# 1 lunar month = 29.53 days. Where did 28 days come from?

Why are we told from childhood that the Moon goes from full to full in 28 days when the reality is a full day and a half longer? I sense a conspiracy.

I was never told that. I don’t think that it’s even a common misconception. Maybe it’s because it’s exactly four weeks?

It’s because it takes 28 days for the moon to go around the earth with respect to the sun, and 29.53 days with respect to the (relatively nonmoving) stars.

That makes sense, Niply. Thanks!

27.3 days wrt the distant stars.

29.5 days wrt to the sun

Orbit of the Moon

I always heard 28 days as a “rough approximation”.

yeah that makes sense, sounds good,great answer, makes the question asker happy. But no, of course the moon’s orbit is not exactly 28 days , with respect to ANYTHING, not in any frame of reference…
I like how the yahoo gives the question asker the right to judge the correct answer… (there’s an XKCD panel for this… not using the same example though.) OJ simpson asks “I am I guilty” ? there a billion answers saying yes, one saying no, so upvotes the one answer saying no, from then on, yahoo questions then says “The best answer is No… according the person who asked the question”.

The XKCD panel was about the upvotes on products just due to fast delivery or cheap price… not actually testing of the product. But similar concept… upvote from the wrong person… the insipid upvote vs the truth.

The actual answer is that its a practical number of days… practicality arises from being easily able to divide it into half, and quarter… so that points at factorization… non-trivial factorization…

Year of 12 months… Factors 6,4,3,2
Month of 4 weeks - 2
Month of 28 days… 14,7,4,2
For that reason it was preferable to say a month was 28 days, rather than 29.
There have been many systems for keeping the calendar in line with the stars, mostly by having fudge days added to various months… and at the end of the year… but thats far away from what the OP asked.

Yip. You might naively think it was done to make it evenly divisible by weeks, but the 7-day week comes from the idea that there are 28-day months. At least, according to Wikipedia.

Rounding? to the nearest week?

A lot of people seem to be convinced a month is exactly four weeks, and can run into a little trouble when planning a budget on that premise.

I see it like this: The OP asked a question he/she didn’t know the answer to. A plausible, simple, but severely wrong answer was provided by somebody. The OP says “Great, thanks.” Then somebody comes along with the right, more complete answer, but the OP is long gone, having had his/her ignorance reinforced. Actually, the OP is now more ignorant that he/she was at the outset; IMO it’s better to know you don’t know an answer than to wrongfully believe a wrong answer.

That’s hardly the spirit of SDMB or GQ at work. *That’s * IMO what **Isilder **was kvetching about. Although I agree the post was a bit hard to follow with run-on sentences & several examples in play.

I suspect this http://xkcd.com/937/ was the relevant xkcd

If you go out and look up at the sky every night, it doesn’t take a lot of knowledge (or a telescope) to notice a few things. First, you’ll see the exact same stars, night after night, in the exact same pattern, pinwheeling across the sky in perfect formation, never changing relative to each other (or at least the changes are so small you can’t see them). Then, you begin to notice there are seven exceptions to this rule. #1 is the sun itself, which moves across the sky once a day and outshines everything else, but it also shifts a bit with the seasons. #2 is the moon, which moves across the sky once a day but the time of day when it rises and sets shifts and it also changes shapes. #3 through #7 are smaller points of light called wanderers (or planetia in Latin): Mars, Mercury, Jupiter, Venus, and Saturn.

Coincidentally, if you look at the moon, you’ll notice that it goes through phases like 1st quarter, full moon, last quarter, new moon, and each phase tells you not only the shape of the moon but also what time of day it will rise and set. For example, a full moon rises when the sun is setting and sets when the sun is rising, but a 1st quarter moon rises in the middle of the day and sets in the middle of the of the night. So you start paying attention to how long it takes to go through each phase and whoah, it’s about 7 days.

That’s why we have 7 days in a week, and each of the 7 days is named after one of the 7 things which move across the sky, the sun, the moon, and the 5 planets. And then every 4 weeks, you’ve gone though the phases of the moon and it starts over. That’s a good way of keeping time.

But, unfortunately, it’s not exactly 7 days, it’s more like 7.4, so instead of four weeks exactly, you get 4 weeks plus a day and a half. But to the average caveman, that’s hardly significant.

Then came all the arguing about different kinds of calendars, trying to make the months match up with the years. Unfortunately, the number of full moons in a year is more than 12 but less than 13, so no matter how you do it, it doesn’t come out even. One solution is to have 12 months in some years and 13 months in other years. Another solution is to have months that don’t quite match up with the moon. Now, here we are, faced with two additional problems. First, we have centuries of tradition which we stick to even if they don’t make much sense (like how we insist that July and August both have 31 days but September only gets 30). Second, we sit in houses with electric lights and go for drives at night in motorized vehicles moving under electric street lamps, and we hardly ever just look up at the night sky. So your average citizen nowadays knows even less about the night sky than the cavemen did.

On a side note, the average length of the human menstrual cycle is very close to 28 days (not 29.5). It has been theorized that menstrual cycles were related to phases of the moon but this theory has been debunked. It’s just a coincidence. Still, if your goal is to keep track of things in your life, such as how much time has gone by, which day next week is that big family reunion we’ve been planning, will my spouse and I be having sex tonight, if I go out hunting how much moonlight will there be, et cetera, then using 7 days in a week and 4 weeks in a month is a convenient way to measure time.