﻿ Calendar History -- Years Converted From Julian Period Day

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## Years Converted From Julian Period Day

From a given Earth Julian Period Day, it is possible to obtain the Julian-1 Year, the Gregorian year, and even the Martian year.

Before we get started on the converstions, let's explain a few things.

### Defining the Julian Period Day

The (Earth) Julian Period began when day zero started at 12:00 P.M. on January 1, 4713 B.C. Julian-1 date time. If you look at it this way, besides the fact that a new day began when it was mean noontime at the zero degree longitude meriidian on Earth, and astronomers preferred it to help date the nights easier (instead of dating the nights a split of day 1/2 or 31/1, etc.), at that point, except for the time zone 12 hours ahead of the meridian, it was January 1 all over the world at that point, with January 2 beginning in the time zone 12 hours ahead where it was midnight there. It was also midnight the same date in the time zone 12 hours behind the zero degree meridian.

The International Date Line split the time zone on the 180 degree of the meridian so that the western half of that time zone was a day ahead of the eastern half of that time zone.

So to go eastwards from the 180 degree meridian, begin at the eastern half of that time zone. The Julian Date would be -0.5 local time there. On the western half of that time zone, the Julian Date would be 0.5 local time there. On the zero degree meridian, it is 0.0 local time. Going eastward from the 180 degree meridian and around the world in 24 time zones, you get these values:

1. 180 degrees West: -0.5 (midnight)
2. 165 degrees West: -0.458 (1am)
3. 150 degrees West: -0.417 (2am)
4. 135 degrees West: -0.375 (3am)
5. 120 degrees West: -0.333 (4am)
6. 105 degrees West: -0.292 (5am)
7. 90 degrees West: -0.25 (6am)
8. 75 degrees West: -0.208 (7am)
9. 60 degrees West: -0.167 (8am)
10. 45 degrees West: -0.125 (9am)
11. 30 degrees West: -0.083 (10am)
12. 15 degrees West: -0.042 (11am)
13. 0 degrees: 0.0 (noon)
14. 15 degrees East: 0.042 (1pm)
15. 30 degrees East: 0.083 (2pm)
16. 45 degrees East: 0.125 (3pm)
17. 60 degrees East: 0.167 (4pm)
18. 75 degrees East: 0.208 (5pm)
19. 90 degrees East: 0.25 (6pm)
20. 105 degrees East: 0.292 (7pm)
21. 120 degrees East: 0.333 (8pm)
22. 135 degrees East: 0.375 (9pm)
23. 150 degrees East: 0.417 (10pm)
24. 165 degrees East: 0.458 (11pm)
25. 180 degrees East: 0.5 (midnight)
26. 195 degrees East: 0.542 (1am)
27. 210 degrees East: 0.583 (2am)
As you go west and cross through a time zone, you subtract one hour, but when you go east and cross thorough a time zone, you add one hour. When you cross the International Date line going east, you subtract one day, but going west, add one day.

When it is any time at the zero degree meridian, just add the decimal number to that time zone's Julian Period Days to get the local Julian Period day for the area. Note that I included two extra degrees East zones, 195 and 210, for computing local Julian Period Days for those zones that are advanced 13 and 14 hours respectively and west of the International Date line.

### The Starting Point of the Julian Period Day

To expand on the starting point of the Julian Date calendar, Julian Period Day starts at noon. Julian Period Day 0 began at noon on January 1, 4713 B.C. Julian-1, or January 1, -4712 Julian-1 decimalized for mathematical operations. On the Gregorian calendar, Julian Period Day 0 began at noon on November 24, 4714 B.C. Gregorian, or November 24, -4713 Gregorian decimalized for mathematical operations.

Also note that the Julian-1 and Gregorian calendars skip year zero because there was no such Roman numeral zero, so the year after 1 B.C. was 1 A.D.

For the Julian Period Day that matches January 1, 4713 B.C. Gregorian, we have to add 38 days to the Julian Period Day 0.

So, Julian Period Day 38 began at noon on February 8, 4713 B.C. Julian-1, or February 8, -4712 Julian-1 decimalized for mathematical operations. On the Gregorian calendar, Julian Period Day 38 began at noon on January 1, 4713 B.C. Gregorian, or January 1, -4712 Gregorian decimalized for mathematical operations.

The Julian Period Year is simply the Julian Period Day divided by the constant 365.25. Julian Period Day 0.000 in this calendar would be January 1, 0 Julian Period Calendar. In this calendar, there is a year zero. If there was such as thing as a Gregorian Period Calendar, the matching date would look like November 24, -1 Gregorian Period Calendar.

### Converting between Julian-1 Year, Gregorian Year and Julian Period Day

CAUTION: The Julian-1 year is NOT the Julian Period Year!

To convert from the Julian Period Day to the Julian Period Year...

Julian Period Day ÷ 365.25

JPY = JPD / 365.25

To convert from the Julian Period Year to the Julian Period Day...

Julian Period Year × 365.25

JPD = JPY * 365.25

To convert from the Julian Period Day to the Julian-1 Year...

(Julian Period Day ÷ 365.25) − 4712

Julian-1-Year = (JPD / 365.25) - 4712

To convert from the Julian-1 Year to the Julian Period Day...

(Julian-1 Year + 4712) × 365.25

JPD = (Julian-1-Year + 4712) * 365.25

To convert from the Julian Period Day to the Gregorian Year...

((Julian Period Day − 38) ÷ 365.24218967) − 4712

Gregorian-Year = ((JPD - 38) / 365.24218967) - 4712

To convert from the Gregorian Year to the Julian Period Day...

((Gregorian Year + 4712) × 365.24218967) + 38

JPD = ((Gregorian-Year + 4712) * 365.24218967) + 38

To convert from the Julian-1 Year to the Gregorian Year...

((((Julian-1 Year + 4712) × 365.25) − 38) ÷ 365.24218967) − 4712

Gregorian-Year = ((((Julian-1-Year + 4712) * 365.25) - 38) / 365.24218967) - 4712

To convert from the Gregorian Year to the Julian-1 Year...

((((Gregorian Year + 4712) × 365.24218967) + 38) ÷ 365.25) − 4712

Julian-1-Year ((((Gregorian-Year + 4712) * 365.24218967) + 38) / 365.25) - 4712

### Julian Period Days Into The Future

Julian Period Day 0 began at noon on January 1, -4712 (4713 B.C.)

Julian Period Day 1461 began at noon on January 1, -4708 (4709 B.C.)

Julian Period Day 1721058 began at noon on January 1, 0 (1 B.C.)

Julian Period Day 2457402 began at noon on January 1, +2016 (2016 A.D.)

To get the Julian Day Number between two leap years on the Julian-1 (not the Gregorian) calendar:

((higher year) − (lower year)) × 365.25

Note: This formula works only on years that are multiples of four.

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