Why is the number seven so special?

Lucky seven.  Seven chakras.  Seven virtues.  Seven deadly sins.  The seven days it took the God of Abraham to create the world (including rest day). Seven wonders of the world.  Seven days in a week.  What is it with all these sevens?

Let’s start by taking a look at the rainbow, with its seven colours: red, orange, yellow, green, blue, indigo, and violet:

Close up of a rainbow over Whitby as viewed from my then workshop window.

When you look at a real one as opposed to a cartoon, that division into seven is not at all obvious and certainly not evenly spaced. Red and yellow are clearly there… but where exactly does the red become orange as opposed to red, and where does the orange become yellow?  There is no distinct boundary.

As for the greeny bluey purpley end of the spectrum… well there are several shades of green, not a whole lot of blue, and if you can find a boundary between indigo and violet then you have much better colour vision than me.

Isaac Newton painted by Godfrey Kneller and looking non too happy about the number of colours in his rainbow.

As far as the spectrum and rainbows are concerned we have Sir Isaac Newton to thank/blame for dividing it into seven… although originally he specified only five colours… adding orange and indigo later on so as to… wait for it… match the number of colours with the number of musical notes in a major scale.

I kid you not, and if you haven’t heard that before I’m guessing you expected something rather more scientific, especially when you consider that there’s no particular reason why there should be seven notes in a musical scale either; that’s just a western tradition and other cultures have different numbers of notes in their musical scales.

So why WAS this icon of 17th century science, so hung up on the number seven that he felt it necessary to squeeze a couple more colours into his spectrum?

Well, I suggest that ultimately, it all comes down to there being seven days in a week, as that seems to pre-date pretty much anything else:

The seven day cycle that we call a week, is widely attributed to the Babylonians. However it’s possible that a similar cycle was developed independently by other cultures.

Chakra Pendant from my AndysEtsyShop

Why? Because like many civilisations before the invention of television, the Babylonians spent the dark evenings making little Babylonians and/or observing the stars.

All cultures that have done this have noted patterns amongst the stars, and have watched them move across the sky during the course of the night.  Thus they have observed that whole sky seems to move as a single mass… and the following night, the same patterns, make the same journey, all over again.

Over a period of weeks it becomes apparent that the entire sky shifts a little further north or south… and observation over a period of years makes it apparent that this is repeats annually, which we now know is due to the earth’s axis being at an angle to the plane of its movement around the sun (it’s also what causes the changing seasons).

This was not merely curious, but potentially useful, because by observing these shifts in relation to landmarks, it was possible for the ancients to chart their progress through the year, and thus predict how much more chilly weather they’d need to endure before planting crops, and when might be the best time to book their summer holidays in order to get the best deals.

However, in amongst this uniform movement of the night sky there are a few celestial bodies that follow paths of their own. The sun and moon are obvious examples because although they have their observable and predictable paths across the sky, they move independently of the seemingly fixed backdrop of stars.

With the naked eye, it is possible to identify five more such celestial deviants, giving a total of seven… and the theory goes that the Babylonians set up a seven day cycle in their honour, and thus invented “the week”… which also makes for a rather nice division of the moon’s (approximately) 28 day cycle.

Mercury, Venus, Mars, Jupiter, Saturn – not to scale

Those other five celestial bodies are: Mercury, Venus, Mars, Jupiter and Saturn.  However those are the Roman names for them because, as you might expect, when the Romans took over and adopted the seven day cycle, they changed the names to those of their own gods.

Thor fights for a four day week on behalf of the Norse gods.

As time went on, the system came into use in Northern Europe where, although we kept the Roman names for the planets, we changed some of the names of the days of the week to honour the Teutonic gods. Thus we now have:

Sun’s-day
Moon’s-day
Tyr’s-day (Tyr being the Norse god of war)
Woden’s-day (Woden = Odin a.k.a. The All-Father)
Thor’s-day (The blonde guy from The Avengers)
Freyja’s-day (Some say Frigg’s-day but it’s arguable that she was the same person anyway)
Saturn’s-day

So, with a couple of thousand years worth of seven day cycles having gone on before he was born, and all of the other stuff that had been tied into the number seven by then, it’s hardly surprising that Sir Isaac Newton considered the number seven to be somehow special, as indeed we continue to do today.

Additional Notes

  1. A particularly interesting occurrence of the number seven, because it is in no way tied into any of the above and occurs independently, is that there are seven “crystal systems” (triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic), which relate to the arrangement of atoms within crystals.
  2. It always surprises me how many people aren’t aware that opposing faces on six sided dice add up to seven. 1+6, 2+5, 3+4. That’s how they are made. Also: if you roll a pair of dice, your safest bet is that you will roll a 7, as that’s the most likely outcome (there are more possible combinations that add up to 7 than any of the other possibilities).

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