See what's up in the night sky for September 2020, including stargazing events and the moon's phases, in this Space.com gallery courtesy of Starry Night Software.
Tuesday, Sept. 1 from 9:55 to 11:13 p.m. EDT — Moon occults star Tau Aquarii
In the eastern evening sky on Tuesday, Sept. 1, observers using binoculars and backyard telescopes (red circle) in the eastern half of North America can see the almost-full moon occult the medium-bright (magnitude 4.05) star designated Tau Aquarii or 71 Aquarii. That star marks the western knee of Aquarius, the Water-Bearer. In the Great Lakes region, the left-hand edge of the moon will cover the star at approximately 9:55 p.m. EDT (or 01:55 GMT on Sep 2). The star will reappear from behind the opposite limb of the moon at about 11:13 p.m. EDT. Ingress and egress vary based on your latitude, so start watching a few minutes before the times quoted above — or use Starry Night or another planetarium app to look up the exact times for your town.
Wednesday, Sept. 2 at 5:22 GMT — Full Corn Moon
The September full moon, traditionally known as the "Corn Moon" and "Barley Moon", always shines in or near the stars of Aquarius and Pisces. In most years, the September full moon is the Harvest Moon — but October's full moon will happen closest to the equinox — so it will have that honor. Full moons are positioned opposite the sun in the sky, so they rise at sunset and set at sunrise. Since the full phase will officially occur in the wee hours of Wednesday in the Americas, the moon will already look full when it rises on Tuesday evening.
Wednesday, Sept. 2 evening — Asteroid Pallas changes direction
On Wednesday, Sept. 2, the main belt asteroid designated (2) Pallas will complete a retrograde loop that began in May — causing it to temporarily stop moving through the background stars. On this night, the magnitude 10 asteroid will be located halfway up the southwestern evening sky — about a palm's width to the right (or 6.25 degrees to the celestial west of the medium-bright star Zeta Aquilae, which marks the western wingtip of Aquila, the Eagle. After tonight, it will return to a regular eastward motion through the stars.
Thursday, Sept. 3 — Northern winter begins on Mars
On Thursday, Sept. 3, the southern polar axis of Mars will reach its maximum tilt of 24 degrees towards the sun, triggering the solstice, and the beginning of winter in Mars' Northern Hemisphere. Mars' longer year means that its seasons are longer, too — slightly more than five months. Viewed in amateur telescopes from our vantage point on Earth, Mars' southern polar cap will shine as a bright, white spot on the red planet (although your telescope's optics may flip Mars upside-down).
Saturday, Sept. 5 overnight — Bright moon dances with Mars
When the bright, waning gibbous moon rises in the east at about 9:45 p.m. local time on Saturday, Sept. 5, it will be positioned only a finger's width to the right (or 1 degree to the celestial southwest) of Mars. That's close enough to appear together in binoculars and telescopes at low magnification (red circle). As the duo crosses the sky together during the night, the diurnal rotation of the sky, and the moon's eastward orbital motion, will combine to shift the moon clockwise around Mars — placing it above the planet by sunrise on Sunday morning. Since the pair will not set in the west until mid-morning on the 6th — skywatchers have a chance to see Mars in the morning daytime sky using binoculars and backyard telescopes — by using the moon as a reference. Observers in central and northeastern South America, Cape Verde Islands, northern Africa, and southern Europe will see the moon occult Mars around 05:00 GMT on Sunday.
Sunday, Sept. 6 overnight — Gibbous moon and Uranus
When the bright, waning gibbous moon rises in the east at about 10 p.m. local time on Sunday, Sept. 6, it will be positioned several finger widths to the lower right (or 4 degrees to the celestial south) of blue-green Uranus. That magnitude 5.71 planet is visible in binoculars (red circle) and backyard telescopes — if you know where to find it. This bright moon will be too bright for dim-planet hunting. Note the positions of the brighter stars in Cetus (below the moon) and Aries (above Uranus), and use them to find Uranus on a night when the moon isn't nearby.
Wednesday, Sept. 9 all night — Mars Stands Still
On Wednesday, Sept. 9, Mars will cease its eastward motion through the distant background stars of Pisces, and commence a retrograde loop (red path with labeled dates) that will last through opposition in October, and end in mid-November. To see Mars' motion, note its position compared to the stars around it and check back on subsequent evenings to see the difference.
Thursday, Sept. 10 at 9:26 GMT — Last quarter moon
At its last quarter phase at 9:26 GMT on Thursday, Sept. 10, the moon will rise around midnight and remain visible in the southern sky all morning. At this phase, the moon is illuminated on its western side, towards the pre-dawn sun. Last quarter moons are positioned ahead of the Earth in our trip around the sun. About 3½ hours later, Earth will occupy that same location in space. The week of moonless evening skies that follow last quarter will be ideal for observing deep sky targets.
Friday, Sept. 11 post-midnight — Crescent moon meets Messier 35
When the waning crescent moon rises shortly after 12 a.m. local time on Friday, Sept. 11, it will be positioned several degrees to the celestial west of the large open star cluster in Gemini known as Messier 35 or the Shoe-Buckle. During the rest of the night, the moon's orbital motion (green line) will carry it closer to the cluster — bringing it just a finger's width to the right of Messier 35 before dawn. To see the cluster's stars more easily, hide the bright moon just beyond the right edge of your binoculars' field of view (red circle).
Friday, Sept. 11 overnight — Neptune at opposition near Phi Aquarii
On Friday, Sept. 11, Neptune will be directly opposite the sun in the sky. At opposition, Neptune will be closest to us for this year — 4 light-hours or 28.9 Astronomical Units from Earth. It will shine at a slightly brighter magnitude 7.8, and will be visible all night long in good binoculars (red circle) and backyard telescopes in a dark sky. Around opposition, Neptune's disk size will grow to 2.4 arc-seconds. Throughout September, Neptune will be located among the stars of northeastern Aquarius, about two degrees to the left (or celestial east) of the naked-eye star Phi (φ) Aquarii.
Saturday, Sept. 12 evening — Jupiter completes its retrograde loop
On Saturday, Sept. 12, Jupiter will appear to stop moving with respect to the distant stars — marking the end of a westward retrograde loop (red path with labelled dates:times) that began in mid-May. After tonight, Jupiter will resume its regular eastward motion in northeastern Sagittarius, and will begin to reduce its 8-degree separation from Saturn.
Sunday, Sept. 13 at 9:08 p.m. EDT — Algol at minimum brightness
Algol, also designated Beta Persei, is among the most accessible variable stars for skywatchers. Its naked-eye brightness dims noticeably for about 10 hours once every 2 days, 20 hours, and 49 minutes because a dim companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star, reducing the total light output we receive. On Sunday, Sept. 13 at 9:08 p.m. EDT (or 1:08 GMT on Monday), Algol will reach its minimum brightness of magnitude 3.4. At that time, for observers in the Eastern time zone, the star will sit 12 degrees above the northeastern horizon. Five hours later, at 2:08 a.m. EDT (or 6:08 GMT), Algol will be high in the eastern sky, and will have brightened to its usual magnitude of 2.1.
Monday, Sept. 14 pre-dawn — Old moon meets Venus near M44
When the waning crescent moon rises at about 3:15 a.m. local time on Monday, Sept. 14, it will be positioned a few finger widths to the left (or 4 degrees to the celestial north) of the bright planet Venus. The pair, which will remain visible in the east until sunrise, will fit together into the field of view of binoculars (red circle) and will make a lovely wide-field photograph when composed with some interesting landscape. Your binoculars might also reveal the large open star cluster known as the Beehive or Messier 44, sitting just above and between the moon and Venus. To see the cluster's stars more easily, hide the moon and Venus just below your binoculars' field of view.
Monday, Sept. 14 at 6:57 GMT — Rare double shadow transit with GRS on Jupiter
From time to time, the Great Red Spot (GRS) and the little round, black shadows cast by Jupiter's four Galilean moons become visible in backyard telescopes as they cross (or transit) the planet's disk. Commencing a few minutes before midnight on Sunday night, and continuing during the wee hours of Monday, Sept. 14, observers on the west coasts of the USA and Canada, and the Pacific Ocean, can witness the rare event of a double shadow transit — accompanied by the Great Red Spot! At 11:57 p.m. PDT (or 06:57 GMT) Ganymede's larger shadow and the Great Red Spot will join Io's smaller shadow already progressing across Jupiter's disk. The trio will remain visible until Io's shadow moves off Jupiter at about 1:30 a.m. PDT (or 08:30 GMT). In some regions Jupiter will set during the event, but observers in the Pacific Ocean region can see the entire transit.
Wednesday, Sept. 16 pre-dawn — Morning zodiacal light for mid-northern observers
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. This is sunlight scattered by interplanetary particles concentrated in the plane of the solar system. During a two-week period that starts just before the September new moon, look above the eastern horizon for a broad wedge of faint light rising from the horizon and centered on the ecliptic (marked by a green line), which extends below Venus toward the bright star Regulus in Leo. Don't confuse the zodiacal light with the Milky Way, which is positioned further to the southeast.
Thursday, Sept. 17 at 11:00 GMT — New moon
At its new phase, the moon is travelling between Earth and the sun. Since sunlight is only reaching the far side of the moon, and the moon is in the same region of the sky as the sun, the moon will be completely hidden from view for about a day. Since this new moon is occurring a day before perigee, the moon's minimum distance from Earth, tides will be larger around the world.
Friday, Sept. 18 after sunset — Crescent moon and Mercury
After sunset on Friday, Sept. 18, sharp-eyed observers might spot the very slim crescent moon sitting just above the western horizon, and a slim palm's width to the upper right (or 5 degrees to the celestial north) of Mercury. The moon and Mercury will both fit into the field of view of binoculars (red circle) — but ensure that the sun has completely disappeared from view before using them.
Monday, Sept. 21 after sunset — Mercury moves past Spica
Low in the west-southwestern sky after sunset on Monday, Sept. 21, Mercury's rapid orbital motion (red curve) will bring it very close to Spica, Virgo's brightest star. Look for brighter Mercury sitting just 40 arc-minutes (about 1.3 times the diameter of a full moon) to Spica's right (celestial west), allowing both objects to fit into the field of view of a backyard telescope at medium magnification. On the following evening, Mercury will climb to sit a similar distance above Spica. Observers viewing from southerly latitudes will be able to see the duo more easily.
Tuesday, Sept. 22 pre-dawn — Venus passes Vesta
In the eastern pre-dawn sky on the mornings surrounding Tuesday, Sept. 22, very bright Venus will overtake and pass the slower-moving main belt asteroid Vesta. Venus will be dropping sunward while Vesta climbs in the opposite direction. At closest approach on Tuesday morning, Venus will be positioned about two finger widths to the right (or 2 degrees to the celestial south) of Vesta. Magnitude -4.14 Venus will outshine magnitude 8.16 Vesta by more than 8300 times! To see how both objects move compared to the stars around them, try to view the event on several mornings — ideally before 6 a.m. local time, when the sky will be starting to brighten.
Tuesday, Sept. 22 at 13:31 GMT — Equinox
On Tuesday, Sept. 22 at 13:31 GMT, the sun will cross the celestial equator moving 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.
Wednesday, Sept. 23 at 9:55 p.m. EDT — First quarter moon
When the moon completes the first quarter of its orbit around Earth at 9:55 p.m. EDT on Wednesday, Sept. 23 (or 1:55 GMT on Thursday, Sept. 24), the relative positions of the Earth, sun, and moon will cause us to see it half-illuminated — on its eastern side. At first quarter, the moon always rises around noon and sets around midnight, so it is also visible in the afternoon daytime sky. The evenings surrounding first quarter are the best ones to see the lunar terrain when it is dramatically lit by low-angle sunlight.
Thursday, Sept. 24 evening — Gibbous moon near Jupiter
The moon's monthly visit with the gas giant planets Jupiter and Saturn will commence on the evening of Thursday, Sept. 24. As the evening sky darkens, the bright planet Jupiter will become visible several finger widths to the upper left (or 4.5 degrees to the celestial northeast of) the gibbous moon in the southern sky. The moon and Jupiter will fit into the field of view of binoculars (red circle). Look for somewhat dimmer Saturn sitting off to their upper left (east). By the time they set soon after midnight local time, the moon will slide east, closer to Jupiter — and the diurnal rotation of the sky will raise Jupiter above the moon. This conjunction will make a beautiful wide field image when composed with some interesting foreground scenery.
Friday, Sept. 25 evening — Bright moon below Saturn
The moon's monthly visit with the gas giant planets Jupiter and Saturn will continue on Friday, Sept. 25. After 24 hours of eastward motion, the bright waxing moon will sit several finger widths to the lower left (or 3.6 degrees to the celestial south of) yellowish Saturn in the southern sky after dusk. The moon and Saturn will fit into the field of view of binoculars (red circle). Look for much brighter Jupiter sitting to their right (west). By the time the moon and Saturn set shortly before 1 a.m. local time, the moon will be farther from Saturn — and the diurnal rotation of the sky will lower Saturn to the moon's right. This conjunction will make a beautiful wide field image when composed with some interesting foreground scenery.
Sunday, Sept. 27 all night — Sinus Iridum's Golden Handle
On Sunday night, Sept. 27, the terminator on the waxing gibbous moon will fall just west of Sinus Iridum, the Bay of Rainbows. The circular 155 mile (249 km) diameter feature is a large impact crater that was flooded by the same basalts that filled the much larger Mare Imbrium to its east — forming a rounded handle-shape on the western edge of that mare. The "Golden Handle" effect is produced by way the slanted sunlight brightly illuminates the eastern side of the prominent Montes Jura mountain range surrounding the bay on the north and west, and by a pair of protruding promontories named Heraclides and Laplace to the south and north, respectively. Sinus Iridum is almost craterless, but hosts a set of northeast-oriented dorsae or "wrinkle ridges" that are revealed at this phase.
Tuesday, Sept. 29 evening — Saturn stands still
On Tuesday, Sept. 29, Earth's faster orbit will cause Saturn to appear to stop moving with respect to the distant stars. The temporary pause in motion (red path with labelled dates:times) marks the end of a westward retrograde loop that began on May 11. After dusk, look for the yellowish, magnitude 0.46 planet in the lower part of the southern sky among the stars of northeastern Sagittarius — seven degrees east of much brighter Jupiter.