How do I use Herschel's Perpetual Calendar?
Use the Day Number and Month together
and also the Day of Week and Year of Century
together. The two are linked by the same letter in the body of the table.
Leap years are shown as
2012
and you must use the columns with Jan
and Feb for such years.
Here are some examples:
On what day of the week was July 4 1998?
Use the Day Number 4 and Month July
to find a letter from the table: F.
Use the Year 1998 and the letter
F to find a row for the Day of Week.
The row for F in the column above 1998 is labelled
Saturday.
So July 4 1998 is a Saturday.
Which months in 2004 have a Friday 13th?
Use the Day of Week Friday and
Year 2004 to find a letter from the
table: A.
Use the Day Number 13 and find the letter
A in its row. The appropriate
column is headed Aug, Feb.
Since 2004 is a leap year (it is shown as 2004
and not as 2004)
then Feb is valid as
February of a leap year.
The 13th is a Friday only in February and August in 2004.
On which years will 4 May fall on a Saturday?
We know the Day Number 4 and the
Month May so use these to look up
a letter in the table: D.
Use the row labelled Saturday and locate the letter
D
in it.
The years under the appropriate column are:
...,
2002, 2013, 2019, 2024, 2030, ...
4 May will be a Saturday in 2019, 2024, 2030, ...
But I thought 2000 was not a leapyear?
There seems to be an erroneous rule about leapyears that is found on some pages on the Web and
in some software. The correct rule is
that a leapyear is a year which is an exact multiple of 4
IF the year starts a new century, for example 1900, 2000 and 2100, then it
must be a multiple of 400 to be a leapyear otherwise it is not.
Therefore 1900, 2100 and
2200 are not leapyears but
1600, 2000,
2400,... are leapyears.
Eric's Treasure Trove
of Science has an entry under
The Gregorian Calendar which has some more historical information on calendars.
The Julian Calendar
The old Julian Calendar had been commissioned by Julius Caesar in 46 BC
and was based on research undertaken by the Egyptian
Sosigenes.
The calendar until then had 10 months of 30 days each. We still see the remnants of
this in the names of the months:
September (7), October (8), November (9) and December (10)
Julius Caesar introduced a year of 365 days with
two extra months named after himself (July) and the first Roman emperor Augustus Caesar
(August). Augustus insisted that his month had the same number of days as Julius's
(31 days each) and so February was shorted to accommodate this to make 365 days in total.
It also introduced the rule
that every fourth year was to be a leapyear with the simple rule that
every year which is a multiple of 4 has an extra day (a leapyear).
The reason for this is that
there are not exactly 365 days in a year
 the time it takes the earth to revolve around the sun in its orbit  but there are
roughly 365.25 days so we need to catch up on the extra 0.25 (=1/4) of a day by adding in a
whole day every 4 years.
The Gregorian Calendar
In fact, even this figure of 365.25 is not accurate enough  it
is too big by about 0.0078 days (that's about 5 mins 37 secs).
This difference was noticed by astronomers and by 1582
when the date on the calendar was out of line with the date "in the stars"
by about 10 days,
Pope Gregory decided to correct it. He ordained that October 5th 1582 should be called
October 15th
to remove the extra 10 days. Also, since 0.0078 is quite close 0.0075 which is
is equal to the handy fraction 3/400, then 3 days in every 400 should not be leapyears.
Gregory's system incorporated this in
the simple rule that loses 3 years in every 400:
A new century year (a multiple of 100) is only a leapyear if it is a multiple of 400
Thus 1800 and 1900, although divisible by 4 are not
leapyears, and neither are 2100, 2200 and 2300. On the other hand, the year 2000,
being a multiple of 400 is a leapyear as are 1600, 2000 and 2400.
This system is called the Gregorian Calendar in honour of Pope Gregory and is
now used worldwide.
Italy, France, Spain and Portugal adopted Gregory's new calendar in 1582 and other
Catholic countries followed later. In Great Britain, it was adopted in 1752
and by then 12 days had to be removed from the calendar, not 10, in September,
causing riots in the streets of London because people thought they had lost
12 days of their lives!
Mark Bader has
the details from the original 1751 Act of Parliament under King George II bringing in the
Gregorian calendar from September 1752
in the UK and "his Majesty's Dominions...".
The 20^{th} century saw its adoption in China, Bulgaria, Turkey, Romania and the former
USSR and Yugoslavia. The latest country to adopt it was Greece in 1923.
We still see remnants of the old calendar in some parts of Europe in that Christmas Day has
retained its former "orthodox" date and is 12 days out  being celebrated on January 6th
instead of December 25th.
More Links
 More on
John Herschel (18371921),
the son of John F W Herschel (17921871) and
grandson of Sir William Herschel (17381822)
who discovered Uranus in 1781. For this discovery William has knighted and made court astronomer to George III.

A picture
of Col John Herschel's
Perpetual Almanack
in the form of a tube (containing a spirit level).
Instead of using letters to coordinate the Month and Date with the Year and Day as we do on this page,
the tube was turned so that the chosen year (the range is roughly 18721924)
aligns with a chosen month and then that month's calendar will be shown
with the days of the month (131) corresponding
to the days of the week (SunSat).
The original is in the Herschel Collection
at the National Maritime Museum, Greenwich, London.
 "On a new form of calendar by which the year or month or monthday
or weekday may be readily found when the other three components of a date are given"
Captain J Herschel R.E., Philosophical Magazine Series 4 (1874) vol 47, issue 323, pages 357358.
This is his original paper on the tabular version of his perpetual calendar as is shown here
above but he used the symbols ×, +, =, *, ‖, ‡, § where we use letters
A,B,C,D,E,F,G .
Here he is Captain in the Royal Engineers which he joined in 1856 and then went on to be Colonel
as inscribed on the tube (see the picture link above).
There are more details about calendars on
The Home Page for Calendar Reform.
Dr Ron Knott
May 1998 (updated 2 April 2015)