Why Do Some Months Have 30 or 31 Days While February Has 28?

[mreq]

Hy.
Why some months have 30 and some 31 days , and february 28?
Thanks!

 

[HallsofIvy]

Why not? This has nothing to do with physics or mathematics- it is mostly due to the Roman calendar- a matter of history.

 

[russ_watters]

Google:

When Rome emerged as a world power, the difficulties of making a calendar were well known, but the Romans complicated their lives because of their superstition that even numbers were unlucky. Hence their months were 29 or 31 days long, with the exception of February, which had 28 days. However, four months of 31 days, seven months of 29 days, and one month of 28 days added up to only 355 days. Therefore the Romans invented an extra month called Mercedonius of 22 or 23 days. It was added every second year.

http://www.infoplease.com/ipa/A0002061.html
...it evolved slightly from there.

 

[mreq]

Can be each day associated with the position of the sun ?

 

[russ_watters]

Not sure what you mean.

 

[mreq]

I mean if 15th of june the sun have the same position or something close to 15th of july and to 15th of august and so on.

 

[blank.black]

mreq do you mean to ask if there is a calendar based on various alignments of the heavenly bodies?

 

[mreq]

What i want to know it's if there is a connection between sun position and the number of the day? Let's say 1 june 2000, 1 june 2001, 1 june 2002, 1 june 2003 etc.
If the sun coordinates are the same.

 

[Janus]

What i want to know it's if there is a connection between sun position and the number of the day? Let's say 1 june 2000, 1 june 2001, 1 june 2002, 1 june 2003 etc.
If the sun coordinates are the same.

Close, but not exact. The mean Tropical year is 365.24219 days long, which means that after four years, the Sun has drifted almost 1 day in position. This is why we have leap years; We add an extra day to the year every four years to tweak the Sun's position and calendar day back into sync. This however over compensates a bit, so our present calendar the Gregorian one, omits the leap day for years that are evenly divided by 400.( Thus the year 2000, which normally should have been a leap year by the four year rule, was not.)
The previous calendar, the Julian, did not have this slight correction, so when Britain and the American colonies switched to the Gregorian calendar in 1751, it was 11 days out of sync with the Sun. As a result of the switch, Sept 2 was followed by Sept 14 to re-align the date and Sun.

As you can imagine, this was disconcerting to some. Some people thought that several days of their lives were being taken away, and some landlords wanted to charge a full month's rent for September, while their tenants argued that they should only be charged for 19 days, etc.

 

[mreq]

Ok.
How about 1th june 2000 and 1 th june 2005 ? Is the position of the sun the same ?

Let's took for example 10 february 1564. Judging by the position of the sun what day should coincide with that by the gregorian calendar.

 

[russ_watters]

As Janus said, close but not exact. How exact do you want to get?

The previous calendar, the Julian, did not have this slight correction, so when Britain and the American colonies switched to the Gregorian calendar in 1751, it was 11 days out of sync with the Sun. As a result of the switch, Sept 2 was followed by Sept 14 to re-align the date and Sun.

Interesting, I had never heard that. I wonder how astronomy software deals with that? I would suspect they ignore such historical issues and just apply the modern calendar backwards.

 

[KalamMekhar]

As Janus said, close but not exact. How exact do you want to get? Interesting, I had never heard that. I wonder how astronomy software deals with that? I would suspect they ignore such historical issues and just apply the modern calendar backwards.

Looks like they just apply the modern calendar backwards.

 

[russ_watters]

Heh, duh, I should have realized how easily I could test that!
[...and Starry Night works the same way.]

 

[KalamMekhar]

I think it would be pretty funny if it had "this date does not exist" or something of the sort.

 

[russ_watters]

Agreed. But if any unscrupulous high school students read this thread, they may go start arguing with their teachers about what date certain historical events happened on. Magna Carta? June 15, 1215? Naah.

 

[KalamMekhar]

Heh, duh, I should have realized how easily I could test that!
[...and Starry Night works the same way.]

I bet I can guess who manufactures your telescope. Hahahaha

 

[Janus]

Agreed. But if any unscrupulous high school students read this thread, they may go start arguing with their teachers about what date certain historical events happened on. Magna Carta? June 15, 1215? Naah.

Well, not that early, as the Gregorian calendar was not introduced until 1582. However, the adoption was not universal. Countries slowly changed over; Russia used the Julian calendar until 1918 and Greece was the last to make the switch in 1923.

 

[russ_watters]

I bet I can guess who manufactures your telescope. Hahahaha

Maybe/mabye not. When I bought Starry Night my primary scope was by one manufacturer and now I have a new one with a label that belies the fact that the OTA and mount are repackaged products from still two more manufacturers! So I've got a lot of major labels covered!

 

[russ_watters]

Well, not that early, as the Gregorian calendar was not introduced until 1582.

Isn't that the point? What does it really mean to say that the Magna Carta was signed on June 15, 1215? According to the people who signed it? According to our new calendar scrolled backwards? And don't even get me started on Christmas. It is bad enough that it isn't known when exactly Jesus was born, but why was December 25th chosen? According to the Wiki it may be because that was the date of the winter solstice in the Roman calendar. But if that's the case, that means calendar changes have moved Christmas so that it is now 4 days later.

 

[mgb_phys]

I think it would be pretty funny if it had "this date does not exist" or something of the sort.

The unix cal command

$ cal 9 1752
September 1752
S M Tu W Th F S
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30

 

[mreq]

If i take a date let's say 15th january 1540. What was the Earth position on the orbit (regarding the sun) that day, and now in 2010 when the Earth is in the same position ?

And another question is Where on the orbit is let's say february ?

Thanks!

 

[russ_watters]

Positions are calculated with the Earth as the reference.

You haven't tried the program yet, have you?

 

[mreq]

Hmm...
So there isn't a fixed point ?

P.S. Are this things possible with some software ? Which one ?

 

[russ_watters]

Hmm...
So there isn't a fixed point ?

P.S. Are this things possible with some software ? Which one ?

mreq, you have another thread open where people suggested software to you! Try it!

Based on the vagueness of the questions you are asking and your inability to properly convey what you are looking for or why, it doesn't appear you really know what you are looking for. So the best thing for you to do is to try the software, see what information it gives you and see if it is of value to you. We can't spoon-feed this to you if you don't even know what you want!

 

[russ_watters]

Lets try i tthis way too:

If i take a date let's say 15th january 1540. What was the Earth position on the orbit (regarding the sun) that day, and now in 2010 when the Earth is in the same position ?

On Jan 15, 1540 at noon, from the earth, the sun is at:

RA: 20h, 54.17m
DEC: -17deg 29.54m

Was this information helpful to you?

 

[mreq]

I'm new to astronomy! So i need to learn terminology. And that 's what i do all day on google. Because i don't know where to find it! And i want things to move faster.

 

[mreq]

How the Earth movement was calculated ? How they know the point of the start at each ear ?

 

[Janus]

The Tropical year, which the calendar is based upon, goes from equinox to equinox (fall to fall or spring to spring). The equinoxes, are when the tilt of the Earth is neither towards or away from the Sun, and the Sun passes directly overhead on the Equator. They corresponds with the days where we get 12hrs of light and 12 hrs of dark. Generally the spring equinox signals the start of the tropical year. (In earlier times, the calendar year also started around this time, on April first. After it was switched to January 1st, to be close to the Winter Solstice, some people mistakenly continued to celebrate the new year on April 1st. They were derisively called "April Fools", and thus was born the tradition of April's Fools Day.)

So the Earth's position is generally measured from the spring equinox.

 

[mreq]

The Tropical year, which the calendar is based upon, goes from equinox to equinox (fall to fall or spring to spring). The equinoxes, are when the tilt of the Earth is neither towards or away from the Sun, and the Sun passes directly overhead on the Equator. They corresponds with the days where we get 12hrs of light and 12 hrs of dark. Generally the spring equinox signals the start of the tropical year. (In earlier times, the calendar year also started around this time, on April first. After it was switched to January 1st, to be close to the Winter Solstice, some people mistakenly continued to celebrate the new year on April 1st. They were derisively called "April Fools", and thus was born the tradition of April's Fools Day.)

So the Earth's position is generally measured from the spring equinox.

So the position on the orbit is not the same the next year ?

 

[Janus]

So the position on the orbit is not the same the next year ?

It depends on what you use as a "year". A mean sidereal year is the time it takes for the Earth to return to the same position relative to the stars and is 365 days 6 hrs 9 min 9.7676 sec long.

A mean tropical year is 365 days 5 hrs 48 min 45 s long.

A calendar year is either 365 or 356 days long depending on whether or not it is a leap year or not.

So on average, the tropical year is ~20 min shorter than the sidereal year. So the seasons and the position of the Earth relative to the stars drift slowly out of sync. The difference is small and comes to about 1/37 the width of the Moon per year.

I said "on average" because due to various influences on the Earth by other bodies in the Solar System, the exact duration of these "years" can fluctuate slightly.

 

[mreq]

I want to know how that point on the orbit it's calculated.
Because i set the time to june 10th 1564 (location randomly) and the sun it's in taurus constelation.
Thanks!

 

[Janus]

I want to know how that point on the orbit it's calculated.
Because i set the time to june 10th 1564 (location randomly) and the sun it's in taurus constelation.
Thanks!

The point is simply calculated like this: You start from some date, say Jan 1, 2000, where you have made a precise observation of the Earth-Sun position relative to the Stars. Then taking the length of the Mean sidereal year, you calculate how many Sidereal years have passed since June 10th 1564. You'll get some whole number and a fraction. Multiply that fraction by 360° and you have how many degrees difference between the present position of the Earth and the one you want to calculate . (to be more accurate, you factor in the fact that the Earth travels at differ speeds at different points of its orbit.) Its just a matter of starting from a known position and date and crunching the numbers.

That being said, I do not believe that WWT does this. It is more designed for showing real time positions than calculating ancient ones. The reason I say this is that it shows the Sun in the same position in the zodiac for 6/10/1564 as it does for 6/10/2010, and it shouldn't. The Sun should be at the borderline between Cancer and Taurus in 1564.

For that, you might try a program called Skyglobe. It will calculate positions going far back or forward in time. It also compensates for the switch between Julian and Gregorian calendars in 1582.

Skyglobe has a nice feature where you can jump forward or backward by centuries, allowing you to watch the Sun drift through the zodiac due to the precession of the equinoxes.(there will be a jump due to the switch of calendars in 1582.)

 

[mreq]

Skyglobe seems to be old!
Isn't there any software to be truly profesional ? What's so hard ? (i mean like there is movie editor - which do everything ...it should be a astronomy software...)

 

[Janus]

Skyglobe seems to be old!
Isn't there any software to be truly profesional ? What's so hard ? (i mean like there is movie editor - which do everything ...it should be a astronomy software...)

So what if its old, as long as it does the job? It not like the methodology for determining the relative positions has changed. The only real difference is the interface. Sure with WWT you can zoom in and see those pretty images of the object, but that is just eye candy if all you need to know is its position.

At least Skyglobe allows you to backtrack the positions over long periods of time. I don't know how many astronomical program you are going to find that will do that; It's not as if it would be a high priority.

 

[mreq]

Another thing. If i take let's say 19 february in a bisec year, then on the next year 19 february it's not on the same position on the orbit as it was last year.
Where is that on the orbit?

 

[Janus]

We already covered that back in post #30. The Sidereal year is 365 days, 6 hr, 9 min, 9.7676 sec long. Thus in a non-leap year. the Earth will be 6 hr, 9 min, 9.7676 sec short of a complete orbit the next Feb 19. This works out to just about 1/4 of a degree or half the width of the Moon.

However, during a leap year, which is 366 days long, the year is longer than it takes for the Earth to complete an orbit by some 3/4 of a degree. So what you would get is the Earth falling behind by 1/4 of a degree for each of 3 years and then making that up in the fourth year. There will still be a minor drift caused by the difference between Tropical and sidereal year (again, as noted in post 30), but this would take years to notice.

 

[Integral]

We already covered that back in post #30. The Sidereal year is 365 days, 6 hr, 9 min, 9.7676 sec long. Thus in a non-leap year. the Earth will be 6 hr, 9 min, 9.7676 sec short of a complete orbit the next Feb 19. This works out to just about 1/4 of a degree or half the width of the Moon.

However, during a leap year, which is 366 days long, the year is longer than it takes for the Earth to complete an orbit by some 3/4 of a degree. So what you would get is the Earth falling behind by 1/4 of a degree for each of 3 years and then making that up in the fourth year. There will still be a minor drift caused by the difference between Tropical and sidereal year (again, as noted in post 30), but this would take years to notice.

This last bit of drift is compensated for with the century leap years, that is a century year (1800, 1900 etc. ) is NOT a leap year even though it is obvioulsy a multiple of 4, unless it is still divisible by 4 after dividing by 100. Thus the year 2000 was a leap year making it part of a 400 year correction cycle. I somehow feel cheated because the rare event that occurred in 2000 meant that we maintained the leap year cycle that we have all become familiar with. Since the current calendar system was established in the 1750's this is the first century which was a leap year.

 

[Janus]

This last bit of drift is compensated for with the century leap years, that is a century year (1800, 1900 etc. ) is NOT a leap year even though it is obvioulsy a multiple of 4, unless it is still divisible by 4 after dividing by 100. Thus the year 2000 was a leap year making it part of a 400 year correction cycle. I somehow feel cheated because the rare event that occurred in 2000 meant that we maintained the leap year cycle that we have all become familiar with. Since the current calendar system was established in the 1750's this is the first century which was a leap year.

Actually, I was referring to the difference between Tropical and Sidereal years. This drift is what is left over after all the calendar manipulations, which are designed to keep the calendar in step with the Tropical(seasonal) year. In other words, the drift due to the precession of the equinoxes. It works out to be about 1.4° per century.

 

[mreq]

So, Janus, if I want to calculate the sideral years from the point of year 0 how to do it ?
Thanks!

 

[Janus]

Our calendar has no year zero. But if you want to figure out how many sidereal years occur between the same date in different calendar years, you would:

Multiply the number of years between the dates by 365.
Add in the number of leap days that occurred during the period.
Divide this by 365.256363051

 

[mreq]

Janus please if you know another software, because Skyglobe it's not running on my pc.
Thanks!

 

[Chronos]

In ancient times, people relied on lunar cycles. 13 lunar cycles [28 days] equals 364 days. That was accurate enough unless your civilization persisted for centuries. After a few hundred or so years, you realized this clock was just a hair off [assuming you trusted your ancestors]. The Romans had this thing about the number 13, so they 'fixed' the calendar - probably just to annoy the encroaching barbarians.