Our Night Sky: September 2021

Depending on your point of view, Autumn either starts on 1 September (meteorological autumn) or the 22 September (astronomical autumn). The latter is when the Autumnal Equinox takes place and when the sun is directly above the earth’s equator. On that date, day and night are of equal length and the Sun rises due east and sets due west. With this, the dark nights return and, because it doesn’t get too cold outside, September is a great month for stargazing. According to NASA, the Equinox is a prime time for Northern Lights so, if you want to catch a glimpse of them, do keep an eye out for “aurora alerts” on social media.

Tioram Light Show - Castle Tioram, Doirlinn, Moidart
This image of an amazing show of the Northern Lights over Castle Tioram was taken on a clear and starry night a few days after the Autumnal Equinox of 2019

The Constellations

​This month the sun sets around 8:00 pm and the stars and the constellations become visible from about 9:00 pm onwards. As with July and August, the best way to navigate your way around the night sky is to use the Summer Triangle, which is made up of the three bright stars of Vega, Altair and Deneb. Start off by finding Vega, a really bright white star, which will be directly overhead. Next is Altair, which you will find halfway between Vega and the southern horizon. Finally, there is Deneb, which will be almost directly above Altair and across from Vega.
 
Once you have found the Summer Triangle, you can start to look for some of the constellations in the southern sky. Start off with Vega, the constellation of Lyra’s (the Lyre) brightest star. Below it, you will see the constellation’s 4 other main stars. These form a small parallelogram and make up the body of the Lyre – the sky’s only musical instrument.  

Next, look at Deneb, which marks the top of the Northern Cross, or the constellation of Cygnus (the Swan), which flies down the Milky Way with its wings outstretched. Deneb marks the tail of the Swan and Albireo, a fairly faint star at the bottom of the constellation, marks its head. If you have a telescope, or even powerful binoculars, take a look at Albireo and you’ll see that it’s a double star of contrasting colours. Typically, people see them as yellow and blue, or more accurately yellowish and bluish.

​Finally, looking at Altair, which you will find the constellation of Aquila (the Eagle) because Altair is its brightest star. Altair also has outstretched wings, though they are not as obvious at the wings of Cygnus. In between these two flying birds is the constellation of Sagitta (the Arrow), a dim but distinctive shape which is reasonably easy to pick out. Another small but easily seen constellation called Delphinus (the Dolphin) is nearby and it also looks like what it’s meant to be. 

The Constellation of Cassiopeia
Image Credit: Till Credner [CC Share Alike 3]

​The two better-known constellations of Capricornus and Aquarius are below Delphinus in the lower part of the sky. They are well known because they are in the Zodiac, the path of the planets but, as they have no very bright stars, you might not be able to pick them out.  Capricornus is the Water-Goat and Aquarius is a human, the Water-Carrier.

Southern Night Sky - September 2021
Created using SkyPortal Mobile App

Northern Night Sky - September 2021
Created using SkyPortal Mobile App

​As with August, September is a good month to view the Milky Way. If you trace a line down from Deneb and through Altair, you should be able to make out the Milky Way’s cloudy core to the right of Jupiter and Saturn, which you’ll find shining bright just above the southern horizon. The cloudy core will be visible up until about 10:00 pm. When you look at the Milky Way, you’ll notice a black band running down it’s middle, between the stars. William Herschel, the first astronomer to map our Galaxy, thought that this was a hole in space, but it is actually a dark swathe of sooty dust, known as the Great Rift in Cygnus which runs along the disc of our Galaxy.
 
If you look north, you’ll see the same stars as you would in any other month, but their orientation varies from month to month. By September the familiar Plough asterism, which many people recognise, is down over to the north-west. Follow its two right-hand stars upward, veer a little bit to the right and you’ll find Polaris, the Pole Star. If you look over to the north-east you can see Capella. Contrast its colour with that of Vega which is virtually overhead at this time of year. Capella is a yellow giant star, while Vega is a slightly bluer star than the Sun. The contrast is obvious if you look from one to the other.

The Moon

​At the very start of the month, we just miss the last quarter Moon as it was on 30 August, while there is a new Moon (no Moon) on 7 September. So, if it’s clear on the evening of 9 September, you will find the slightest of crescent Moons setting in the west at around 8:00 pm with a bright Venus to its left. However, it may be difficult to see the moon on 9 September as it will be lying close to where the sun has set. You'll probably find it easier to see it on the following evening, 10 September, as it will have moved slightly to the left and will be sitting further from the sun and to the left of Venus. 

The first quarter Moon (a half moon) is on 13 September and on 16 September it will be sitting just below Saturn. On 17 September, you will find the moon below and to the right of Jupiter, while on 18 September it will be below and to the left of this gas giant, the largest planet in our Solar System.

​This month's full moon is on the night of 20-21 September, becoming 100% full at just before 1 am on the morning of 21 September. It will rise over in the east at around 8:00 pm, with Jupiter well over to its right. On the night of 25-26 September, you will find the moon passing beneath and close to the Pleiades (The Seven Sisters), one the closest open star clusters to Earth.  September’s full moon is called the Corn Moon because this is when crops are gathered at the end of the summer season. At this time of year, the Moon appears particularly bright and rises early, thus letting farmers continue harvesting into the night. This moon is also sometimes named the barley moon, and it is often the nearest full moon to the autumnal equinox, earning it the title of 'harvest moon'.

​The Planets

When it is getting dark, you may be able to spot Venus hanging low down on the western horizon just before it sets at around 8:30 pm.  You may also catch Mercury, the smallest and innermost planet in our Solar System because it reaches maximum separation from the Sun on 14 September.
 
However, if you are looking for planets, you will have more success when looking to the south-east at dusk because two of our Solar System’s gas giants, Jupiter and Saturn will rise from there at about 7:30 pm.  Jupiter is the brighter of the two planets and will be sitting to the left of Saturn. Jupiter sets at around 4:00 am, while Saturn sinks below the horizon at about 2:30 am.
 
If look at Jupiter through binoculars, you’ll see its four largest moons. They are Io, Europa, Ganymede, and Callisto and are known as the Galilean moons. With a telescope, you’ll also be able to see the dark belts of Jupiter crossing its disc, and maybe, if you are lucky, the storm known as the Great Red Spot. If your binoculars or telescope magnify by about 40 or 50 times, you be able to see Saturn’s famous rings and its biggest moons when you look at it. The largest, Titan, is enormous. In fact, it’s 40% more massive than the planet Mercury and 80% more massive than our own moon.
 
Neptune will be above the horizon all night long and you will find it to the left of Jupiter in Aquarius. It will be at its closest to Earth on 14 September, but it is our Solar System’s most distant planet, so "close" actually means 2,688 million miles away. Also, it is not particularly bright, so you will need binoculars or a telescope to see it. Its near twin in size, Uranus, can be found further over to the east in Aries and will be just visible to the naked eye, although you’ll have a better chance of seeing it if you use binoculars or a telescope.
 
Mars is too close to the Sun to be visible this month.

Meteor Showers

The annual Perseid Meteor Shower put on a good show last month and those of you who were lucky enough to have clear skies around its peak on the night of 12-13 August will have seen them shooting across the sky. However, September is not a good month for meteor showers, so we will have to wait until October for the chance to see some more. The Draconis will peak on the night of 8-9 October and the Orionids on the night of 20-21 October.

Autumnal Equinox

The Autumnal Equinox is when night and day are nearly exactly the same length (12 hours). It also marks the start of astronomical autumn and when the nights start to become longer than the days. Although the autumnal equinox referred to as a day by many people, it is actually the exact moment in time when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun. For 2021, this will be at 8:21 pm on Wednesday 22 September.
 
The Autumnal Equinox also paves the way for increased chances to see aurora borealis displays. According to NASA, the equinoxes are prime time for Northern Lights, because the geomagnetic activity that causes them is more likely to take place in the spring and autumn than in the summer or winter. In addition, we tend to have more clear nights in spring and autumn so this, combined with more geomagnetic activity, may be the reason why I tend to have captured most of my Northern Light images in September/October and March/April. Finally, if you do want to catch a glimpse of the “Merry Dancers”, be sure to keep an eye out for “aurora alerts” on the web and on social media. Good sources for forecast and alerts are AuroraWatch UK, Glendale Skye Auroras and Aurora Research Scotland.