Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during September 2015.

The Stars

To the south and moving westward as night progresses you may see the Summer Triangle: the bright stars Deneb (in Cygnus), Vega (in Lyra) and below them Altair (in Aquila). Towards the south later in the evening you may spot the great square of Pegasus - adjacent to Andromeda and M31, the Andromeda Nebula. To the north lies "w" shaped Cassiopeia and Perseus. Between the two, close to the Milky Way, try to spot the Perseus Double Cluster with a goods pair of binoculars. You might also spot M33 may also be visible on a transparent night with a good pair of binoculars.

The Planets

• Jupiter reached superior conjunction on August 26th, and now rises shortly before the sun. It will be best seen at month's end, 18 degrees above the northeast horizon at sunrise. With a disk increasing to 31 arcsecond disk, you should be able to see its equitorial bands and 4 Galilean moons.
• Saturn can be seen after sunset low in the southwest. It lies in eastern Libra, moving slowly away from the wide double star Alpha Librae as it shines with a magnitude of +0.6. One hour after sunset at the start of the month it will lie just 10 degrees above the horizon with a 16.4 arc second disk. By month's end it will only be a few degrees elevation at this time so early this month is really our last chance to observe it for a month or so as it passes behind the Sun. The ring system, now opened out to 24.3 degrees to the line of sight, should still be visible along with Titan, its largest satellite.
• Mercury can be seen just above the western horizon for the first few days of the month reaching greatest elongation from the Sun on the 4th of September shining at magnitude +0.1. It will be lost in the twilight by mid-month before it passes in front of the Sun (Inferior Conjunction) on the 30th.
• Mars is a pre-dawn object, and lies in Leo not far from Regulus, Alpha Leonis. On the 25th the salmon-pink planet will lie just 47 arc minutes from the blue star making a very nice colour contrast. Shining at magnitude +1.8 its disk is just 3.8 arc seconds across so no details will be seen of its surface. Seen best towards the end of the month, it will then rise around 3 hours before the Sun.
• Venus, rises in the east-northeast in the pre-dawn sky an hour and a half before the Sun at the start of September but this increases to four hour by month's end as Venus moves further away in angle from the Sun. Shining at a magnitude -4.8 during the third week of the month month it will show a thin crescent, 9% illuminated, 52 arc second disk as the month begins.

The Moon

On September 4th and 21st you may spot The Alpine Valley, a cleft across the Appenine mountain chain. It is about 7 miles wide and 79 miles long and a thin rill runs along its length which is quite challenging to observe.

Highlights

• Neptune came into opposition on the 29th of August, so will be seen well this month. Its magnitude is +7.9 so Neptune is easily spotted in binoculars lying in the constellation Aquarius as shown on the chart. 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.
• Look east before dawn on 5th September: the magnitude +0.9 star Aldebaran will be occulted by the bright limb of the Moon as it passes in front of the Hyades cluster at around 05:30 BST. Given a mount tracking Aldebaran with a telescope, you may see it reappear from behind the unlit lunar disk at 07:10 in the bright daylight sky. Be warned: the times will vary by a few minutes depending on where you live in the UK so be watching Aldebaran for perhaps 10 minutes before the two stated times.
• All clear before dawn on 10th September given a good low unobstructed horizon towards the northeast you may spot an 8% lit, thin crescent Moon just 2 degrees to the upper left of Venus shining at magnitude -4.6.
• Looking southwest one hour after sunset on 18th September (20:45 BST), Saturn will be seen 4.5 degrees to the lower left of a waxing crescent Moon.
• About one hour before dawn on the morning of the 25th, Mars will be seen less than one degree just to the left of Regulus in Leo. Shining brightly up to their right will lie Venus, dominating the morning sky whilst lying well below Mars will be seen Jupiter - a very nice planetary grouping.
• Before dawn on 28th September, from 01:12 BST until 06:22 BST we will witness a total eclipse of a Harvest Supermoon! It will display the largest apparent angular diameter of the year (33.5 arc minutes) and the eclipse lasts for three hours and twenty minutes with totality starting at 03:11 BST and ending at 04:23. At the mid point of the eclipse at 03:47 BST the Moon will then lie at an elevation of 27 degrees above the southwest horizon. As the Moon leaves the umbral shadow 05:27 BST it will lie some 15 degrees above the horizon in the dawn sky.

Southern Hemisphere

Haritina Mogosanu from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during September 2015.

The Milky Way

At the beginning of springtime here, the Milky Way spans the sky from north to south going through Zenith. From there, the fish hook of Maui slowly starts to drag the Milky Way down from the sky and towards the western horizon, all night long so that night after night the center of the galaxy appears lower and lower in the evening sky. Here, in the Southern Hemisphere, we are very lucky to see the Milky Way in all its brightness and beauty.

The Stars

Many of the brightest stars are scattered along or near the Milky Way. Starting from North is Albireo, the beautiful orange and blue double star, in the constellation of Cygnus or the Northern Cross. On the left hand side and close to the Milky Way lays Vega, due north at dusk and setting in the late evening. On the right hand side, a celestial dolphin flips from the galactic river, revealing its two famous stars Sualocin and Rotanev- the anagram of the astronomer Nicolaus Venator! Just a few degrees higher up in the sky than the dolphin, the Eagle is flying Altair towards the galactic river. Keep lifting your head up and follow the Milky Way. With a telescope powerful enough, you will find Pluto right there near the Teapot’s handle in Sagittarius, but at 14 magnitude you would need a large aperture telescope (of 10 inches or more) to see it, as well as great star charts to pinpoint it. If Pluto is not an easy target, the fish hook of Maui in Scorpius is home of the bright star red giant Antares or Rehua in Maori. In between Sagittarius and Scorpius there are beautiful deep sky objects and now is a great time in the southern hemisphere to hunt for them. They include the Lagoon Nebula, the Omega Nebula, also known as the Horseshoe Nebula or Swan Nebula; and the Trifid Nebula.

To find directions in the Southern Hemisphere all we need to do is to follow the arch of the Milky Way. On it, midway down the southwest sky almost opposite Altair, are ' The Pointers ', Beta and Alpha Centauri. They point down to Crux the Southern Cross. There are about 27 ways to find South here and most of them involve the Southern Cross. As a circumpolar constellation, at the beginning of the spring's evening sky, Crux appears almost in the 3 o'clock position on the 60 degrees declination circle. Inside it, the brilliant Jewel Box, discovered by Nicolas Louis de La Caille and baptised so by John Herschel, or NGC 4755, is an open cluster of stars. At the center of it, a blue giant, a red giant, and another blue giant star align to make the more modern asterism of the 'traffic light'. But of course you will need a telescope to see this.

Globular Clusters

• Omega centauri, the largest globular cluster in the Milky Way can be seen as a fuzzy star roughly near the Southern Cross. In telescopes it appears like very delicate lace and it takes skill to use peripheral vision to see it but it's worth the effort.
• The 47 Tucanae globular cluster hides in the constellation of the exotic bird the Toucan. But once again the Southern Cross can help. Imagine that Crux could as a giant arrowhead, you will see it pointing left to a star, Achenar, on the other side of its declination circle. Following the arrow head, takes us between two fuzzy clouds, two thirds from Crux towards Achenar. These are the Magellanic Clouds and 47 Tucanae is located beside the Small Magellanic Cloud. In ideal conditions it should look almost the size of the full Moon.

The Moon and Planets

In Maori the Moon is called Marama which literarily can mean the white (ma) light (ma) coming from the sun Ra. The harbinger of this spring is a supermoon. A supermoon happens when the full moon coincides with when the Moon is closest to Earth, also known as perigee. Supermoons occur every fourteenth full Moon.

Mercury Whiro and Saturn Pare -a -Rao are bright planets in the evening sky. At the beginning of the month Mercury is making its best evening sky appearance of the year, low in the west. Cream-coloured Saturn is northwest of the zenith at dusk and midway down the western sky by late evening. Brilliant Venus, 'Kopu Rere Ata' is the 'morning star' for Maori, rises in the east two hours before the Sun. A telescope shows Earth-sized Venus shining as a thin crescent from 60 million km away. On 21 of September Venus displays its greatest illuminated extent as the morning "star" This means that for the next several mornings, our morning star Venus will be shining at or near its greatest brilliancy.