“Sun * Moon * Venus * Mercury * Mars * Jupiter * Saturn
Our sky-watching, hunter-gathering ancestors had 7 marked out as a number of enormous importance for tens of thousands of years. For this is the number of the visible planets—‘the five wanderers,’ plus the sun and the moon.
This respect for the 7 became ever more ingrained as the first agricultural civilizations allowed for accurate fixed observations from the calendar-keeping priests, whose temples throughout the ancient Middle East were all equipped with star-watching terraces above their cult chambers. It is an intriguing element within the cult of the 7 that the planets are not all visible at once: Mercury and most especially Venus (whose horns are occasionally visible) are the morning and evening stars. Bright Jupiter, luminous Saturn, and the more elusive red Mars belong to the full night. So we have always known that we have been watched, influenced, and enclosed by these 7 who right from the dawn of our consciousness have intriguingly different characteristics and hours of dominance and passageways through the heavens.
Although most of mankind probably now accepts that the earth is a planet which circles around the sun, and the moon is a planet of the earth, the mystery of our 7 encircling heavens still haunts our imagination. But this once immutable number of 7 keeps changing. First we knocked the seven down to five (as the sun and moon were taken off the list), then, in relatively modern times, it grew to nine. Uranus was discovered in 1781, followed by Neptune in 1846, then Pluto in 1930 (though this was later demoted to a dwarf planet to bring us back down to eight planets). So, currently, we have eight planets and five dwarf planets (Ceres, Pluto, Haumea, Makemake, and Eris), as well as five named moons orbiting around Pluto.”
Excerpted from: Rogerson, Barnaby. Rogerson’s Book of Numbers: The Culture of Numbers–from 1,001 Nights to the Seven Wonders of the World. New York: Picador, 2013.