# Calendar system

1. Jun 15, 2010

### mreq

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

Last edited: Jun 15, 2010
2. Jun 15, 2010

### HallsofIvy

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

3. Jun 15, 2010

### Staff: Mentor

....it evolved slightly from there.

4. Jun 16, 2010

### mreq

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

5. Jun 16, 2010

### Staff: Mentor

Not sure what you mean.

6. Jun 16, 2010

### 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.

Last edited: Jun 16, 2010
7. Jul 6, 2010

### blank.black

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

8. Jul 8, 2010

### 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.

9. Jul 8, 2010

### Janus

Staff Emeritus
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.

10. Jul 8, 2010

### 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.

11. Jul 8, 2010

### Staff: Mentor

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.

12. Jul 8, 2010

### KalamMekhar

Looks like they just apply the modern calender backwards.

13. Jul 8, 2010

### Staff: Mentor

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

14. Jul 8, 2010

### KalamMekhar

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

15. Jul 8, 2010

### Staff: Mentor

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.

16. Jul 8, 2010

### KalamMekhar

I bet I can guess who manufactures your telescope. Hahahaha

17. Jul 8, 2010

### Janus

Staff Emeritus
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.

18. Jul 8, 2010

### Staff: Mentor

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!

19. Jul 8, 2010

### Staff: Mentor

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.

20. Jul 9, 2010

### mgb_phys

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

21. Jul 11, 2010

### 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!

Last edited: Jul 11, 2010
22. Jul 11, 2010

### Staff: Mentor

Positions are calculated with the earth as the reference.

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

23. Jul 11, 2010

### mreq

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

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

24. Jul 11, 2010

### Staff: Mentor

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!

25. Jul 11, 2010

### Staff: Mentor

Lets try i tthis way too:
On Jan 15, 1540 at noon, from the earth, the sun is at:

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