Day-related calculations for BC dates

In summary, you need to use a fabricated proleptic Gregorian calendar to calculate dates between different calendric systems.
  • #1
James Bejon
5
0
Dear All,

Apologies if this question isn’t really in the right place. I wanted to check I’ve understood some data correctly. The link below shows lunar cycles all the way back to 2000 BC: http://astropixels.com/ephemeris/phasescat/phasescat.html. Suppose I click on a table and get a date of 1st June 750 BC for some event or other. How could I work out the number of days between that date and, say, 1st June 751 AD? Is it simply 1500 * 365.25? Or do I need to use the length of a tropical year in such calculations.

James.
 
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  • #2
No, you have to understand the intricacies of the changes to the calendar over the years. It changed radically several time. Personally, I'd use the Jewish calendar instead, AD/BC makes date calculations annoying.
 
  • #3
Thanks for the reply. Which calendar to use, though, isn't really my choice to make, as I want to work with the dates as stated here: http://astropixels.com/ephemeris/phasescat/phasescat.html. Are you saying those dates already factor in all the intricate changes you're referring to, and I now need to unwind them?
 
  • #4
These kinds of calculations use Julian days. Otherwise astronomers would go insane.

In the past I've written conversion software between various calendric systems. You have to realize that there are all kinds of gotchas with calendars.

Calendars are political/religious "footballs" kicked around mercilessly for reasons both good and bad. Plus the terminology used by people in that field are arcane since it dates from Roman times and the Middle Ages (IMO). From 1582 until 1923 there were different calendars (Julian vs. Gregorian) in Christian Europe - same for Westernized North America which all got on the same "page" in 1752. Example of the nuttiness: UNIX designers decided that September 1752 was "the" month when the changeover occurred.
Code: 
$ cal 9 1752
  September 1752
Su Mo Tu We Th Fr Sa
  1  2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
Fun huh?

Christian Western Europe and the Colonial Americas a crazy quilt of countries, colonies, cities, and principalities all using one of two calendars:
Julian Calendar (from Roman time), or the Gregorian Calendar (Pope Gregory 1582). Plus in your case the Gregorian calendar did not exit 2000 years ago. You have to use a fabricated Proleptic Gregorian calendar -
https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar
Here is someone with your problem:
http://raywoodcockslatest.blogspot.com/2012/05/million-day-calendar-with-explicit.html

Your problem is a potential headache. Assuming you know the actual date in a known system try:

http://www.hermetic.ch/jgdc/jgdc.htm

The UNIX "cal" program will convert 'old dates' to the Proleptic Gregorian. You can install Cygwin to run Unix under Windows
https://www.cygwin.com/
 
  • #5
Hi Jim. Wow. Thanks a lot for the info. Unless I’ve misunderstood you, though, my problem’s more straightforward than you suggest, isn’t it? As long as I stick with dates provided from the astropixels.com site, I can work out the number of days between them just by equating a year with 365.25 days. It’s only if I have a date provided derived by some other means that life becomes difficult, right?
 
  • #6
That is...for dates pre 16th cent. AD at least
 
  • #7
No! If that is the case just use Julian days like astronomers do. No years, no months, nothing, just days.
The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Greenwich Mean Time, with Julian day number 0 assigned to the day starting at noon on January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar).
From wikipedia - ignore proleptic for now.

Use this instead: http://numerical.recipes/julian.html
Code: 
Get date1 -> JD number1
Get date2 -> JD number2
Get the difference as number of days between by subtraction.
 
  • #8
Gotcha. Thanks a lot. The link's helpful too.
 

1. How do you calculate the number of days between two BC dates?

To calculate the number of days between two BC dates, you would subtract the earlier date from the later date. For example, to find the number of days between January 1st, 1000 BC and January 1st, 500 BC, you would subtract 500 BC from 1000 BC, which equals 500 years. Since there are 365 days in a year, the total number of days would be 500 x 365 = 182,500 days.

2. How do you convert BC dates to the Gregorian calendar?

To convert BC dates to the Gregorian calendar, you would subtract the BC year from the number of years in the Gregorian calendar. For example, to convert the date 500 BC to the Gregorian calendar, you would subtract 500 from 2021 (the current year), which equals 1521. Therefore, 500 BC would be equivalent to the year 1521 in the Gregorian calendar.

3. How do you handle leap years when calculating BC dates?

When calculating BC dates, you can handle leap years by including an extra day in the month of February every four years. However, this was not the case in ancient times, so some historians and scientists use a different system for dealing with leap years in BC dates. It is important to clarify which system is being used when discussing or calculating BC dates.

4. Can you calculate BC dates using negative numbers?

Yes, BC dates can be calculated using negative numbers. For example, instead of writing 500 BC, you could write -500. This is useful for doing mathematical calculations or sorting dates in chronological order.

5. How do you handle different calendars when calculating BC dates?

When calculating BC dates, it is important to consider the different calendars that were used in ancient times. For example, the Roman calendar had 10 months and did not account for leap years, while the Julian calendar had 12 months and did have a leap year system. It is important to research and clarify which calendar is being used when dealing with BC dates in order to accurately calculate and compare dates.

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