My thanks to Vincent Alaniz for sending me this photo and a question, which I will answer here.
Whenever we consider the position of the sun in the sky, then time and direction are closely related. If we know one, we can usually work out the other. This is the logic behind shadow sticks, they help us work out if the sun is rising and therefore east of us, or setting and west of us. And they help us identify that unique moment, when the sun has reached its highest point in the sky, meaning it is true midday.
At this moment the sun will be due south and it will cast shadows due north.
There is a simple way of building a very basic clock using this knowledge and here is the list of materials you will need to construct it:
If you place a stick in the ground and then place the second stick a little way away and due north of the first stick, you have created the simplest clock imaginable. If the shadow of the first stick is to the left of the second then it is morning, if it is to the right then it is the afternoon and if the shadows lie perfectly on top of each other then it is the middle of the day.
This basic sundial needs no winding or batteries, and is more dependable over the long term than any manufactured clock or watch in the world. Put another way, if it stops working you will have more to worry about than accurate time-keeping, as the end of the world as we know it will be nigh. Or a cloud may have passed in front of the sun.