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THE NIGHT SKY FOR SEPTEMBER 2017

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during September 2017.

The Planets

  • Jupiter: Now five months after opposition, Jupiter can still just be seen very low in the southwestern sky after nightfall, lying at an elevation of some 10 degrees 45 minutes after sunset. By month's end it will be at an elevation of just 4 degrees at dusk. With a magnitude of -1.7 and an angular size of ~31 arc seconds it will be at its dimmest and smallest during this year's apparition and is too low for any reasonable telescopic views. At the start of September, Spica, Alpha Virginis, lies some 4 degrees to its lower left. Jupiter, moving eastwards passes 3 degrees to the upper right of Spica on September 11th. Now moving down towards the lower part of the ecliptic, next year it will only have an elevation of 25 degrees when due south whilst for the following two years an elevation of just 18 degrees.

  • Saturn came into opposition back on June 11th and so will be seen in the southwest as darkness falls and sets late evening. It shines initially at magnitude +0.4 falling to +0.5 during the month and has an angular size of ~16.5 arc seconds. With an angle of 26.8 degrees inclination to the line of sight, the rings are virtually as open as they ever can be. Their maximum tilt, at 27 degrees, will come in October - the first time since 2002. It is sad that Saturn, now lying in the southern part of Ophiuchus between Sagittarius and Scorpius, only reached an elevation of ~17 degrees above the horizon when due south, so hindering our view of this most beautiful planet. If imaging Saturn (or Jupiter), Registax 6 has a tool to align the red, green and blue colour images to largely remove atmospheric dispersion from the image. At somewhat over £100 one can purchase the ZWO atmospheric dispersion corrector which uses two, contra rotating, prisms to carry out an even better correction - and which can also be used for visual observing.

  • Mercury. has now become a morning object and will form a very tight grouping with Mars and Regulus, in Leo on the morning of the 5th. They will lie about 15 degrees below Venus. Binoculars will be needed to observe them in the bright twilight but please do not use them after the Sun has risen. Rising in elevation during the first part of the month, by the 10th it will have brightened to zero magnitude and lie just half a degree to lower right of Regulus. Mercury reaches greatest elongation, some 18 degrees from the Sun on the 12th - its best morning apparition this year. On the 14th, it lies 11 degrees to the lower left of Venus whilst, before dawn on the 16th, it closes to just 0.3 degrees from Mars. In the final week of September, moving back towards the Sun, it will be lost in the Sun's glare. .

  • Mars has now become a morning object at the start of its new apparition. Lying in Leo, and still not easily seen in the pre-dawn sky, it forms a tight grouping with Mercury and Regulus on the 5th some 15 degrees to the lower left of Venus. During the month, Mars has a magnitude of 1.8 and an angular size of just 3.6 arc seconds so no details will be seen on its salmon-pink surface. As the month progresses Mars rises higher in the sky before dawn and moves closer to Venus which is now moving back towards the Sun

  • Venus is visible in the east before dawn this month, rising around 2 hours before sunrise. Its magnitude remains at -3.9 during the month as its angular diameter shrinks from 12.4 to 11.2 arc seconds. However, at the same time, its illuminated phase increases from 84 percent to 91 percent - which explains why its magnitude does not change.

  • Highlights of the Month

    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-eastern sky well 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 - A good month to observe Neptune with a small telescope. Neptune comes into opposition - when it is nearest the Earth - on the 2nd of September, so will be well placed to spot this month. Its magnitude is +7.9 so Neptune, with a disk just 3.7 arc seconds across, is easily spotted in binoculars lying in the constellation Aquarius as shown on the charts. It rises to an elevation of ~27 degrees when due south. Given a telescope of 8 inches or greater aperture and a dark transparent night it should even be possible to spot its moon Triton.

    September 5th - before dawn: Mars Mercury and Regulus. Before dawn on the 5th, Mercury and Mars will be seen in a close grouping with Regulus in Leo. A very low eastern horizon will be needed to spot them with the use of Binoculars - but please do not use them after the Sun has risen. [The magnification given by the binoculars reduces the effective brightness of the pre-dawn light.]

    September 12th - before dawn: the Moon closes on the Hyades Cluster. Before dawn on the 12th, the Moon will be seen closing onto the Hyades Cluster in Taurus.

    September 16th - before dawn: three planets below the Moon. Before dawn on the 16th, a thin crescent Moon will be seen high above Venus, Mars and Mercury. Regulus lies between Venus and Mercury.

    September 26th - after sunset: Saturn below the crescent Moon. After sunset on the 26th, Saturn will be seen lying below the Moon.

    September: 12th and 28th: the Alpine Valley. These are good nights to observe an interesting feature on the Moon if you have a small telescope. Close to the limb is the Appenine mountain chain that marks the edge of Mare Imbrium. Towards the upper end you should see the cleft across them called the Alpine valley. It is about 7 miles wide and 79 miles long. As shown in the image a thin rill runs along its length which is quite a challenge to observe. Over the next two nights following the 28th the dark crater Plato and the young crater Copernicus will come into view. This is a very interesting region of the Moon!

    Southern Hemisphere

    Claire Bretherton tells us what we can see in the southern hemisphere night sky during September 2017.

    Kia ora and welcome to the August Jodcast from Space Place at Carter Observatory in Wellington, New Zealand.

    The Planets

    • Saturn: After what felt like a long, cold, wet winter here in Wellington, September marks the start of spring in the southern hemisphere. As we head towards the equinox on the 23rd of the month we see a rapid change in our daylight hours, with our days getting longer and our nights shorter. Equinox means "equal night" because we have the same number of hours of daylight and the same number of hours of darkness at this time of year. By the end of September the Sun won't be setting until nearly half past 7.The mission has revealed the complexity of Saturn's ring system, identified numerous new moons, and provided some of the most stunning images of the solar system that we have ever seen. It has found liquid oceans and a thick atmosphere on Titan, with conditions that may be similar to early Earth, and exploration of the icy moon Enceladus has revealed a hot spot at the southern pole, icy jets spewing out from the surface and a vast ocean below the ice.

    • Neptune: Whilst there are only two evening planets you can see with your own eyes, Neptune is also in our evening skies, with Uranus joining it before 10pm. Both can be picked out with binoculars, and you may even notice a greenish colour to Uranus, but Neptune will be indistinguishable from a faint star.Now is the best time to look for the eighth planet though, as it reaches opposition on the fifth of the month, when it will be directly opposite the Sun in the sky and at its highest in the north at midnight. At around this time the planet will also be at its closest and brightest, shining at magnitude 7.8, and is sat less than a degree just below and to the right of the 3.8 magnitude star lambda aquarii, but you'll still probably need a detailed finder chart to spot it. Unfortunately, the full moon passes close to Neptune just after opposition, so it may be easier to find a week or two later.

    • Stars

      The bright stars Vega and Canopus mark north-south around dusk this month, guiding our eye to the bright band of the Milky Way passing high overhead. Along with the nearby bright stars of Deneb, in Cygnus the swan, and Altair, in Aquila the eagle, Vega forms part of the "winter triangle" as seen here in the southern hemisphere.

      To the south-east of Capricornus is the faint constellation of Piscis Austrinus, the southern fish, with its only bright star, Fomalhaut, marking the mouth of the fish. Fomalhaut is the 18th brightest star in the night sky, and the only, lonely bright star in its vicinity.Follow up observations, however, failed to confirm the planet and left many doubting its existence. It took until 2012 before Fomalhaut b was independently detected and confirmed. Its controversial past has earned it the nickname "the zombie planet", a planet resurrected from the dead.

      Wishing you clear skies from the team here at Space Place at Carter Observatory.

      Compiled by Ian Morison


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