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.
Thursday, October 1 evening—Mercury at Greatest Eastern Elongation
In the western sky on the evening of Thursday, October 4, Mercury (orbit shown in red) will reach its widest separation, 26 degrees east of the Sun. With Mercury positioned well below the evening ecliptic (green line), this appearance of the planet will be a poor one for Northern Hemisphere observers, but offer excellent views for observers near the equator and in the Southern Hemisphere. The optimal viewing times at mid-northern latitudes fall around 7:15 p.m. local time. Viewed in a telescope (inset) the planet will exhibit a waning, half-illuminated phase.
Thursday, October 1 at 21:05 GMT—Full Harvest Moon
Because this full moon occurs closest to the autumnal equinox in 2020, it is also the Harvest Moon. On the evenings around its full phase, the moon usually rises about 50 minutes later each night. But the shallow angle between the horizon and the evening ecliptic on dates around the equinox causes the moon to rise at almost the same time each night – only delayed by 10-20 minutes, depending on your latitude. This lunar phenomenon traditionally allowed farmers to work longer into the evening under bright moonlight when the crops were ready to harvest—hence the name.
Friday, October 2 pre-dawn—Venus Kisses Regulus
When very bright Venus rises in the east at about 3:45 a.m. local time on Friday, October 2, it will be positioned less than a finger's width above (or 41 arc-minutes to the celestial west of) the bright, white star Regulus in Leo. The planet and star will appear together in the field of view of binoculars, or in a backyard telescope at high magnification (red circle). At closest approach, at 23:00 GMT, observers in western Asia can see Venus only 5 arc-minutes from the star. On the following morning, Venus' orbital motion eastward will lower it to a half finger's width below Regulus.
Friday, October 2 all night—Bright Moon meets Mars
When bright, reddish Mars rises in the eastern sky at around 7:45 p.m. local time on Friday, October 2, the gibbous, waning moon will be positioned just two finger widths to the lower right (or 2.25 degrees to the celestial south) of the red planet. The moon and Mars will be close enough to one another to see them together in binoculars (red circle) all night long. By midnight, the diurnal rotation of the sky and the moon's eastward orbital motion will carry the moon to just a finger's width below Mars. At sunrise on Saturday morning, Mars will sit three finger widths to the moon's lower right in the western sky – allowing you to find the magnitude -2.52 planet in the brightening western sky with binoculars. Early on Saturday morning, the moon will occult Mars for observers in southern and southeastern South America, most of western Antarctica, the Ascension Islands, and southwestern Africa.
Monday-Tuesday, October 5-6 all night—Mars Closest to Earth
A week before Mars officially reaches opposition, Earth and Mars will be at their minimum distance apart during the early hours of Tuesday, October 6. At that time, Mars will be 38.57 million miles, 62.07 million km, 0.515 Astronomical Units, or 3.45 light-minutes distant from Earth. After Mars rises in the east on Monday evening, observers with backyard telescopes can expect to see the planet with more detail than it will exhibit for 15 years – especially after midnight, when the planet will climb highest in the sky. Viewed in a telescope Mars' maximum apparent disk diameter will be 22.6 arc-seconds. (For comparison, Jupiter's disk spans about 44 arc-seconds.) Its Earth-facing hemisphere that night will display its bright southern polar cap, the dark Syrtis Major Planum and Tyrrhena Terra regions, and the lighter toned Hellas Planitia region. The planet will not be as close to Earth for another 15 years.
Wednesday, October 7 evening—Draconids Meteor Shower Peaks
The Draconids Meteor Shower, which runs between October 6 and 10 every year, will peak overnight on Wednesday, October 7. This shower, generated by debris dropped by Comet 21P/Giacobini-Zinner, usually delivers relatively few meteors. But it has occasionally been much more prolific. The best time to watch for Draconids will be after dusk, when the shower's radiant in Draco will be sitting high in the northern sky. Unfortunately, a bright, waxing gibbous moon will wash out many of the fainter meteors after it rises at 10 pm local time.
Friday, October 9 at 8:39 pm EDT—Last Quarter Moon
When it reaches its last quarter phase at 8:39 pm EDT on Friday, October 9 (or 0:39 GMT on Saturday, October 10), the moon will rise around midnight and then remain visible in the southern sky all morning. At last quarter, 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.
Sunday, October 11 pre-dawn—Waning Moon Meets Messier 44
When the waning crescent moon rises at about 1 a.m. local time on Sunday, October 11, it will be positioned four finger widths above (or 4 degrees to the celestial west) of the large open star cluster known as the Beehive or Messier 44, in Cancer. During the hours before dawn, the moon's orbital motion (green line) will carry it somewhat closer to the cluster. To see Messier 44's stars more easily, hide the moon just below your binoculars' field of view (red circle).
Tuesday, October 13 all night—Mars at Opposition
Mars will officially reach opposition on Tuesday evening, October 13. On that night, the bright red planet will rise among the stars of western Pisces at sunset, climb to its highest position, 51° above the southern horizon, at 1 am local time, and set at sunrise. At opposition, Mars will shine with a maximum visual magnitude of -2.62. Although it will be slightly farther from Earth (38.57 million miles; 62.07 million km ; 0.415 AU; 3.45 light-minutes) than it was a week prior, Mars will still be an impressive sight in backyard telescopes, showing an apparent disk diameter of 22.57 arc-seconds (Jupiter's disk spans about 42 arc-seconds). Its Earth-facing hemisphere that night will display its bright southern polar cap, the dark Tyrrhena Terra, Cimmeria Terra, and Sirenum Terra regions, and the lighter toned Hellas Planitia region. Mars oppositions occur approximately every 25.5 months.
Wednesday, October 14 pre-dawn—Old Moon near Venus
In the eastern sky for several hours preceding sunrise on Wednesday, October 14, the delicate crescent of the old moon will make a pretty sight with the very bright planet Venus. Keep an eye out for Earthshine – sunlight reflected from Earth that is slightly brightening the moon's darkened region. The moon and Venus will make a lovely photo opportunity, especially when composed with some interesting foreground scenery.
Thursday, October 15 pre-dawn—Morning Zodiacal Light for Mid-Northern Observers
For about half an hour before dawn during moonless periods in September and October every year, the steep morning ecliptic favors the appearance of the zodiacal light in the eastern sky. Zodiacal Light is sunlight scattered from interplanetary particles concentrated in the plane of the solar system. From dark-sky sites during the two-week period from now until the October 31 full moon, look above the eastern horizon for a broad wedge of faint light centered on the ecliptic (marked by green line), which descends through the bright star Regulus in Leo and down past Venus. Don't confuse the zodiacal light with the Milky Way, which is positioned further to the southeast.
Friday, October 16 at 19:31 GMT—New Moon and Large Tides
At its new phase, the moon is travelling 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 will be completely hidden from view. This new moon, occurring only 4.5 hours after perigee (the moon's closest approach to Earth), will trigger large tides around the world.
Saturday, October 17 dusk to 7:43 pm EDT—Rare Double Shadow Transit with the Great Red Spot on Jupiter
From time to time, the Great Red Spot (GRS) and the little round, black shadows cast by Jupiter's four Galilean moons are visible in backyard telescopes as they cross (or transit) the planet's disk. On Saturday evening, October 17 observers in the Eastern Time Zone can witness the rare event of two shadows transiting with the Great Red Spot! As the sky darkens, the diffuse shadow of Callisto, the crisp, round shadow of closer-in Io, and the great Red Spot will all be completing a group transit event that began at 5:25 p.m. EDT. The three objects will rotate off of Jupiter's limb at 7:25 p.m. EDT.
Wednesday, October 21 pre-dawn—Orionids Meteor Shower Peak
The annual Orionids meteor shower is produced when the Earth plows through a cloud of small particles dropped by repeated passages of Comet Halley in its orbit. The shower runs from September 23 to November 27 and will peak between midnight and dawn on Wednesday, October 21. At that time the sky overhead will be moving directly into the densest region of the particle field, producing 10-20 fast meteors per hour. The meteors can appear anywhere in the sky, but will seem to be travelling away from the constellation of Orion. On the peak night, the young, crescent moon will set during evening – leaving the overnight sky dark for meteor-watching.
Thursday, October 22 evening—Half-Moon near Jupiter and Saturn
The moon's monthly visit with the gas giant planets Jupiter and Saturn will occur in the southwestern sky after dusk on Thursday, October 22. The waxing, half-illuminated moon will form a neat triangle below the two planets—bright Jupiter to the moon's upper right, and somewhat dimmer Saturn positioned to the moon's upper left. The trio, which will set in late evening, will offer a lovely wide field photo opportunity when composed with some interesting foreground scenery.
Friday, October 23 evening—Dwarf Planet Ceres Changes Direction
On Friday, October 23, the dwarf planet Ceres will complete a retrograde loop (red path with dates:time) that began in July – causing it to temporarily cease its motion through the background stars. On this night, the magnitude 8.6 object will be located in the lower part of the southern evening sky – about a fist's diameter to the upper right (or 9.5 degrees to the celestial northwest) of the very bright star Fomalhaut. After tonight, Ceres will resume its regular eastward motion through the stars.
Friday, October 23 at 13:23 GMT—First Quarter Moon
When the moon completes the first quarter of its orbit around Earth at 9:23 a.m. EDT (or 13:23 GMT) on Wednesday, October 23, 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-angled sunlight.
Friday, October 23 at 10 p.m. EDT—Lunar X
Several times a year, for a few hours near its first quarter phase, a feature on the moon called the Lunar X becomes visible in strong binoculars and backyard telescopes. When the rims of the craters Parbach, la Caille, and Blanchinus are illuminated from a particular angle of sunlight, they form a small, but very obvious X-shape. The phenomenon called is pareidolia—the tendency of the human mind to see familiar objects when looking at random patterns. At approximately 10 p.m. EDT on Friday, October 23, the Lunar X is predicted to peak in intensity—but the phenomenon will be visible for approximately two hours on either side of that time. This event should be visible anywhere on Earth where the moon is shining in a dark sky during that time window. Simply adjust for your difference from the Eastern Time zone. For the Americas, the Moon will be positioned low in the southwestern sky. The Lunar X is located near the terminator, about one third of the way up from the southern pole of the Moon (at 2° East, 24° South). The prominent round crater Werner sits to its lower right.
Saturday, October 24 evening—Rupes Recta (The Straight Wall)
On Saturday evening, October 24, the pole-to-pole terminator that divides the lit and dark hemispheres of the waxing gibbous moon, will fall just to the left (or lunar west) of Rupes Recta, also known as the Straight Wall, a feature that is very obvious in good binoculars and backyard telescopes. The rupes, Latin for "cliff," is a north-south aligned fault scarp that extends for 65 miles (110 km) across the southeastern part of Mare Nubium, which sits in the lower third of the moon's Earth-facing hemisphere. The feature is always prominent a day after first quarter and day before last quarter. For reference, the very bright crater Tycho sits due south of the Straight Wall.
Tuesday, October 27 all night—Sinus Iridum's Golden Handle
On Tuesday night, October 27, the pole-to-pole terminator that divides the lit and dark hemispheres of the waxing gibbous moon, will fall just to the left (or lunar 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 right (lunar east) – forming a rounded, handle-shape on the western edge of that mare. You can see it easily with sharp eyes and binoculars. A "Golden Handle" effect is produced by the way slanted sunlight brightly illuminates the eastern side of the prominent Montes Jura mountain range that surrounds the bay on the top and left (north and west), and by a pair of protruding promontories named Heraclides and Laplace to the bottom and top, respectively. Sinus Iridum is almost craterless, but hosts a set of northeast-oriented dorsae or "wrinkle ridges" that are revealed in a telescope at this phase.
Thursday, October 29 all night—Bright Moon near Mars
In the eastern sky after dusk on Thursday, October 29, the nearly full moon will be positioned only a few finger widths below (or 4 degrees to the celestial southeast) of Mars—close enough to appear together in most binoculars (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 a generous palm's width to the upper left of the red planet in the western sky by sunrise on Friday morning.
Friday, October 30 around 06:37 GMT—Bright Moon Occults Star Xi1 Ceti
Overnight on Friday, October 30, observers using binoculars and backyard telescopes (red circle) in most of North America can see the almost-full moon occult the medium-bright star designated Xi1 Ceti (or ξ1 Ceti). That magnitude 4.35 star marks the top of the head of Cetus, the Whale. In San Francisco, the leading, left-hand edge of the moon will pass in front of the star at approximately 9:02 p.m. PDT. The star will emerge from behind the right-hand edge of the moon's disk at about 10:24 a.m. PDT. (Those times convert to 05:02 to 6:24 GMT on Saturday, October 31). In the Great Lakes region, the occultation will run from approximately 1:38 a.m. to 2:26 a.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.
Saturday, October 31 at 14:49 GMT—Small Full Blue Hunter's Moon
The full moon of October, traditionally called the Hunter's Moon, Blood Moon, or Sanguine Moon, always shines in or near the stars of Cetus and Pisces. Since it's opposite the sun on this day of the lunar month, the full moon rises at sunset and sets at sunrise. This full moon will occur only 20 hours after apogee, producing the smallest full moon of 2020. When a lunar phase occurs on the first day or two of a month, it can be repeated at month-end. This second full moon of October is colloquially known as a "Blue Moon"—although the moon will not sport any unusual coloration.
Saturday, October 31 all night—Uranus at Opposition
Uranus will reach opposition on Saturday, October 31. On that night it will be closest to Earth for this year at a distance of 1.75 billion miles, 2.81 billion km, or 156 light-minutes. Its minimal distance from Earth will cause it to shine at a peak brightness of magnitude 5.7 and to appear slightly larger in telescopes for a few weeks. At opposition, planets are above the horizon from sunset to sunrise. During autumn this year, the blue-green planet will be located below the brightest stars of Aries, Hamal and Sheratan, while moving slowly retrograde westwards towards the constellation of Pisces.