Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 9: Time and the Calendar

Key Ideas

Timekeeping is the earliest application of Astronomy
Rise of the Modern Calendar: Lunar Calendar; Solar Calendar; Modern Calendar
Divisions of the Year and Day:: Quarter and Cross-Quarter Days; Months, Weeks, Hours, Minutes and Seconds; Modern Time and Timezones

Timekeeping

Astronomical events determine all of the major divisions of the year:: The Year is the time it takes the Earth to orbit once around the Sun
Months are derived from the cycle of Lunar Phases: The Day is the time it takes the Earth to rotate once on its axis relative to the Sun

Solstice and Equinox Holidays

Cross-Quarter Holidays

Lunar Calendars

The Phases of the Moon provide a convenient way to keep track of time.: Easily visible and distinctive; 12 lunar months is 354 days, just 11 days short of a year of 365 days
Oldest recognizable ancient calendars are lunar calendars: Common among nomadic peoples and those without written languages.

Metonic Cycle

The 354 day lunar year is 11 days short of 365 days.: Causes the seasons to drift among the months in lunar calendars
Babylonians discovered the Metonic Cycle: 235 lunar months = 19 solar years
The Babylonians built a complex, but very precise hybrid luni-solar calendar based on the Metonic Cycle.

Lunar Calendars Today

Islamic Calendar:: Purely lunar calendar, 354 days in the calendar year.; Months occur in different seasons (e.g. Ramadan)
Jewish Calendar:: Luni-Solar calendar; Interpolate an extra 13th month every 2-3 years to keep the calendar aligned with the seasons; Repeats on the 19-year Metonic cycle

Solar Calendars

Solar Calendars mark time by the Seasons: Set calendar by Equinoxes and Solstices
Arrival of seasons has practical and/or cultural importances:: Knowing when to plant or harvest; Annual flood of the Nile Valley
Religious festivals associated with specific seasons (e.g. Easter and Christmas)

The Months

In Ancient Rome the year had only ten months and began on March 1st
September, October, November, and December are so named because they were the 7th, 8th, 9th, and 10th months
January and February were added to the end of the year around 700BC, as well as an additional "leap month" named Mercedonius
Julius Caesar and Augustus Caesar later renamed the months of Quintilis and Sextilis after themselves

The Julian Calendar (46 BC)

Caesar asked the Alexandrine astronomer Sosigenes to reform the Roman calendar.
He started with a Solar Year of 365.25 days:: Year divided in 12 months of 30 and 31 days (Feb had 29), adding up to 365 days.; Every 4 years, add 1 day to February to make 366 days in that year.
This "Leap Year" makes up the difference between 365.25 and 365 days per year.

Annus confusionis

Calendar was initiated in 46 BC.: Added 80 extra days to 46 BC; 46 BC had 445 days!
Caesar called 46 BC:: Ultimus annus confusionis; (The final year of confusion)
The Romans called it: Annus confusionis; (The year of confusion)

Almost got it!

Sosigenes and others knew that the year was not exactly 365.25 days long.
True Solar Year is about 365.242199 days: Calendar gets ahead by 1 day per 128 years; Slow slip of the seasons through the Julian Calendar
By the Middle Ages the slip became large, about 10 days

A Moveable Feast

In 325 AD the Council of Nicaea established a formula for computing the day of Easter:: Adopted a fixed March 21 Vernal Equinox; Easter is the first Sunday after the first Full Moon of the Vernal Equinox that does not coincide with Passover

Gregorian Calendar

The Julian Calendar became misaligned with the seasons by 10 days by the 1500s.: Easter was being computed incorrectly, and so celebrated on the wrong day.; Other imporant holy days were also being celebrated on the wrong days.
Pope Gregory XIII appointed a commission to develop an improved calendar.

A New Leap Year Formula

An elegant formula was invented by Aloysius Lilius, an Italian physician:
Use the Julian Leap Year formula: But, a century year is not a leap year unless it is divisible by 400.; 2000 is a leap year, but not 1700, 1800, 1900, or 2100.
Removes 3 days every 400 years: Remaining error only about 3 hours per 400 years.

The Lost Ten Days

Gregory instituted the new calendar in 1582:: Took 10 days out of October 1582 to realign the calendar with the seasons; The day after October 4 was October 15
Adopted by Catholic countries within 2 years: Some rioting over the "lost days," especially over payment of wages and rents.
Adopted all over continental Europe by 1750s.

Still slightly off

The Gregorian Calendar formula is equivalent to a year of 365.2425 days
This is about 0.0003 days longer than the length of the true solar year (365.2422 days): Get ahead of the true solar year by 1 day every 3327 years
The Gregorian Calendar will be ahead of the true Solar Year by 1 day in 4909 AD.

Months and Weeks

The year is also divided into 12 months: 12.4 lunar months (phase cycles) per year; The word Month derived from the word Moon
Months are divided into Weeks:: Each week is divided into 7 days
Seven is for the 7 "planets" visible to the naked eye

Dividing the Day

The 24h day likely originated with the ancient Egyptians, who used a duodecimal or base-12 system
Day and night were divided into 12 hours each, regardless of the time of year
The day began at sunrise or sunset for many cultures
Mechanical time finally led to the introduction of equal-length hours in the 1300s: Minutes and seconds were introduced later as timekeeping became more precise.

Standard Time

Long-distance railroads and telegraph networks required standarized time:: Coordination of interstate railroad schedules; Telegraph stations linked widely separated longitudes instantaneously, but needed to coordinate communications.
Small differences in local time began to matter.

Time Zones

19th Century innovation in US and Canada:: Divide the Earth into Time Zones by longitude from the Prime Meridian; Basic time zones are 15 degrees of longitude apart (360 degrees/24 hours = 15 degrees/hour); Each time zone keeps Local Solar Time for a fixed reference longitude.; All longitudes within that zone use "Zone Time" instead of local solar time.

Added complications...

Actual time zone borders do not follow the meridians.
Cities, counties, and small countries want to use the same time zone.: Some states refuse to have two time zones.; Island nations do not wish to be divided.
These lead to irregular time zone boundaries.

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.