The night sky for September 2019

Northern Hemisphere

Ian Morison tells us what we can see in the Northern Hemisphere night sky during September 2019.

The Planets

• Jupiter, shining on the 1st at magnitude -2.2 and falling to -2 during the month, can be seen in the south as darkness falls. As the month progresses, its angular size drops from 39 to 36 arc seconds. Jupiter, in the southern part of Ophiuchus, ended its retrograde motion on the 11th of August and so is now moving away from Antares in Scorpius initially lying some 7 degrees up and to its left. A highlight gives the times when the Great Red Spot faces the Earth. Sadly it is heading towards the southernmost part of the ecliptic so, as it appears in the twilight, it will only have an elevation of ~13 degrees (from central UK). Happily, its elevation will only have dropped by a degree or so an hour later in full darkness. With its low elevation, atmospheric dispersion will take its toll and an atmospheric dispersion corrector would greatly help to improve our views of the giant planet.
  
  
• Saturn , crosses the meridian, so is highest in the sky, at around 9pm BST as September begins. Then, its disk is ~17.6 arc seconds across and its rings - which are still nicely tilted from the line of sight - spanning some 41 arc seconds across. By month's end it will be best seen at around 8 pm BST when lying just west of south. During the month its brightness falls from magnitude +0.3 to +0.5 whilst its angular size falls to 16.9 arc seconds. Sadly, now in Sagittarius and lying on the south-western side of the Milky Way, it is at the lowest point of the ecliptic and will only reach an elevation of ~14 degrees. As with Jupiter, an atmospheric dispersion corrector will help improve our view.
  
  
• Mercury, passes behind the Sun (Superior Conjunction) on the night of September 3rd/4th so will not be visible this month.
  
  
• Mars, which passes behind the Sun (Superior Conjunction) on September 2nd, lies too close to the Sun to be visible. We will have to wait until the end of October to spot it in the pre-dawn sky at the start of its next apparition.
  
  
• Venus, went through superior conjunction on the far side of the Sun on the 14th August. By month's end it will set in the west south-west 30 minutes after sunset but will be very difficult to see due to the fact that the ecliptic is at a shallow angle to the horizon and so Venus will have a very low elevation. Binoculars and a very low horizon will be needed, but please do not use them until after the Sun has set.
  
  

Highlights

• September - observe Saturn. Saturn which reached opposition on the 9th of July, is now low (at an elevation of ~14 degrees) in the south as darkness falls lying above the 'teapot' of Sagittarius. Held steady, binoculars should enable you to see Saturn's brightest moon, Titan, at magnitude 8.2. A small telescope will show the rings with magnifications of x25 or more and one of 6-8 inches aperture with a magnification of ~x200 coupled with a night of good "seeing" (when the atmosphere is calm) will show Saturn and its beautiful ring system in its full glory.The thing that makes Saturn stand out is, of course, its ring system. The two outermost rings, A and B, are separated by a gap called Cassini's Division which should be visible in a telescope of 4 or more inches aperture if seeing conditions are good. Lying within the B ring, but far less bright and difficult to spot, is the C or Crepe Ring.
  
  
• September - Find the globular cluster in Hercules and spot the "Double-double" in Lyra There are two very nice objects to spot with binoculars in the south-western sky after dark this month. Two thirds of the way up the right hand side of the 4 stars that make up the "keystone" in the constellation Hercules is M13, the best globular cluster visible in the northern sky.Just to the left of the bright star Vega in Lyra is the multiple star system Epsilon Lyrae often called the double-double. With binoculars a binary star is seen but, when observed with a telescope, each of these two stars is revealed to be a double star - hence the name!
  
  
• September, late evening: the Double Cluster and the 'Demon Star', Algol. Later in the month is a good time to look high in the Southeast towards the constellations of Cassiopea and Perseus. Perseus contains two interesting objects; the Double Cluster between the two constellations and Algol the 'Demon Star'. Algol is in an eclipsing binary system. Normally the pair has a steady magnitude of 2.2 but every 2.86 days this briefly drops to magnitude 3.4. Visible times of the eclipse are (in UT): on the 12th at 23:43 and the 15th at 20:31..
  
  
• September 5th to 9th - midnight: Find Neptune. These nights are a great time to find the blue planet Neptune as it is very close to the 4th magnitude star Phi Aquarii. With a magnitude of 7.8, large binoculars or a small telescope will be required to spot it. A medium aperture telescope will reveal Neptune's disk showing a hint of blue grey. With such a telescope, you might also be able to spot its 14th magnitude Moon Triton. On the night of the 5th/6th Neptune lies just 13 arc seconds from Phi Aquarii! .
  
  
• September - evening: find the 'Coathanger'. Looking upwards after dark you should spot the three stars making up the 'Summer Triangle'. The lowest is Altair in Aquilla, up to its right is Vega in Lyra and over to its left is Deneb in Cygnus. With binoculars sweep upwards about one third of the way from Altair towards Vega. You should spot a nice asterism, formally 'Brocchi's Cluster' but usually called the Coathanger. It is formed of a straight line of six stars below which is a 'hook' of four stars. A pretty object.
  
  
• August 9th - evening: Jupiter near the Moon. In the evening towards the south-west, Jupiter will be seen down to the lower left of the Moon, a day after first quarter.
  
  
• September 8th: Two Great Lunar Craters This is a great night to observe two of the greatest craters on the Moon, Tycho and Copernicus, as the terminator is nearby. Tycho is towards the bottom of Moon in a densely cratered area called the Southern Lunar Highlands. It is a relatively young crater which is about 108 million years old It has a diameter of 85 km and is nearly 5 km deep. At full Moon - seen in the image below - the rays of material that were ejected when itwas formed can be see arcing across the surface. Copernicus is about 800 million years old and lies in the eastern Oceanus Procellarum beyond the end of the Apennine Mountains. It is 93 km wide and nearly 4 km deep and is a classic "terraced" crater. Both can be seen with binoculars.
  
  

Haritina Mogosanu and Samuel Leske from the Carter Science Centre in New Zealand speaks about the Southern Hemisphere night sky during September 2019.

• A bit about SeptemberSeptember comes from Latin word “septem”, which means “seven.” This is because in the old Roman calendar it was the 7th month, rather than 9th as it is today. Old Roman calendar used to only have 10 months until Julius Caesar introduced a new Julian calendar with 12 months. September has 30 days and marks the Autumn season in the northern hemisphere, and Spring in the Southern Hemisphere. This is the time of harvest and when many schools start their new school year in the northern hemisphere. Here, in New Zealand it is the month when we celebrate the September equinox, when the day is equal to the night.
  
  
• What’s the Sun up to? The Sun rises at 6:47 am on the first day of September and earlier and earlier every day so that on the 28th of September it will rise at 6:01 AM. However, the clock will shift by one hour so on the 30th of September it will rise at 6:57 am. The sun sets at 5:55 PM on the 1st of September and later and later 7:24 PM on the 31st of August. The days are getting longer.In September, the Sun transits first the zodiacal constellations of Leo, and then moves into Virgo on the 17th of September where it stays until October 31st. The zodiacal constellations are those stars visible behind the plane of our solar system, about 8 degrees each side of the ecliptic. This is why we say they form a band in the sky, called the Zodiacal Band. Since the Sun is transiting both the space we call Leo and Virgo it means we cannot see the stars in these constellations, they are behind the Sun.
  
  
• It's dangerous to look into the Sun!! Of course, if you have solar telescope, that is well maintained and is designed for looking at the Sun, then you can look at the Sun.The Sun in Virgo means only one thing: opposite the Sun (that 180 degrees on the other side of the zodiacal band) is Pisces. Pisces will rise just after sunset and be visible all night long.
  
  
• The Milky Way and Zodiacal Light In September the constellation of scorpius is the Fishhook of Maui that drags the Milky Way down from the sky here in Aotearoa. In addition to the Milky Way, if you are stargazing from somewhere with very dark skies, you can spot what is called the “Zodiacal Light” It’s a cone-shaped light that stretches from low on horizon along the ecliptic. The ecliptic marks the plane of our solar system bearing the zodiacal constellations. The ecliptic is a great circle on the celestial sphere representing the Sun's apparent path during the year, so called because lunar and solar eclipses can only occur when the moon crosses it. The zodiacal light is the light we see reflected from dust and ice particles in the plane of our own solar system! How cool is that? So in the sky we can see both the galaxy that we inhabit and the solar system. Two completely different scales!
  
  
• Planets
  
  
• Jupiter We continue to see Jupiter near constellation of Scorpius throughout the month of September in the evening sky.
  
  
• Saturn We also can enjoy the view of Saturn this month again. Near Sagittarius, Saturn with its magnificent rings continues to grace us with its presence. You can easily see the rings through a telescope here at Space Place but unfortunately you cannot discern the rings with just your eyes.
  
  
• Venus You can also catch a view of the planet Venus just after the sun sets later this month.
  
  
• Mercury Venus is also joined by Mercury in September. Although much fainter, you can see Mercury paired close with Venus later in September also right following sunset. Mercury is the closest planet to the sun in our solar system and can be difficult to observe but it’s possible if you time it right!
  
  
• Neptune In the late evening and morning sky, you can see the farthest planet from the sun in the solar system, Neptune in the eastern skies this month! Don’t try looking for it with your naked eye, as it is the only planet in our solar system not visible to naked eye, but with some help from telescopes or binoculars, you can see this ice giant planet and it will look like a bluish dot. Quite a delight to see!
  
  
• Uranus Lastly, Uranus is also a morning planet this month. Uranus' name is derived from Greek word for ouranos for “heavens” or “starry sky”. Uranus has a multitude of unique features including but not limited to its axis around which it spins being almost parallel to the solar system plane rather than perpendicular. In that sense it spins sideways around the sun, like a bicycle wheel. It would take a whole separate podcast to talk about Uranus but just know when you’re looking at this greenish and bluish planet that it is quite remarkable!
  
  
• Scorpius, Centaurus and Southern CrossAt this time of the year, in Aotearoa, the Māori names for Scorpius is Te Matau A Maui - the fishhook of Maui that drags the Milky Way from the sky all night long. The constellation Scorpius has a magnificent red supergiant star Antares. It is impossible to miss on a clear night. It looks quite reddish, just like planet Mars!
  
  
• Centaurus South of Scorpius you can find the constellation of Centaurus, a creature that is half-human and half-horse in Greek mythology. Although the constellation itself is more difficult to discern, it contains two very well known star systems in the southern hemisphere: alpha and beta centauri. Alpha Centauri is the closest star system to Earth! It’s about 4.37 light years away so it takes light about 4.37 years to reach it. As a reference, it takes about 8 minutes for light to reach us from the Sun! It is a triple star system and there was an exoplanet discovered orbiting Proxima Centauri, one of the three stars in this system.
  
  
• Circumpolar objects to New Zealand Circumpolar are objects that rotate around the celestial pole. These objects are above the horizon at all times in a given latitude. For instance Cassiopeia is circumpolar from Europe but here in Wellington we cannot even see it, here on the other hand we have the Southern Cross with the pointers that are circumpolar.The Diamond Cross and the False Cross are circumpolar too. Canopus and Achernar are also circumpolar. The same for the Magellanic Clouds, Omega Centauri, 47 Tucanae, the Jewel Box, the Southern Pleiades, the Gem Cluster and Omicron Velorum.
  
  
• Crux Alpha and Beta Centauri can be used as pointers to what is arguably the most well-known constellation in the southern hemisphere, the Southern Cross or Crux. In September, in the evenings, you will find the southern cross in the south western part of the sky. So just after sunset is at 3 o’clock position heading down followed by the pointers. Canopus would be at the same time grazing the southern horizon so hard to see from hilly wellington. Achernar and the two magellanic clouds would be in the south eastern part of the sky.
  
  
• Bright stars on the Ecliptic Very close to the ecliptic are Spica in Virgo early in the month. Spika means “head of grain” from Latin, it’s the grain that Virgo is holding. We can also see stars Zubenelgenubi and Zubeneschamali in Libra, and Nunki in Sagittarius. The ecliptic intersects the Milky Way in Scorpius.
  
  
• Stars in the Milky Way Starting from the centre of the Galaxy, going North are Shaula, the stinger of Scorpius, Atria in Triangulum Australe.
  
  
• Other Bright stars: In the north, we can see the bright star Altair in Aquila, the constellation of the eagle, a triangle-shaped constellation in north-eastern skies. Canopus, the brightest star in the southern hemispheres continues to shine bright and can be seen near horizon in southern skies.
  
  
• From Wellington New Zealand, Haritina Mogosanu and Samuel Leske wish you a fantastic September.