The Twilight Zone – A Guest Post by Julian Delf

As our next assignment is “Dusk to Dawn” I thought it might be interesting to look at dusk and dawn and how they relate to sunset, sunrise, twilight and the golden and blue hours. We will look specifically at dusk, dawn is just the reverse.

So, we might think dusk is some time around sunset to when it gets dark, but there is a precise scientific definition of what it is. Contrary to what we might think dusk isn’t actually a period of time, it is a point in time. To make things slightly more complex there is more than one level of dusk. In looking at the definition we need to consider events related to the “true horizon”. At many locations the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the “visible horizon”.

So, the first event to consider is sunset, this is the time at which the trailing edge of the Sun crosses the true horizon. When the Sun is low to the horizon the atmosphere refracts the light, which means the Sun may still appear to be on the horizon when it has actually already set. After sunset we enter “Civil Twilight”. This continues until the centre of the Sun is 6º below the true horizon and this is the point of “Civil Dusk”. From this point on we are in “Nautical Twilight” until the centre of the Sun is 12º below the true horizon and this is the point of “Nautical Dusk”. After this we move into “Astronomical Twilight” and that continues until the centre of the Sun is 18º below the true horizon and this is the point of “Astronomical Dusk”. After this point we are in Night, this continues until the Sun has risen to 18º below true horizon and this is the point of “Astronomical Dawn” and we follow the same steps e.g. “Astronomical Twilight” to “Nautical Dawn”, etc.

When we look at the times these events occur and how long they last, it gets a bit tricky as the path of the Sun depends on our Latitude and the time of the year as these change the angle the Sun rises and sets. If we look at our next meeting (6th June) we have :

Sunset 21:05, Civil Twilight length 37min, Civil Dusk 21:42, Nautical Twilight length 48min, Nautical Dusk 22:30 Astronomical Twilight length 60min, Astronomical Dusk 23:30 – start of Night

There are many websites that show these times over the year, a good one is :

https://www.timeanddate.com/sun/canada/owen-sound

The changes in the position and path of Sun also affect the photographic Golden Hour and Blue Hour. As well as the Sun’s position these hours are also dependent on the local weather and atmospheric conditions. Unlike the descriptions above these don’t have precise technical description and there are a few different interpretations of how to calculate them.

The Golden Hour, or Magic Hour, relates to a period sometime before sunset till sunset, or just after sunset. If we use the information from the site http://www.golden-hour.com/ then it is taken as the time the Sun is 6º above the horizon till sunset. A different site uses an end point of the Sun being 4º below the horizon. Looking at the timings for 6th June we have :

Evening Golden Hour Starts (6º above the horizon) 20:21, ends (sunset) 21:05, duration 44mins. The Blue Hour relates to a period where the golden light has ceased and the light is mainly blue. In this example it is taken from near the end of Civil Twilight to near the end of Nautical Twilight, a good site for Blue Hour information is http://www.bluehoursite.com/. Looking at the timings here for the 6th June we have :

Night Blue Hour Starts 21:27, ends 22:17, duration 50mins

However other sites may give different timings based on different calculations. There is a matching “hours” in the morning. Below is a graphic that shows all the above points.

 

 Screen Shot 2017-05-15 at 11.41.42 AM

 

 

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