Fortnight

“Fortnight” is such an interesting word. It’s still used in British English, but it sounds anachronistic to American ears, like “four score.”

If it sounds like it’s a blend of the two words “fourteen” and “night”, that’s because it very much is. Fourteen nights, or two weeks, is the length of time in one fortnight.

Well, okay, but… why nights, and why fourteen?

The answer comes from the same place: the moon.

If you saw a new moon, 29 or 30 nights would go by before you would see a new moon. When we think of a lunar cycle today, this is pretty much what we think of today. This is called a synodic month, and synodic means “meeting.”

That’s because the moon “meets” where the sun lines up with the earth, and that’s why there is a new moon. The moon is tidally locked with the earth, so we always only see one side of the moon. When the sun is lined up “behind the moon”, we have a new moon every 29.53 days.

Our ancestors also paid an awful lot of attention to the night sky. It provided a handy calendar, to be sure, but it also gave us amusement and entertainment at a time when there wasn’t a lot of competition for eyeballs.

Stories formed based on these stars, planets, and moons. Some of these stories became deeply held religious beliefs over many, many generations.

In addition to the synodic month, there’s another incredibly reliable cycle that happens. It’s called the sidereal month, and “sidereal” comes from the Latin “sidus”, meaning star. Our ancient ancestors would compare the amount of time it took for the Moon to return to the same position, as compared to the stars.

This turned out to be a shorter amount of time—27.3 days, give or take.

Okay, so there were two types of cycles our ancestors noticed, and maybe it makes sense that 28 days “split the baby”, making neither option entirely accurate… but that’s not terribly satisfying, is it? And, it’s speculative to boot.

There’s one more reason for “fortnight”, though. First, it’s easy to divide 28 nights by two, so you would end up with 14 as the midway point. This halfway marker was extremely useful for people who wanted to measure something longer than a day, but still shorter than a lunar month.

Even ancient superstition played a role: the number 7 was considered to be lucky—the Romans had 7 hills forming the basis of the very city itself, for instance—and dividing 28 by 4 gives you a lucky 7 day week. Double your lucky number, and you get a fortnight.

It's important to acknowledge that tracking the exact phases of the lunar cycle could have also been more technically challenging for ancient timekeepers without precise instruments, so picking a nice easily divisible number made sense.

Do you ever use the term “fortnight”? Is the explanation of how we ended up using this word plausible? Let me know what you think!