Night sky guide for September 2017

There are no major (Class I) meteor shower peaks during this month. Instead, we have two minor ones (Class II), Aurigids on September 1 and September Epsilon Perseids on September 9, with an estimated number of meteors per hour of 6 and 5, respectively. We'll also have 8 weak variable showers (Class IV) but all of them are expected to have less than 2 meteors per hour.
The Moon will reach its full phase (Sun - Earth - Moon) at 07:04 UTC on September 6, 2017.
The best time of the month to observe faint objects such as galaxies and star clusters because there is no moonlight to interfere - New Moon - will be on September 20 when the Earth, Moon and Sun all lie in a roughly straight line, with the Moon in the middle, appearing in front of the Sun's glare.
On September 22 - September Equinox - the Sun will be above the horizon for exactly half the time everywhere on Earth. According to the astronomical definitions of the seasons, this day marks the beginning of autumn in the northern hemisphere and of spring in the southern hemisphere.

• September 1 - Conjunction of the Moon and Pluto - 19:06 UTC. The Moon, 11 days old, and Pluto will share the same right ascension, with the Moon passing 2°31' to the north of Pluto. The Moon will be at mag -12.3, and Pluto at mag 15.0, both in the constellation Sagittarius. The pair will be too widely separated to fit within the field of view of a telescope but will be visible through a pair of binoculars.
  
• September 2 - Conjunction of Mars and Mercury - 19:20 UTC. Mars and Mercury will share the same right ascension, with Mars passing 4°05' to the north of Mercury. Mars will be at mag 1.8, and Mercury at mag 2.8, both in the constellation Leo. The pair will be too widely separated to fit within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars.
  
• September 5 - Neptune at Opposition - 05:13 UTC.  At around the same time that Neptune passes opposition, it also makes its closest approach to the Earth – termed its perigee. It will be brighter than any other time of the year and will be visible for much of the night, reaching its highest point in the sky at around midnight local time. This happens because when Neptune lies opposite the Sun in the sky, the solar system is lined up so that Neptune, the Earth and the Sun form a straight line with the Earth in the middle, on the same side of the Sun as Neptune. This is the best time to view and photograph the blue giant. However, due to its extreme distance from Earth, it will only appear as a tiny blue dot in all but the most powerful telescopes. Find it in the constellation Aquarius. Over the weeks following its opposition, Neptune will reach its highest point in the sky four minutes earlier each night, gradually receding from the pre-dawn morning sky while remaining visible in the evening sky for a few months.
  
• September 6 -  Close approach of the Moon and Neptune - 05:00 UTC. The Moon and Neptune will make a close approach, passing within 0°44' of each other on September 6. The Moon will be at mag -12.6, and Neptune at mag 7.8, both in the constellation Aquarius. The pair will be a little too widely separated to fit comfortably within the field of view of a telescope but will be visible through a pair of binoculars.
  
• September 6 - Full Moon - 07:04 UTC. The Moon will reach full phase at 07:04 on September 6. At this time in its monthly cycle of phases, the Moon lies almost directly opposite the Sun in the sky, placing it high above the horizon for much of the night. The sequence of full moons through the year are often assigned names according to the seasons in which they fall. This month's will be the third to fall in northern hemisphere's summer of 2017 – the Fruit Moon. This full moon was known by early Native American tribes as the Full Corn Moon because the corn is harvested around this time of year. Over the nights following September 6, the Moon will rise around an hour later each day, becoming prominent later in the night. Within a few days, it will only be visible in the pre-dawn and early-morning sky. By the time it reaches last quarter, a week after full moon, it will rise at around midnight and set at around noon. At the exact moment when the Moon reaches full phase, it will lie at a declination of -08°03' in the constellation Aquarius, and so will appear high in the sky at all but the most extreme latitudes. It will be visible at all latitudes between 71°N and 88°S. Its distance from the Earth will be 384 000 km (238 464 miles).
  
• September 6 - Asteroid 89 Julia at opposition - 15:43 UTC. Asteroid 89 Julia will be well placed for observation, lying in the constellation Pegasus, well above the horizon for much of the night. Regardless of your location on the Earth, 89 Julia will reach its highest point in the sky at around midnight local time. At around the same time that 89 Julia passes opposition, it also makes its closest approach to the Earth. This happens because when 89 Julia lies opposite to the Sun in the night sky, the solar system is lined up so that 89 Julia, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as 89 Julia. On this occasion, 89 Julia will pass within 1.102 AU of us, reaching a peak brightness of magnitude 9.0. Nonetheless, even at its brightest, 89 Julia is a faint object beyond the reach of the naked eye or binoculars; a telescope of moderate aperture and a good star chart are needed.
  
• September 9 - Conjunction of the Moon and Eris - 10:37 UTC. The Moon and Eris will share the same right ascension, with the Moon passing 8°12' to the north of 136199 Eris. The Moon, 19 days old, will be at mag -12.6 in the constellation Pisces, and Eris at mag 18.8 in the neighboring constellation of Cetus.
  
• September 12 - Mercury at greatest elongation west - 09:11 UTC. Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.4. Its orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is very difficult to observe most of the time. It is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation. These apparitions take place alternately in the morning and evening skies, depending whether Mercury lies to the east of the Sun or to the west. When it lies to the east, it rises and sets a short time after the Sun and is visible in early evening twilight. When it lies to the west of the Sun, it rises and sets a short time before the Sun and is visible shortly before sunrise. This is the best time to view the planet since it will be at its highest point above the horizon in the morning sky. Look for the planet low in the eastern sky just before sunrise.
  
• September 13 - Mercury at dichotomy - 03:34 UTC. Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.5. Mercury's phase varies depending on its position relative to the Earth. When it passes between the Earth and Sun, for example, the side that is turned towards the Earth is entirely unilluminated, like a new moon. Conversely, when it lies opposite to the Earth in its orbit, passing almost behind the Sun, it appears fully illuminated, like a full moon. However, at this time it is also at its most distant from the Earth, so it is actually fainter than at other times. Mercury shows an intermediate half phase - called dichotomy - at roughly the same moment that it appears furthest from the Sun, at greatest elongation. The exact times of the two events may differ by a few hours, only because Mercury's orbit is not quite perfectly aligned with the ecliptic.
  
• September 15 - Conjunction of the Moon and Ceres - 21:56 UTC. The Moon and Ceres will share the same right ascension, with the Moon passing 5°14' to the south of Ceres. The Moon will be 25 days old. The Moon will be at mag -11.0 in the constellation Cancer, and Ceres at mag 8.9 in the neighboring constellation of Gemini.
  
• September 16 - Conjunction of Mercury and Mars - 18:25 UTC. Mercury and Mars will share the same right ascension, with Mercury passing 0°03' to the north of Mars. Mercury will be at mag -0.9, and Mars at mag 1.8, both in the constellation Leo. The pair will be close enough to fit within the field of view of a telescope, but will also be visible to the naked eye or through a pair of binoculars.
  
• September 18 - Conjunction of the Moon and Venus - 00:57 UTC. The Moon, 28 days old, and Venus will share the same right ascension, with the Moon passing 0°32' to the south of Venus.The Moon will be at mag -9.6, and Venus at mag -3.9, both in the constellation Leo. The pair will be a little too widely separated to fit comfortably within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars.
  
• September 19 - Conjunction of the Moon and Mercury - 23:23 UTC. The Moon and Mercury will share the same right ascension, with the Moon passing 0°01' to the south of Mercury. The Moon will be at mag -8.4, and Mercury at mag -1.0, both in the constellation Leo. The pair will be close enough to fit within the field of view of a telescope, but will also be visible to the naked eye or through a pair of binoculars.
  
• September 20 - New Moon - 05:31 UTC. At new moon, the Earth, Moon and Sun all lie in a roughly straight line, with the Moon in the middle, appearing in front of the Sun's glare. In this configuration, we see almost exactly the opposite half of the Moon to that which is illuminated by the Sun, making it doubly unobservable because the side we see is unilluminated. Over coming days, the Moon will rise and set an hour later each day, becoming visible in the late afternoon and dusk sky as a waxing crescent which sets soon after the Sun. By first quarter, in a week's time, it will be visible until around midnight. This is the best time of the month to observe faint objects such as galaxies and star clusters because there is no moonlight to interfere.
  
• September 21 - Conjunction of the Moon and Makemake - 12:18 UTC. The Moon, one day old, and Makemake will share the same right ascension, with the Moon passing 26°00' to the south of Makemake. The Moon will be at mag -8.4 in the constellation Virgo, and Makemake at mag 17.0 in the neighboring constellation of Coma Berenices.
  
• September 22 - September Equinox - 19:53 UTC. On this day, the Sun will be above the horizon for exactly half the time everywhere on Earth. According to the astronomical definitions of the seasons, this day marks the beginning of autumn in the northern hemisphere and of spring in the southern hemisphere.
  
• September 23 - Conjunction of the Moon and Haumea - 22:44 UTC. The Moon and dwarf planet Haumea will share the same right ascension, with the Moon passing 24°13' to the south of Haumea. The Moon will be at mag -9.9 in the constellation Virgo, and Haumea at mag 17.4 in the neighboring constellation of Bootes.
  
• September 24 - NGC 55 well placed for observation. Across much of the world NGC 55, a barred irregular galaxy in Sculptor will be well placed for observation. It will reach its highest point in the sky at around midnight local time. At a declination of -39°10', it is easiest to see from the southern hemisphere but cannot be seen from latitudes much north of 30°N. At magnitude 8.0, NGC55 is quite faint, and certainly not visible to the naked eye, but can be viewed through a pair of binoculars or small telescope.
  
• September 26 - Makemake at solar conjunction - 07:04 UTC. Dwarf planet Makemake will pass very close to the Sun in the sky as its orbit carries it around the far side of the solar system from the Earth. At closest approach, Makemake will appear at a separation of only 27° from the Sun, making it totally unobservable for several weeks while it is lost in the Sun's glare. At around the same time, Makemake will also be at its most distant from the Earth - receding to a distance of 53.39 AU – since the two planets will lie on opposite sides of the solar system. If Makemake could be observed at this time, it would appear at its smallest and faintest on account of its large distance. It would measure 0.0 arcsec in diameter. Over following weeks and months, Makemake will re-emerge to the west of the Sun, gradually becoming visible for ever-longer periods in the pre-dawn sky. After around six months, it will reach opposition, when it will be visible for virtually the whole night.
  
• September 27 - 47-Tuc well placed for observation. Across much of the world the second brightest globular cluster in the sky, 47 Tuc (NGC 104), in Tucana will be well placed for observation. It will reach its highest point in the sky at around midnight local time. At a declination of -72°04', it is easiest to see from the southern hemisphere but cannot be seen from latitudes much north of 2°S. At magnitude 4.0, 47-Tuc is tricky to make out with the naked eye except from a dark site, but is visible through a pair of binoculars or small telescope.
  
• September 27 - Close approach of the Moon and Saturn - 00:35 UTC. The Moon, 7 days old, and Saturn will make a close approach, passing within 3°27' of each other. The Moon will be at mag -11.5, and Saturn at mag 0.3, both in the constellation Ophiuchus. The pair will be too widely separated to fit within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars.
  
• September 29 - Conjunction of the Moon and Pluto - 03:03 UTC. The Moon and Pluto will share the same right ascension, with the Moon passing 2°27' to the north of Pluto. The Moon will be at mag -12.0, and 134340 Pluto at mag 15.0, both in the constellation Sagittarius. The pair will be too widely separated to fit within the field of view of a telescope but will be visible through a pair of binoculars.