Most people know that the Sun rises in the east and sets in the west, but they may not realise that this is quite a generalisation. In fact, the Sun only rises due east and only sets due west on two days of the year; the days of the Spring Equinox and the Autumnal Equinox which, for 2021, are March 20 and September 22. To illustrate this, Figures 1 and 2 show the location of the rising and setting sun at the Spring Equinox and Autumnal Equinox, respectively.

​Once it has risen due east and set due west at the Autumnal Equinox (Figure 2), the Sun rises and sets a tiny bit further south each day until the Winter Solstice, the day on which it rises as far to the southeast as it ever does and sets as far to the southwest as it ever does (See Figure 4). It then changes direction and begins moving north each day, eventually rising due east and setting due west on the Spring Equinox (Figure 1) before continuing northwards until the Summer Solstice, when it rises as far to the northeast as it ever does and sets as far to the northwest as it ever does (Figure 3).

​Once it has risen due east and set due west at the Autumnal Equinox (Figure 2), the Sun rises and sets a tiny bit further south each day until the Winter Solstice, the day on which it rises as far to the southeast as it ever does and sets as far to the southwest as it ever does (See Figure 4). It then changes direction and begins moving north each day, eventually rising due east and setting due west on the Spring Equinox (Figure 1) before continuing northwards until the Summer Solstice, when it rises as far to the northeast as it ever does and sets as far to the northwest as it ever does (Figure 3).

​Image 1, at the top of this blog, was taken at Resipole during sunset on the day after the Autumnal Equinox of 2019, with the camera facing southwest down Loch Sunart towards the Isle of Carna and the hills of Morvern.  If you look closely, you can see a patch of intense colour behind the wooded hillside on the right-hand side of the image. This was the spot where the sun was setting due west and for me, such a sight is one of the most welcome of the year because it marks the return of spectacular sunsets to this part of the Loch.

​With the passing of the autumn months and the movement of the setting sun towards the south, these sunsets become more and more intense. They are something that I’ll never tire of photographing even though I have taken countless photos of them from this spot. I can honestly say that I’ve never seen the same mix of colours twice. Each sunset is different because of the varying position of the sun and the infinite range of possible cloud formations that reflect the light from the sun and produce the kaleidoscope of colours.
 
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. 

Image 2 above was taken 4 days after the Autumnal Equinox of 2019, when I had a client out with me for some night photography tuition. We had spent the first part of the night preparing for and taking photographs of the Milky Way rising above Loch Sunart and while we were doing this a very high Aurora Alert came through on my phone. We quickly packed up our camera gear and headed up to Acharacle and set it up again on the jetty there is on Loch Shiel. It is a perfect spot to look for and photograph the Aurora Borealis because it faces directly towards northern horizon that has only a few distant and low-lying hills on it, meaning that the view north is clear of any obstructions. We had a very productive time there and also at another couple of locations nearby and came away with some great shots of the “Merry Dancers”.
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Finally, although the 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, the Autumnal Equinox will be at 8:21 pm on Wednesday 22 September and the Spring Equinox was at 9:37 am on Saturday 20 March.

​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.  

​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.

​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.

​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. 

​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'.

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.

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.

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.

"Are they really that green colour and can you see it?”

The short answer to this question is yes, but our eyes can see only see the Northern Lights as a grey or bluish grey dancing light shapes in the sky. This is because of how our eyes work in the dark compared to how they work in daylight. 
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You see, the retina in the human eye contains two types of photoreceptors, rods and cones. It has about 6 to 7 million cones and they provide the eye with the ability to distinguish colour. However, these cones only work effectively when it is light. On the other hand, the eye has about 120 million rods and these are far more effective at helping us to see in low light and near dark conditions, but they cannot distinguish colour. So, as it gets dark and the cones stop working, our eyes move from enabling us to see in full colour to enabling us to only see in black and white. This means that anything we look at in the dark, including the Northern Lights will only be seen in black and white and shades of grey.

On the other hand, the sensors that you find in digital cameras do not suffer from this limitation and are extremely effective at picking up both light and colour in the dark. This means that they pick up the colours in the Northern Lights that were present at the time meaning that the colours you see in the images produced by a camera are not a result of any photo editing. 

The auroral colours that are present are actually a result of collisions between gaseous particles in the Earth's atmosphere and charged particles released from the sun's atmosphere. Three colours tend to be present in the aurora, with each colour depending on the type of gas particles that are colliding. Green is the most common as most solar particles typically collide with our atmosphere at an altitude of around 60 to 150 miles where there are high concentrations of oxygen and it is particle collisions involving oxygen that produce a pale yellowish-green colour.

Blue and purple can also be present, but far less frequently than green as they tend to only appear when solar activity is high enough to cause particle collisions in our atmosphere at an altitude of 60 miles or less. At these heights, it is a reaction with nitrogen that causes the aurora to be tinged with purple or blue.