# A Seventeen-Month Lunar Calendar

A very simple rule-based lunar calendar in which the lunar year always has 17 months and one in 50 lunar years is a leap year.

## Introduction

The Lunar Calendar in The Invisible Landscape has a lunar year always of 13 months. The odd numbered months have 30 days and the even numbered months have 29 days. This provides a lunar year of 384 days, which is a little too long. To be accurate, such a calendar must drop a day from a year, once every 10 or 9 lunar years, in a complicated sequence.

Extending this year to 17 months makes it more accurate and it need only be corrected once every 50 lunar years, by adding a day.

This forms the basis of a very simple rule-based lunar calendar.

## Rules

1. All lunar years have 17 months.
2. All odd-numbered months have 30 days.
3. All even-numbered months have 29 days, except the eighth month of a leap lunar year, which has an extra day.
4. The leap lunar years are those lunar years whose number is divisible by 50.
5. Lunar year 1 began with 1757-09-14 CE = JD# 2363048.

## Accuracy

The calendar repeats over a cycle of 50 lunar years consisting of 850 months lasting a total of 25101 days. This is the same as the 52-yerm cycle of my Yerm Calendar, so is equally accurate in the long term.

The seventeen month calendar is simpler than the Yerm Calendar, because its 'years' are of equal length. But the Yerm Calendar follows the phases of the moon more closely as shown here.

## Dates of New Lunar Years

New lunar years occur on the following dates:

```Lunar Year  NY Gregorian   JD#       Yerm Date (afternoon)
1        1757-09-14   2,363,048   28(02(01
51        1826-06-06   2,388,149   28(02(01
101        1895-02-25   2,413,250   28(02(01
151        1963-11-18   2,438,351   28(02(01

175        1996-11-11   2,450,399   01(01(01
176        1997-03-28   2,450,901   02(01(01
177        1999-08-12   2,451,403   03(01(01
178        2000-12-26   2,451,905   04(03(01
179        2002-05-12   2,452,407   05(03(01
180        2003-09-26   2,452,909   06(03(01
181        2005-02-09   2,453,411   07(05(01
182        2006-06-26   2,453,913   08(05(01
183        2007-11-10   2,454,415   09(05(01
184        2009-03-26   2,454,917   10(07(01
185        2010-08-10   2,455,419   11(07(01
```
I chose the epoch, so that most months and the first month of every lunar year would have a Yerm Calendar month beginning on the first day.

Also I chose the epoch so that the first month has the same moon as the first month of a Hebrew year (5518 AM) and an Islamic year (1171 AH).

## Lunar Year and Solar Year

Eight lunar years is just one and a half days short of eleven solar years. 243 lunar years is less than one hour short of 334 mean tropical years.

243 lunar years have an average of 121990.86 days, which equal 334 years of 365.2420958 days each.