Transcript How long is a year, anyway?

A Matter of Time
Celebration of Inquiry 2004
Louis Keiner, Dan Ennis
Coastal Carolina University
How long is a year?
Or first: what is a year?
How do we measure a solar year?
Year Animation
(If the Shockwave animation does not play automatically,
right click on the box and select ‘Play’)
We find that the year is about 365.25 days long.
What happens if your calendar year
is not the right length?
If your calendar year is shorter than a solar year…
If your calendar year is longer than a solar year…
Months
• Lunar Month = 29.5306 days
– (new moon to new moon)
• 12 lunar months 354.3672 days.
– << 365 days!
What’s an astronomer/astrologer/priest to do?
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•
•
•
Greek – lunar, add 90 days each 8 years.
Jewish – lunar, add a month each 3 years.
Islamic – lunar, no corrections.
Chinese – lunar, add 7 months during 19 years.
Of course the Romans……
Roman Calendar I - c. 735 B.C.E
10 Months – 304 days
1. Martius
2. Aprilus
3. Maius
4. Junius
5. Quintilis
6. Sextilis
7. September
8. October
9. November
10. December
Roman Calendar II - c. 700 B.C.E
12 Months – 354 days
Add a month as needed
to keep the seasons
correct.
1. Januarius
2. Februarius
3. Martius
4. Aprilus
5. Maius
6. Junius
7. Quintilis
8. Sextilis
9. September
10. October
11. November
12. December
Julius Caesar
Or rather, “Julius,” as in
Caesar. In 45 B.C. (or at
least what we call 45 B.C.
using our calendar), Julius
Caesar
replaced
the
chaotic Roman calendar
with a system based on a
year of 365.25 days—close
to the actual length. The
.25 days was dealt with by
having a “leap” year every
four years.
Roman Calendar III - c. 46 B.C.E
12 Months – 365 days
Add a day every four
years to keep the
seasons correct.
1. Januarius 31
2. Februarius 30
3. Martius 31
4. Aprilus 30
5. Maius 31
6. Junius 30
7. July 31
8. Sextilis 30
9. September 30
10. October 31
11. November 30
12. December 31
The year is exactly
365.242190
days long
(Well, not exactly…a Tropical Year was exactly
365.242190 days long in the late 1990s. To get the
exact number for 2004, or 1804, or 2204, we need…)
365.2421896698  0.00000615359T  7.29x1010 T2  2.64x1010 T3  ...
days, where
T
JD  2451545
36525
JD=Julian Date
The Julian
Calendar
“loses” one
day every
128 years.
A Little Math and a Pope
By the sixteenth-century, Europe--using the Julian calendar--was in
something of a crisis.

First, the Julian calendar had been “losing” days for centuries.
 Second, the Christians liked dating things from the birth of Christ (the
Romans of Caesar’s time, for obvious reasons, did not).
 Third, the date of Easter, for reasons to complicated to relate, was
calculated using a combination of the lunar cycle and the vernal
equinox. Every year Easter got a little further from the first day of
spring.
Pope Gregory XIII decided enough was enough.
The Gregorian Calendar
1. Nine days--October 5-14, 1582--were
struck from the calendar. This would
allow Europe to “catch up.”
2. The Julian leap year rule was changed.
Leap years on all century years not
divisible by 400 were eliminated.
3. The Gregorian calendar--accurate to one
day every 3300 years—seemed to be the
solution.
So why was it unpopular?
TRANSITION SLIDE
The “Georgian” Calendar was
understood to be the “Roman
Catholic” calendar…it was adopted
rather quickly by predominantly
Roman Catholic countries…..
Italy—1582
France—1582
Spain—1582
Portugal—1582
Bohemia—1584
Moravia—1584
Poland—1586
Hungary—1587
Countries with a mixture of
Catholics and Protestants
split—the more Catholic areas
went with Gregory, but places
where Protestants held
authority stuck with the Julian
system. Thus in the late 1500s
both Switzerland and Germany
had two calendars. It goes
without saying that Gregory’s
calendar was not a hit in areas
that were Greek Orthodox …or
Muslim.
Which Brings
Us To
England…
What Day is it in Great Britain?
By the mid-eighteenth century
the
Julian
(English)
calendar was eleven days
behind
the
Georgian
Calendar (which by now
most
of
Europe
had
adopted).
July 1, 1750 in England was
the same day as July 12,
1750 on the continent.
Wait—there’s more…
HAPPY NEW YEAR?
The Gregorian calendar marks the New Year (as we know) on January 1st.
The Julian calendar marks the new year on March 25. So for about three months each year
England and the continent were on different years:
Thus
January 1, 1750 in Great Britain=
January 12, 1751 on the continent.
English Trade was suffering…
How can you
enforce a contract
schedule a trading voyage
calculate interest
When you disagree on the date?
Lord Chesterfield and The Calendar Act
Finally, in 1751 (Julian and
Georgian!) Parliament acted. The
actions proscribed by the calendar
act were complicated:
1. March 24, 1751 was by custom New Year’s Eve, so that made
March 25, 1752, New Year’s Day in Great Britain. This was the last
March New Year in British history.
2. Then 1752 was allowed to proceed normally until September.
September 1, 1752 was followed not by September 2, but instead
by September 14, 1752.
3. The year 1752 ended not on March 24, as had been the English
practice, but on December 31. Thus January 1 marked the first day
of 1753.
4. Then Britain simply followed standard Georgian practice.
Oddly enough, that meant the year 1752 was only 293 days long.
TheCalendar Act provoked a storm of protest. There were
The
Riots
supposedly
riots in Oxfordshire, London and Greenwich. The
Georgian calendar was associated with Roman Catholicism. Some
protested the Government’s “stealing” of days. Laborers noted that
they were paid daily wages but had to pay monthly rents.
A contemporary ballad put it this way:
In seventeen hundred and fifty-three
The style was changed to Popery,
But that it is lik'd, we don't all agree;
Which nobody can deny.
When the country folk first heard of this act,
That old father style was condemned to be rack'd,
And robb'd of his time, which appears to be fact,
Which nobody can deny…
William
Hogarth
produced an
engraving in
1754 in
which a mob
outside a
window
waved a
banner
saying “Give
Us Back Our
Eleven
Days!”
Britain’s American Colonies had to adopt the change as well, but the new
calendar was welcomed by many colonial merchants as a means to simplify
trade.
Ben Franklin, who supported the
change, took a more philosophical
point of view, saying:
“It is pleasant for an old man to
be able to go to bed on
September 2, and not have to
get up until September 14.”
The Calendar Act solved the problem
for the moment, but there are still
issues to be resolved. For example,
the actual year is not 365.25 days
long—if so, the Georgian calendar
would be perfectly accurate. It is
instead closer to 365.242190 days
long. So even as we speak our
calendar is falling behind…bit by bit...