Time of day It’s easy to see when it gets light and dark, but it’s harder to keep track of how far through the day it is. This is where the sun is helpful – or rather, the shadows it makes. Over 5,000 years ago, in about 3500 BCE, the ancient Egyptians started putting up tall stones, called obelisks, and using their shadows to tell the time.
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The shadow is longest at the beginning and end of the day ay when the sun is low in the sky. At noon, the sun is right overhead, so the shadow is very short. The shadow gets shorter – and then longer – as the day goes on.
There’s never any shade at lunchtime!
early morning
noon
late evening
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Shadows changing their length is crazy enough – but they change direction too! The Egyptians and Babylonians soon worked out how to use this movement to track time passing – they invented sundials. This Egyptian sundial is called a “gnomon”. You have to hold it with the crosspiece to the east in the morning, and to the west in the afternoon. Where the shadow falls shows you how far through the day you are. Clever! Half of the stick is in shadow, so it’s halfway between midday and nightfall. This end is pointing west.
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The markings on a Roman sundial sunrise morning: six divisions
sunset
afternoon: six divisions shadow shows the time
noon
Fact Sundials were so useful that by 30Â BCE, there were over 13Â different types.
This Roman sundial stands on a pillar.
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To make it easier to talk about time, the Romans came up with a name for the 12 parts of the day – “horae” which means “hours”. But there is one big difference between Roman horae and modern hours. Let’s meet for a snack at the fourth hour.
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Because the earth is tilted, the length of daylight changes through the year. Days are longer in the summer and shorter in winter. So when a sundial divided the daylight into 12 equal parts, it meant the parts changed length too, depending on the season. A sundial hour in winter can last only 40 minutes, while a summer one can last 75 minutes! It’s the fourth hour.
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