It’s all because our eyes contain two types of photoreceptors (cones and rods) and how they work in the dark compared to how they work in daylight.
 
Each retina 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, there are about 120 million rods. 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 only seeing in black and white. This means that anything we look at in the dark, including the Northern Lights, will only be seen in black, white and shades of grey.
 
However, 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. They can easily pick up the greens, reds, blues and purples that are present in the aurora, meaning that the colours you see in digitally taken images of the aurora are not a result of any photo editing but are, in fact, real.
 
The auroral colours are a result of collisions between gaseous particles in the Earth's atmosphere and charged particles released from the Sun's atmosphere. The colours present in the aurora depend on the type of gas particles the charged solar particles collide with. Green is the most common because most solar particles collide with our atmosphere at an altitude of around 60 to 150 miles, where there are high concentrations of oxygen and these collisions with oxygen produce a pale yellowish-green colour.
 
Blue and purple can also be present, but far less frequently than green because they only appear when solar activity is strong 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.
 
On rare occasions, red can be present in the aurora, but only when the Sun’s activity causes solar particles to react with oxygen at altitudes above 150 miles or so. At this height the oxygen is less concentrated and is “excited” at a higher frequency or wavelength than the denser oxygen lower down in the atmosphere and so produces a purely red aurora.
 
To give you an idea of the difference between what we can see and what the camera sees, I’ve prepared two versions of another image taken that night, but at Castle Tioram rather than Loch Shiel. These two versions are below, with the one on the left showing what the camera saw and the one on the right showing what I saw when I was taking the photograph. 

Don’t despair though, as but there is a way you can experience the colours for yourself. Many modern phone cameras have a “Night Mode” setting that allows you to take bright, sharp and noise-free photos, even in the darkness of the night. So, the next time you hear of the aurora happening, head to a place with a clear view of the northern horizon, put you phone camera in “Night Mode” and use it to help you see the colours of the ‘Na Fir-chlis’.

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.