# Why the number of days in a month?

1. Jul 28, 2009

### tgt

In the calender used by Westerners, why do some month have 30 days and some have 31 days. Was it arbitrary decided? Or has it got something to do with the moon?

I do know why Feburary usually has 28 days but every 4 years, it has 29 days. It is to account for the fact that the earth needs 365 and a quarter days to spin a full turn around the sun. However, why choose Feburary to have that extra day every 4 years?

2. Jul 28, 2009

### TheStatutoryApe

http://en.wikipedia.org/wiki/Gregorian_calendar

I have not found anything specific so far but it looks as though the gregorian calendar is a more accurate reinvention of the julian calendar.

http://en.wikipedia.org/wiki/Julian_calendar

The only difference between the two seems to be the frequency of leap days. The Julian calendar is apparently a reform of the roman calendar by Caesar and kept the same (or approximately the same) scheme for month lengths.

http://en.wikipedia.org/wiki/Roman_calendar

Here they talk about the month lengths more in depth.

3. Jul 28, 2009

### Stripe

not too sure but i'm assuming that the longer months make up for the shorter ones, as i don't think it would be 100% the same every time (the moon revolves)

4. Jul 28, 2009

### Helios

The Gregorian calendar is a botch in that the leap days are not spread as evenly as possible. If they were, then the spring equinox would tend to stay fixed to MAR 21. Instead it jitters all over.
Most protestant countries rejected it, as they rejected the authority of the church of Rome. England and U.S. were of the last hold-outs to resist this calendar. One should look at John Dee's ( 1527 - 1609 ) calendar refprm proposal. It would have been superior to the Gregorian calendar.

Stripe, the longer months do not make up for the shorter ones, it's the other way around.

5. Jul 28, 2009

### tgt

So they would like all month to have 31 days?

Why would Feburary only have 29 days max? Why not just delete a 31 day month to make it 30 and make Feb have either 29 or 30? Or even better delete two 31 day months and make Feb 30 or 31 days? This would make every month have either 30 or 31 days.

6. Jul 28, 2009

### TheStatutoryApe

This is in part because of the leap days. Solar, lunar, and daily cycles do not match up exactly. So we have a lunar calander that was adjusted to more accurately reflect the yearly cycle and then reinvented to be a solar calander that had to be adjusted yet again when they were able to better measure the length of a year which according to the gregorian calander is "365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds" and is still slightly off.

7. Jul 28, 2009

### Helios

It's not "they", It's Julius Caesar. Take it up with him. He reformed an even elder calendar and likely had some predilictions that we don't know about.

8. Jul 28, 2009

### Jimmy Snyder

The Julian calendar used a figure of 365.25 as a result of which over the centuries, the holidays started to slip in the seasons. The Gregorian calendar used a figure of 365.2425 which as you say is slightly off. Herschel proposed a change to the calendar that would use a figure of 365.24225 days. Unless this proposal is implemented, the calendar will be off by about 1 day every 4000 years. If it is implemented, there will still be a discrepency. However, rather than make further adjustments to the calendar, it may be that the people of some future era will simply adjust the orbit and/or rotation of the Earth.

9. Jul 28, 2009

### rolerbe

A simpler, less historically driven system would have 13 months of 28 days each. One additional *special* day each year would be new years (that's it, not monday, tuesday, etc., just "new years"), preferably on the winter solstice. Then on leap years, there would also be a special day "leap day" on the summer solstice. both days would be holidays.

10. Jul 28, 2009

### CRGreathouse

January and February were Julian additions from a block of non-month 'winter days'. Why they chose to split it as 31/28(29) rather than 30/29(30) was probably a superstitious choice to not spite the god Ianus. (The februatio festival was unlikely to take the same sort of retribution as a scorned god.)

11. Jul 28, 2009

### CRGreathouse

Intercalary days have been a popular solution to uneven months and years. Personally, I prefer systems without them -- the regularity of "Monday always follows Sunday" and the like is more important to me than having the same number of days per month.

12. Jul 28, 2009

### mgb_phys

According to wiki
But I have never heard of it anywhere else.

13. Jul 28, 2009

### jobyts

Since religion comes into play, people would be more willing to change the rotation of the earth than to accept others' calendar.

14. Jul 28, 2009

### Jimmy Snyder

England accepted it before the American colonies gained independence, so the US never really accepted it, they just inherited the British acceptence. It hadn't been accepted by Russia at the time that they sold Alaska to the US, so I suppose you could call that a holdout. I don't think it was ever accepted by the Russian Empire, but I'm not sure about that. I think it was the Kerensky govt. that did.

15. Jul 28, 2009

### rolerbe

Just when you think you've had an original thought...

and then there's religion. My favorite is a law passed in Arkansas or some other of those midwestern states way far from civilization (i.e. the east coast) where they decreed Pi to be 3.

16. Jul 28, 2009

### mgb_phys

Indianna, it wasn't a religous thing it was just a spectacularly confused crank that managed to get in front of the legislature. Compared to some of the things that have resulted from spectacularly confused cranks getting in front of a legislature it was relatively harmless.

http://www.agecon.purdue.edu/crd/Localgov/Second Level pages/Indiana_Pi_Story.htm

17. Jul 28, 2009

### maverick_starstrider

I'm not american but is not the west coast also considered a heart of "civilization" and liberalism?

18. Jul 28, 2009

### mgb_phys

Not in 1897

19. Jul 28, 2009

### tiny-tim

originally 29 and 30 days only

Originally, all the months (except February) had either 29 or 30 days, then Julius Caesar enlarged all the 29s to 30 or 31.

February stayed at 28.

That was from the table following http://en.wikipedia.org/wiki/Julian_calendar#Julian_reform

That was an English lunar calendar: I think England had a normal solar calendar for most purposes.

20. Jul 28, 2009

### mgb_phys

Re: originally 29 and 30 days only

But it wasn't really lunar, there are nearer 12 lunar months in a year, this was just a counting sets of 4 weeks.
Yes I'd never heard of it before.

It still applies sometimes, I remember as a student the difference between private landlords that charged rent per 'calender month' and the university that charged per 4 weeks = nearly 10% difference in a year.

21. Jul 29, 2009

### maverick_starstrider

Oh, I'd heard that anecdote about setting Pi to 3 before but I always assumed it was a modern thing.

22. Jul 29, 2009

### Helios

This is likely a very rare time or the only time that someone brings up calendars. So, I'll show the calendar I invented.
It's very simple. Days are arranged in groupings of 21 or 22 days called terms. Thirty-three terms are grouped together into an alternating sequence of 709 days as shown below.

21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21 22 21

This basic 709-day period, so arranged, repeats perpetually. A year equals seventeen consecutive terms. The mean year equals 365 8/33 days. The mean term equals 21 16/33 days.

http://www.helios.netne.net/ivymike.htm

23. Jul 31, 2009

### BobG

The Moon's synodic period, the time it takes to return to the same position relative to the Sun, is 29.5 days, or close to the length of a month. The sidereal period is only 27.3 days, but the Sun made a more obvious reference point than the stars. If you establish a calendar based on lunar months, you wind up with about 355 days. If you establish a calendar based on the Sun, you wind up with about 365 days (plus the extra .25 days). Different civilizations dealt with this in different ways. You could have 12 months lasting a total of 355 days, plus 10 special days; or just round to 30 and have 360 days, plus 5 dead days (which were usually unlucky days to be born); or some other strange combination trying to fit both the Sun and Moon into the same calendar. (It's more than coincidence that a circle has 360 degrees - the stars shift almost 1 degree per day).

If you're basing the calendar on the Sun, but try to force the lunar months in there anyway, there has to be an imbalance in the length of the months if all four of the key points (vernal equinox, summer solstice, autumnal equinox, and winter solstice) are all to fall on or about the 21st of the month. Notice there are more days between the summer side of the equinoxes than the winter side of the equinoxes (unless you live in Australia which does everything upside down). That's because the Earth reaches perihelion during the first week of January and apohelion during the first week of July (usually July 4th, creating an unusual coincidence for Americans).

Just an annoying side note, those people that say the longest day of the year is June 21 are wrong. That's just the day with the longest period of sunlight. The longest day of the year, from local noon to local noon, is when the Earth is at perigee during the first week of January.

24. Jul 31, 2009

### mgb_phys

It also depends where you live.
If you are in the desert and are herders the exact time of year isn't such a big deal. Being able to tell the date (with practice to a day) by looking at the moon is great.

If you are further north and farming it's important to know when it's time to plant. If you rely on the weather and plant too early or late you starve.