The
Hebrew calendar is a lunar calendar based on
19-year cycles. The
following sections explain the principles of
this calendar.
The
Torah (Old Testament) inscribes to follow
the lunar calendar, which contains exactly
29 days, 12 hours, 44 min, 3 1/3 seconds.
July 21, 1969, a retro reflector array
left on the Moon by Apollo 14 astronauts.
A 2-foot wide panel studded with 100
mirrors pointing at Earth. Lunar laser
ranging works by firing pulses of laser
light at reflectors on the Moon's surface
and catching the returning photons.
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A laser pulse shoots out of a telescope
on Earth, crosses the Earth-moon divide,
and hits the array. Because the mirrors
are "corner-cube reflectors,"
they send the pulse straight back where
it came from.
It able them to track exactly a full
mouth cycle. The results came to be
exactly 29 days, 12 hours, 44 min, 3
1/3 seconds. God most likely know what
he was talking about.
Units of
Time
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Thecalculations
in the subsequent sections are based on an ancient
form of measuring time. Each day is divided
into 24 hours, which are each divided into 1080
parts
A month is defined as the period from one new
moon to the next, corresponding to the astronomical
definition of a siderial month. For these calculations
the length of a month is taken to be exactly
29 days, 12 hours, 44 min, 3 1/3 seconds .
The
Biblical date of creation is used as the basis
for the Hebrew calendar. Reverse calculation gives
the time of the first new moon of Year 1 as 5
hours and 204 parts from the beginning of the
second day of the week (Monday). This date corresponds
to October 7, 3761 BCE, using the Julian (not
Gregorian) calendar. For later arithmetic convenience,
we will define the preceding Sabbath as day number
0, so that this date, 1 Tishrei of Year
1, is day number 2.
Types
of Years
A year in theHebrew
calendar is normally twelve months: Tishrei,
Cheshvan, Kislev, Tevet, Sh'vat, Adar, Nisan,
Iyar, Sivan, Tammuz, Av, and Elul. An extra
month (Adar is replaced with Adar I and Adar II)
is intercalated in seven of every nineteen years
(years 3, 6, 8, 11, 14, 17, and 19). Every 19-year
cycle thus contains 235 months. These two cycles
of years and months are so close that they diverge
at the rate of only 4.5 days every 1000 years.
Because
a lunar month is not a whole number of days
(29 days....) the length of the months can be
either 29 or 30 days to equal out. A typical
year alternates 30- and 29-day months, beginning
with Tishrei, which is 30 days. In leap
years, Adar I is 30 days and Adar II is 29 days.
To further bring the calendar into alignment
with the lunar cycle, a year may need to be
shortened or lengthened by one day: Kislev
may be shortened to 29 days, or Cheshvan
may be lengthened to 30 days. Thus, there are
six types of years:
- Defective common years, with 353 days
- Regular common years, with 354 days
- Excessive common years, with 355 days
- Defective leap years, with 383 days
- Regular leap years, with 384 days
- Excessive leap years, with 385 days
An
ordinary year has 353, 354, or 355 days. The
three lengths of the years are termed, "deficient,"
"regular," and "complete,"
respectively.
An
ordinary year has 12 months, every month starts
on the day of a new moon.
The months and their lengths are:
| Name |
Number |
Length |
Gregorian
Equivalent |
| Nissan |
1 |
30
days |
March-April |
| Iyar |
2 |
29
days |
April-May |
| Sivan |
3 |
30
days |
May-June |
| Tammuz |
4 |
29
days |
June-July |
| Av |
5 |
30
days |
July-August |
| Elul |
6 |
29
days |
August-September |
| Tishri |
7 |
30
days |
September-October |
| Cheshvan |
8 |
29
or 30 days |
October-November |
| Kislev |
9 |
30
or 29 days |
November-December |
| Tevet |
10 |
29
days |
December-January |
| Shevat |
11 |
30
days |
January-February |
| Adar |
12 |
29
or 30 days |
February-March |
| Adar
II |
13 |
29
days |
March-April |
The
month Adar I is only present in leap
years. In non-leap years Adar II is
simply called "Adar."
Note that in a regular year
the numbers 30 and 29 alternate; a
complete year is created by adding
a day to Heshvan, whereas a deficient
year is created by removing a day
from Kislev. The alteration of 30
and 29 ensures that when the year
starts with a new moon, so does each
month. |
A year is a leap year if the number year mod
19 is one of the following: 0, 3, 6, 8, 11,
14, or 17.
The
years are counted since the creation of the
world, which has taken place in 3761 B.C.E.
In that year, AM 1 started (AM = Anno Mundi
= year of the world). In the year C.E. 2000
we have witnessed the start of Jewish year
AM 5760.
The Jewish day does not begin at midnight, it
begins at sunset (or when three medium-sized stars
should be visible, depending on the religious
circumstance). Sunset marks the start of the 12
night hours, whereas sunrise marks the start of
the 12-day hours. This means that night hours
may be longer or shorter than day hours, depending
on the season.
The first day of the Jewish calendar year is Rosh
Hashanah, the first of Tishri. It starts on the
day of the new moon that occurs about 354. (Leap
year would have different status)
Besides Rosh Hashanah being the Jewish New Year,
there are some other new years within Judaism.
Such as the 15 of Shevat: well known as Tu B'shevat.
- The New Year for trees, when fruit tithes should
be brought.
The first of Nisan: is the New Year for Kings.
Nisan is considered the first month, although
it occurs 6 or 7 months after the start of the
calendar year. And at last the first of Elul:
New Year for Animal Tithes (Taxes).
A
typically Jewish calendar looks like this
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