Aberdeen Harbour as you will know if you have done the other caches in this series is the UKs oldest business having been granted a royal charter by King David in 1136. This cache takes you to the entrance to the harbour on its southern side. Here you will find two tall octagonal white towers placed some distance apart these are the leading lights. At the cache location is the forward light tower and if you look inland up the street towards Torry you will see the rear light tower.
The leading lights were designed and installed in 1842 and are the world's second oldest cast iron light towers. Even to this day they play a vital role in ensuring the safety of vessels entering and leaving the harbour. Each of the light towers has a red and a green light. Vessels need to align themselves so that the two lights line up when they do that they are then sailing up the exact centre of the channel and thus are navigating in the deepest, safest part of the channel.
In 2013 the harbour channel was widened and the leading lights no longer marked the centre of the channel so there was an engineering challenge to move the historic tower next to the cache so that they once again aligned with the centre of the channel. A time lapse video was made of the move which you can see here.
The red and green lights are not as you might expect, red for stop and green for go, but are instead are directional. Red meaning vessels may enter the channel from the sea to enter the harbour but may not enter the channel if heading towards the sea. Green meaning vessels may enter the channel when heading sea bound but may not enter the channel from the sea heading harbour bound. Both red and green - no movement to the channel in either direction is permitted.
The cache location gives a different perspective to see vessels heading in and out of the harbour and gives good views across to Fittie and back towards the city.