YSS - Intro. to Observational Astrophysics (ASTR 205).

Class # 2The Science of Astronomy

Professor: José Maza

June 6, 2012

Calendar

Motion of the Sun: the day (24 hours) and the year, a bit more than 365 days.

The week and the month have a different origin. The month has it origin in the motion of the Moon. In 29,5 days it goes from new Moon to full Moon and back.

The lunar month can be divided into four weeks, seven days each, corresponding to the seven celestial bodies going around the Earth: Mercury, Venus, Mars, Jupiter, Saturn, the Moon and the Sun.

The week has an astrological origin. It was believed that the seven celestial bodies (Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn) governed every hour of the day.

If the first hour of one day was governed by Saturn, the second was governed by Jupiter, the third by Mars, the fourth by the Sun, the fifth by Venus, the sixth by Mercury and the seventh by the Moon.

In the eighth hour the cycle was restarted. Then Saturn governed that day the first,

eighth, fifteenth and twenty-second; the twenty-third would be governed by Jupiter and the last one by Mars.

The following hour, the first of the next day would be governed by the Sun.

It was adopted that the day would by marked by the planet governing the first hour.

Julian Calendar:

Romans adopted a year with 12 months of 29.5 days on average (30 & 29), totalizing 354 days, 11¼ days short of a tropical year (the year of the seasons).

Every second year they added an extra month during February, the last month of the year.

The roman year started on March, continued with April, May, June, Quintilis, Sixtilis, September, October, November and December;

The year ended with Januarius and Februarius. On one occasion an extra month of 22 days was added and the following time the extra month had 23 days.

The years lasted 354, 376, 354 and 377 that is 1.461 days in four years, resulting in an average of 365¼ days.

The extra months were ordered by the Supreme Roman Pontifex, making it confusing when the Empire grew large and it was difficult to communicate the extra month throughout the Empire.

The month of the roman year were:

Martius Quintilis November Aprilis Sextilis December Majus September Januarius Junius October Februarius

In the year 46 B.C. Julius Cæsar in Rome, with the advice of Sosigenes, an Alexandrian astronomer, adopted a year with twelve months with 365¼ days on average, that is, three years in a row with 365 days and another of 366.

Months of 29 and 30 days become of 30 and 31, giving away the 11 extra days.

February until then the last month of the year did not get an extra day and remained with 29 days.

The year 46 B.C. (better named year 708 from the foundation of Rome) got 3 extra months completing 445 days, for that reason it has been named “the confusion year”.

Starting 45 B.C. the Julian calendar reigned.

The Roman Senate decided to changed the name of the fifth month of the old year to honor Julius Caesar (JULY).

The Julian reform consisted of: 1.- The beginning of the year was moved

to January 1 to make it coincide with the appointment of the consuls.

2.- The duration of the old roman lunar year was adjusted to the solar year, producing month of the following duration:

Januarius 31 Majus 31 September 31 Februarius 29 Junius 30 October 30 Martius 31 Quintilis 31 November 31 Aprilis 30 Sextilis 30 December 30

With Julius Caesar the lunar aspect of the calendar came to an end.

The calendar became strictly solar. The month was preserved as an

intermediate unit but it does not have a relationship with the Moon any more (the new Moon or the full Moon can be at any time of the month).

3.- Every four years an extra day is added in February.

We believe the extra day is added at the end of February.

The Romans added the extra day after February 23.

That was the sixth day before to the “calendas of March” (it was called “ante diem sextum calendas martias”) and the extra day was called “ante diem bissextum calendas martias”, that is the sixth day counted twice.

The extra day was called bissextus and finally the year with that extra day was called bisiesto (in Spanish) leap year (in English).

Julius Cæsar

After the murder of Julius Caesar in the hands of Brutus in 44 B.C. the Julian rule was not followed; they added an extra day every three years.

By the year 8 B.C. they have added 13 leap years instead of the 10 years of the Julian rule.

The new roman Emperor Augustus Cæsar did not add extra days until the year 8 A.D. in order to have the spring starting on March 25th as it was common in Rome.

The Roman Senate decided to named "August" the sixth month, that had 30 days.

In order that the month of Augustus is "not less than" that of Julius, the Roman Senate decided that the month of August would have 31 days, getting the extra day from February that had 29 days and was degraded to 28, rising to 29 only once every four years.

For this reason we have two consecutive months with 31 days (July & August).

The Senate change from 31 to 30 the seventh month (September) and the ninth (November), rising from 30 to 31 the eighth and tenth.

The calendar was left in the following form:

Januarius 31 Majus 31 September 30 Februarius 28 Junius 30 October 31 Martius 31 July 31 November 30 Aprilis 30 August 31 December 31

That order and length of the months is used until today.

In the year 525 A.D. the abbot of Rome Dionisius the Exiguous introduced the usage of counting the years from the birth of Christ instead of doing it from the foundation of Rome.

He called year 754 from the foundation of Rome year one after and the year 753 the year one before Christ.

Dionisius did not considered the year zero in our chronology because the Romans did not have the concept of the number zero.

It took a lot for the Christian era to get massively used, being England the first to use it in 705, France, in the year 742 and Germany in 876.

In Portugal it was adopted in the year 1422. Even the Pope only adopted it in 1431.

Further studies indicate Dionysius was wrong on his “appreciation” of the year of the birth of Christ.

His source associated Christ’s birth with the years of Augustus government (the 18th year) but Dionysius failed taking into account 4 years that Augustus ruled in a triumvirate (as Octavius).

Therefore Dionisius should have identified year 750 after the foundation of Rome as year 1 in Christ era.

That is why we find the paradox that, in our chronology, Christ was borne the year 4 B.C.

A detailed discussion situates the year of Christ’s birth between 7 B.C. and 4 B.C.

What is absolutely out of discussion is that the day of Christ’s birth was not December 25.

December 25 was a Roman holiday celebrating the end of the solar migration to the south (boreal winter solstice).

The first Christians in Rome used that occasion to celebrate Christ.

Emperor Constantine converted the Empire to Catholicism and adopted December 25 as the day of birth of Christ and also adopted the celebration of Eastern (Nicea Council).

Gregorian Calendar

In the year 325, the Council of Nicea, fixed the celebration of Eastern the weekend after the first full Moon occurring at or immediately after March 21, the date of the beginning of the spring.

Between 45 B.C. and 325 A.D. the beginning of the spring has moved from March 24 to March 21.

The Julian year, 365 days and 6 hours is 11 minutes and 14 seconds longer than the tropical year (the year of the seasons).

That small difference accumulates an error of one day every 128 years, accumulating 3 days from Julius Cæsar until the Council of Nicea.

The strict implementation of the Julian calendar throughout the Middle Ages made it accumulate a large error.

By 1550 spring was starting on March 11, ten days off.

The problem for the Church was the proper celebration of Eastern.

The Julian Calendar was used until 1582 when Pope Gregory XIII, with the advice of Luigi Lilio and Christopher Clavius, ordered the reform of the calendar.

Pope Gregory XIII

The new Gregorian calendar consisted of: Drop 10 days of the calendar: Thursday

October 4th of 1582 would be followed by Friday October 15.

In order to prevent further disagreements due to the extra day every 128 years the Gregorian calendar dropped 3 leap year every 400.

According to the new rule leap years are those divisible by 4. Those years divisible by 100 are not leap years unless they are divisible by 400.

In this way the years end a century 1700, 1800 and 1900 were not leap years (they were so in the Julian calendar) but the year 2000 was a leap year.

The Gregorian year is 365 days and 97/400 that is 365,2425 days (365d 5h 49m 12s) just 26 seconds too long. The tropical year is 365,242193 (365d 5h 48m 45,4s).

This error accumulates one day in 3.250 years.

The Gregorian calendar was adopted immediately in all catholic countries, like Spain, Portugal, France and Italy.

Germany changed the calendar in 1700. England resisted until 1752.

England in 1752 they went from September 2nd to September 14th.

In Russia they never change the calendar, so they were 13 days off at the beginning of the XX century.

Only after the October Revolution (November in our calendar) of 1917, the Soviet Union adopted the western calendar.

Claudius Ptolemy (~100-~170) Geocentric Universe.

“Almagest” A.D. 150 Nicholas Copernicus (1473-1543) In 1543 he published “De Revolutionibus

Orbium Caelestium” (Concerning the revolution of the Heavenly Spheres).

Heliocentric Universe. Tycho Brahe (1546-1601) Big improvement in the art of observations Errors dropped from ±10’ to less than ±1’

Johannes Kepler (1571-1630). He used Tycho´s observations to study the motion of Mars and formulated the laws of planetary motion in 1609.

Galileo Galilei (1564-1642). He studied the free fall and formulated the inertia principle.

He built the first telescope and made many great astronomical discoveries (craters of the Moon, four satellites of Jupiter, Sunspots, etc.).

Isaac Newton (1643-1727) “Philosophiæ Naturalis Principa

Mathematica”, published in 1687 contains the law of universal gravitation, the laws of mechanics and infinitesimal calculus.

William Herschel (1738-1822) Robert Kirchhoff (1824-1887) Jacobus Kapteyn (1851-1922) Harlow Shapley (1885-1972) Edwin Hubble (1889-1953)