizon for a broad wedge of faint light rising from the horizon and centered on the ecliptic (marked by green line). It will be brightest to the left of the bright star Procyon. Don't confuse the zodiacal light with the Milky Way, which is positioned nearby in the southeastern sky.
Wednesday, September 8 - Crescent Moon above Mercury (after sunset)
Look low in the western sky for a brief period after sunset on Wednesday, Sept. 8 to see the very young crescent moon (only 4.8%-illuminated) positioned a slim palm's width to the upper right (or 5 degrees to the celestial north) of Mercury. The magnitude 0.0 planet will become apparent as the sky darkens beyond 7:30 p.m. local time and will set 30 minutes later. The moon will take an additional half hour to set. This conjunction will be a challenge to see from mid-northern latitudes, but quite easy from southerly locations. Use binoculars (red circle) only after the sun has completely set.
Thursday, September 9 - Young Moon between Venus and Vesta (after sunset)
In the west-southwestern sky after sunset on Thursday, Sept. 9, look for the young crescent moon shining several finger-widths to the upper right (or 4 degrees to the celestial northwest) of the bright planet Venus. The main belt asteroid designated (4) Vesta will be there, too - positioned about double the moon's distance from Venus. The magnitude 7.7 asteroid will be somewhat difficult to see unless you are viewing it from a southerly latitude, where skies darken quickly after sunset. Tonight, use binoculars (red circle) to hunt for Vesta only a moon's diameter above the medium-bright star 80 Virginis. The moon will dance away from them after Thursday, but Venus and Vesta will continue to travel eastward together through the stars of Virgo. Compare their positions with Virgo's bright star Spica.
Saturday, September 11 - Asteroid Pallas at Opposition (all night)
On Saturday, Sept. 11, the main-belt asteroid designated (2) Pallas will reach opposition and its minimum distance from Earth for this year. On the nights near opposition, Pallas will rise at sunset and set at sunrise, and shine with a peak visual magnitude of 8.55. That's within reach of backyard telescopes, but wait until the asteroid has risen higher for the best view of it — about 9 p.m. local time or later. Pallas will be situated in western Pisces, several finger-widths to the right (or 4 degrees to the celestial southwest) of the medium-bright star Gamma Piscium. Nearby Neptune will reach its own opposition several days from tonight.
Sunday, September 12 - Half-Moon above Antares (evening)
Look low in the southwestern sky on Sunday evening, Sept. 12, for the waxing, nearly half-illuminated moon shining several finger-widths to the upper right (or 3 degrees to the celestial north) of Scorpius' very bright, reddish star Antares, the "Rival of Mars". The duo will share the field of view in binoculars (red circle). Off to their right (west), try and spot the will vertical row of scorpion's fainter white claw stars Graffias, Dschubba, and Pi Scorpii. Grab a photo of the pretty scene as soon as it gets dark. That part of the sky will set after 10 p.m. local time.
Monday, September 13 - First Quarter Moon (at 20:39 GMT)
When the moon completes the first quarter of its journey around Earth on Monday, Sept. 13 at 4:39 p.m. EDT, or 20:39 GMT, 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 surrounding 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 the lit and dark hemispheres.
Monday, September 13 - Lunar X in Early Evening (peaks at 7 p.m. EDT)
Several times a year, for a few hours near its first quarter phase, features on the moon called the Lunar X and Lunar V become visible in strong binoculars and backyard telescopes. The Lunar X is located on the terminator south of the crater La Caille, about one-third of the way up from the southern pole of the Moon (at 2 degrees East, 24 degrees South). The "V" is located near the crater Ukert (at 1 degree East, 14 degrees North). On Monday, Sept. 13 those letters are predicted to start developing by 5 p.m. EDT (or 21:00 GMT), peak in intensity around 7 p.m. EDT (or 23:00 GMT), and then gradually fade out. That peak will be during waning daylight for observers in the eastern Americas - but you can observe the moon in a telescope during daytime, as long as you take care to avoid the sun. The Lunar X and V will be observable anywhere on Earth where the moon is visible, especially in a dark sky, between about 21:00 GMT on Sept. 13 and 01:00 GMT on Sept. 14.
Monday, September 13 - Mercury at Greatest Eastern Elongation (after sunset)
After sunset on Monday, Sept. 13, Mercury (orbit shown in red) will be just hours away from its widest separation, 27 degrees east of the Sun, and its maximum visibility for the current apparition. On Tuesday evening it will be almost as elongated. With Mercury positioned well below the evening ecliptic (green line) in the west-southwestern sky, this appearance of the planet will be a poor one for Northern Hemisphere observers but will offer excellent views for observers near the equator and in the Southern Hemisphere. The optimal viewing times at mid-northern latitudes fall around 7:30 p.m. local time. Viewed in a telescope (inset) the planet will exhibit a waning, half-illuminated phase.
Tuesday, September 14 - Neptune at Opposition (all night)
On Tuesday, Sept. 14, Neptune will reach opposition. At that time Neptune will be closest to Earth for this year — a distance of 2.69 billion miles, 4.33 billion km, 4 light-hours, or 28.9 Astronomical Units. The blue planet will shine with a slightly brighter magnitude 7.8. Since it's directly opposite the sun in the sky, it will be visible all night long in good binoculars (red circle) if your sky is very dark, and backyard telescopes. Your best views will come after 9 p.m. local time, when the blue planet has risen higher. Around opposition, Neptune's apparent disk size will attain 2.4 arc-seconds and its large moon Triton will be most visible (inset). Throughout September, Neptune will be located among the stars of northeastern Aquarius, about 4 degrees to the left (or celestial east) of the naked-eye star Phi Aquarii (φ Aqr).
Wednesday, September 15 - Watch Algol Brighten (at 9:40 p.m. EDT)
Algol, also designated Beta Persei, 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's visual brightness dims and re-brightens noticeably. This happens because a companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star, reducing the total light output we receive. Algol normally shines at magnitude 2.1, similar to the nearby star Almach (aka Gamma Andromedae). But when fully dimmed, Algol's brightness of magnitude 3.4 is almost identical to Rho Persei (or Gorgonea Tertia or ρ Per), the star sitting just two finger widths to Algol's lower right (or 2.25 degrees to the celestial south). On Wednesday, Sept. 15 at 9:40 p.m. EDT (or 01:40 GMT on Sept. 16), Algol will be at its minimum brightness and low in the northeastern sky. Five hours later Algol will shine at full brightness nearly overhead in the eastern sky.
Thursday, September 16 - Bright Moon below Saturn (evening)
After the sun sets on Thursday evening, Sept. 16, look in the lower part of the southeastern sky for the bright waxing gibbous moon shining a slim palm's width below (or 5 degrees to the celestial southwest of) the yellowish dot of Saturn - with much brighter Jupiter positioned well off to their left (east). While the pair is crossing the sky during the night, both objects will just fit into the field of view of your binoculars (red circle). Meanwhile the diurnal rotation of the sky will lift the moon to Saturn's left before they set together in the west-southwest at about 2:30 a.m. local time.
Friday, September 17 - Bright Moon Joins Giant Planets (overnight)
After 24 hours of eastward orbital motion, on Friday, Sept. 17 the bright, waxing moon will sit below and between Jupiter and Saturn among the faint stars of Capricornus. The trio will shine in the southeastern sky after dusk, and then cross the southern sky overnight. They'll make a nice wide-field photo when composed with some interesting scenery — immediately after dark and then again before Saturn sets, ahead of the other two, shortly before 3:30 a.m. local time on Saturday morning. The moon will be tucked a little closer on Jupiter's lower left (celestial southwest) by that time.
Monday, September 20 - Full Harvest Moon (at 23:54 GMT)
The September full moon, traditionally known as the "Corn Moon" and "Barley Moon", always shines in or near the stars of Aquarius and Pisces. The indigenous Anishinaabe people of the Great Lakes region call this moon Waatebagaa-giizis or Waabaagbagaa-giizis, the Leaves Turning or Leaves Falling Moon. Full moons always rise around sunset and set around sunrise. Because this is the closest full moon to the autumnal equinox in 2021, it is also the Harvest Moon. On the evenings around its full phase, the moon normally rises about 50 minutes later than the previous night. But the shallow slope of the evening ecliptic (and the moon's orbit) around the equinox causes Harvest Moons to rise at almost the same time each night — only delayed by as little as 10 minutes, depending on your latitude. This phenomenon traditionally allowed farmers to work into the evening under bright moonlight - hence the name.
Tuesday, September 21 - Mercury Speeds Past Spica (after sunset)
In early September, bright Venus passed closely above Virgo's brightest star, Spica. Now, it's Mercury's turn. On Tuesday, Sept. 21, the speedy planet's path (red line with labels) will carry it only a thumb's width below Spica (or 1.4 degrees to the celestial south). That is just close enough for them to share a low-power eyepiece view, but telescopic views will be blurry because they are shining through so much of Earth's distorting atmosphere. The pair will be nearly as close on Monday, too. Magnitude 0.25 Mercury will appear twice as bright as Spica — helpful to know since most telescopes will flip or mirror the binoculars' view (red circle). Start searching the sky above the west-southwestern horizon starting about 20 minutes after the sun has completely set. Skywatchers at southerly latitudes will see the pair more easily - higher, and in a darker sky.
Wednesday, September 22 - Equinox (at 19:21 GMT)
On Wednesday, Sept. 22 at 3:21 p.m. EDT, or 19:21 GMT, the sun's apparent motion along the ecliptic (green line) will carry it across the celestial equator traveling southward, marking the autumnal equinox in the Northern Hemisphere and the beginning of autumn there. On the equinoxes in March and September, day and night are of equal length and the sun rises due east and sets due west (yellow arc).
Thursday, September 23 - Bright Moon Hops Past Uranus (all night)
On Thursday night, Sept. 23, the bright, waning gibbous moon will shine a generous palm's width to the right (or 7.5 degrees to the celestial southwest) of magnitude 5.7 Uranus. By dawn on Friday morning, the moon's orbital motion will carry it closer to Uranus in the west-southwestern sky, and the diurnal rotation of the sky will move it below the planet. When they rise again on Friday night, the moon will sit 5 degrees to Uranus' lower left (celestial east) - close enough for them to share the view in binoculars. While the blue-green dot of Uranus can be seen in binoculars, I recommend noting its location between the stars of Aries and Cetus and seek it out on the weekend, when the bright moon will have moved away from it.
Saturday, September 25 - Bright Moon between the Pleiades and Hyades (all night)
When the waning gibbous moon rises in mid-evening on Saturday, Sept. 25, it will shine several finger widths below (or 5 degrees to the celestial south of) the bright Pleiades star cluster, also known as Messier 45, the Seven Sisters, Subaru, and Matariki. The broad Hyades star cluster that forms the triangular face of Taurus the bull will be located below the moon. The moon and the Pleiades will share the field of view of binoculars (red circle). By dawn, the rotation of the sky will lift the Hyades to the left of the Pleiades, with the moon midway between them.
Tuesday, September 28 - Third Quarter Moon Again (at 9:57 p.m. EDT)
When a lunar phase occurs in the first few days of a calendar month, it can re-occur at month's end. For the second time in September, the moon will reach its third-quarter phase at 9:57 p.m. EDT on Tuesday, Sept. 28 (or 01:57 GMT on Wednesday, Sept. 29). The ensuing week of moonless evening skies will be ideal for observing deep-sky targets.
Wednesday, September 29 - The Andromeda Galaxy (all night)
September evenings feature the Andromeda Galaxy, which is already climbing the northeastern sky after dusk. This large spiral galaxy, also designated Messier 31 (or M31) and NGC 224, lies only 2.5 million light-years from us, and covers an area of sky measuring 3 by 1 degrees (or six by two full moon diameters). Under dark skies, the galaxy 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 a triangle 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).