Seven is a common story-telling number. Seven days in a week, sailing the seven seas, the seven wonders of the world, the seven continents, seven deadly sins, etc.
1. Seven days in a week
2. sailing the seven seas
3. the seven wonders of the world
4. the seven continents
5. seven deadly sins
6. seven sisters
7. the seven canonical examples
There is a whole spectrum of color, but there are only a handful of colors that most humans distinguish and talk about. It has always seemed odd to me that we elevate Indigo into the spectrum of named colors.
However, Newton was fond of the occult, and numerology, and the number seven. He wrote papers about appearances of the number seven in the Bible.
I think that Newton made Indigo an official color just to bring the number up to 7.
The seven colors are a consequence of us having 3 distinct photosensitive pigments in our eyes which aren't evenly spaced in terms of the wavelengths to which they are sensitive. If they were perfectly spaced we'd have 6 colors: Red, Anti-dark-blue=Yellow), Green, Anti-red=Cyan, Dark-blue, and Anti-green=Magenta. However because of the shift, what we consider to be true blue is somewhere between dark-blue and cyan, so we have Indigo which is halfway between True-blue and Red, Orange which is Anti-true-blue, magenta is blue-shifted into violet, and we forego cyan as one of the major colors.
Note that different cultures do not actually agree on 7 colors in the spectrum or which colors they are. For example some languages consider green to be a shade of blue or vice versa, or green to be a shade of yellow. Some only distinguish between hot (red-yellow) and cold (green-blue) colors. There are also some non-spectrum basic colors, for example english considers light-red (pink) and dark-orange (brown) to be basic colors, whereas a dark green is just a shade of green; and this also varies by culture.
Yes, but as has been noted multiple times in this thread, there are more than 6 visible stars in the cluster. They're just not visible at all times of year, and certainly impossible to see for most of us who live in/near cities.
This is not true. The Pleiades cluster looks the same all year round.
Depending on how one defines the limit of what can be seen, there are either 0 stars, or 6 stars, or 9 stars, or hundreds. But only someone with very, very exceptional eyesight under very, very exceptional conditions can actually see seven stars but not nine.
Source: I've been observing the cluster, and the sky as a whole, for decades. I'll draw the locations of the six notable stars just as easily as some will draw the big dipper. And I've never, in decades of observing under many different conditions from different latitudes, ever seen seven stars.
I think the underlying question is, "Can Pleione be observed with the naked eye today?"
> And I've never, in decades of observing under many different conditions from different latitudes, ever seen seven stars.
This really only tells us about you. Even if your vision is 20/20, your experience doesn't preclude the possibility of someone else discerning Pleione. And if they can, that ability can definitely be determined by time of year; which affects not only where the Pleiades are in the sky (e.g. how much air mass you're looking through), but also things like turbulence and humidity.
