Our summer almanac 27.08.2010 – 02.09.2010

From Summer to Autumn
While the Summer Triangle still dominates the night sky, the stars of autumn are rising in the east, most notably the stars of the great constellation of Pegasus, the Winged Horse. The brightest star in the Square of Pegasus is Alpheratz.

The month named after the emperor Augustus ends, and the ninth month of the year begins, carrying a name, September, that means the seventh month, a leftover of the old Roman ten month calendar.

The Ancient Egyptian New Year

August 29 was the New years Day for the ancient Egyptians, celebrated with ceremonies that began when the brightest star in the heavens, Sirius, appeared on top of the point of obelisks that were set up to be precisely aligned with observation points on the ground below. So what was it that was so special about August 29?

The Egyptian calendar, like most calendars, was organized around what is most important to people in their lives and livelihood. For the ancient Egyptians the pattern of their everyday life was shaped by the great African river Nile. The Nile is the longest river in the world. Its source is the river Luvironza in Burundi, and it flows over 4000 miles to the eastern Mediterranean Sea. Because the Nile floods regularly at almost exactly the same time of year, the Egyptians who depended on the annual flood to water and replenish the soil with rich alluvial deposits, were very interested in predicting when this would happen.

When rainfall and the melting snows of the mountains of Ethiopia eventually increased the water level thousands of miles downstream, the ancient Egyptians marked the rise in water using a gauge on the river bank. Using this Nilometer they could then count the days from the high water mark until the next flood, and that would be roughly a year. However, there were periods of time when the floods did not come abundantly, times of climatic variation and drought, and this had a disastrous impact on the lives and welfare of everyone.

Then, later on, the ancient Egyptian astronomers noticed that Sirius, the Dog Star, rises at dawn in a direct line with the rising Sun once a year, and that this astronomical event coincided with the annual flood of the river Nile. This particular astronomical event, and the particular day upon which it occurred became very important as the first day of the month dedicated to the Egyptian god Thoth, and the beginning of a new year.

By timing the appearance of Sirius from year to year the Egyptian astronomers were able to calculate that the length of the year was one quarter of a day longer than 365 days. so the Egyptian year was accurate to a margin of 11 minutes and 24 seconds to the solar year, 2000 years before Julius Caesar was inspired to adopt this calendar for Rome in 46 BC.

Water in space

This week 2 September 2010 ESA’s Herschel infrared space observatory discovered that ultraviolet starlight is the key ingredient for making water in space. It is the only explanation for why a dying star is surrounded by a gigantic cloud of hot water vapour.

Every recipe needs a secret ingredient. When astronomers discovered an unexpected cloud of water vapour around the old star IRC+10216 in 2001, they immediately began searching for the source. Stars like IRC+10216 are known as carbon stars and are thought not to make much water. Initially they suspected the star’s heat must be evaporating comets or even dwarf planets to produce the water. Now, Herschel’s PACS and SPIRE instruments have revealed that the secret ingredient is ultraviolet light, because the water is too hot to have come from the destruction of icy celestial bodies.

What is ultraviolet light?
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays. It is not visible light for human beings and so the name means “beyond violet” (from Latin ultra, “beyond”), violet being the color of the shortest wavelengths of visible light.

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation and that includes visible light, invisible light like Ultra Violet and Infra Red and radio waves. The graphic above sets out the range the electromagnetic spectrum extends from low frequencies used for modern radio to gamma radiation at the short-wavelength end, covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The long wavelength limit is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous.

This image shows a false-colour image of the Sun’s corona as seen in deep ultraviolet by the Extreme ultraviolet Imaging Telescope. The Extreme ultraviolet Imaging Telescope (EIT) is an instrument on the SOHO spacecraft used to obtain high-resolution images of the solar corona in the ultraviolet range. The EIT wavelengths are of great interest to solar physicists because they are emitted by the very hot solar corona but not by the relatively cooler photosphere of the Sun; this reveals structures in the corona that would otherwise be obscured by the brightness of the Sun itself. EIT was originally conceived as a viewfinder instrument to help select observing targets for the other instruments on board SOHO, but EIT is credited with a good fraction of the original science to come from SOHO, including the first observations of traveling wave phenomena in the corona, characterization of coronal mass ejection onset, and determination of the structure of coronal holes, the areas where the Sun’s corona is darker, colder, and has lower-density plasma than average. These were found when X-ray telescopes in the Skylab mission were flown above the Earth’s atmosphere to reveal the structure of the corona.


The Julian Calendar
The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus. A tropical year (also known as a solar year), for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. It has a regular year of 365 days divided into 12 months, and a leap day is added to February every four years. Hence the Julian year is on average 365.25 days long.

The problem with the previous Roman Calendar system was that the average Roman year would have had 366¼ days over four years, giving it an average drift of one day per year relative to any solstice or equinox. So the calendar became more and more out of synchronization with the solar year.

Julius Caesar, having spent some time in Egypt was aware of the the fact that the Egyptian calendar had a fixed year of 365 days, drifting by one day against the sun in four years. When Caesar returned to Rome in 46 BC, according to Plutarch, he called in the best philosophers and mathematicians of his time to solve the problem of the calendar. Eventually, it was decided to establish a calendar that would be a combination between the old Roman months, the fixed length of the Egyptian calendar, and the 365¼ days of the Greek astronomy.

Although the new calendar was much simpler than the pre-Julian calendar, the pontifices added a leap day every three years, instead of every four years. According to Macrobius, the error was the result of counting inclusively, so that the four-year cycle was considered as including both the first and fourth years; perhaps the earliest recorded example of a fence post error. The following problem illustrates the error:
If you build a straight fence 100m long with posts 10m apart, how many posts do you need?

A common intuition is to divide 100 by 10 and thus answer 10. This is incorrect; the fence has 10 sections, but it has 11 posts.

After 36 years, this resulted in three too many leap days. Augustus remedied this discrepancy by restoring the correct frequency. He also skipped three leap days in order to realign the year.

The Julian calendar remained in use into the 20th century in some countries as a civil calendar, but has been replaced by the Gregorian calendar in nearly all countries. The Roman Catholic Church and Protestant churches have replaced the Julian calendar with the Gregorian calendar, but the Orthodox Church (with the exception of Estonia and Finland) still use the Julian calendar for calculating the dates of moveable feasts. Some Orthodox churches have adopted the Revised Julian calendar for the observance of fixed feasts, while other Orthodox churches retain the Julian calendar for all purposes. The Julian calendar is still used by the Berber people of North Africa, and on Mount Athos.

The Julian reform set the lengths of the months to their modern values. However, a 13th century scholar, Sacrobosco, proposed a different explanation for the lengths of Julian months which is still widely repeated but is certainly wrong. He then said Augustus changed this to:
31, 28/29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
so that the length of Augustus would not be shorter than (and therefore inferior to) the length of Iulius, giving us the irregular month lengths which are still in use.

The evidence disproving this theory includes a wall painting of a Roman calendar (shown above) predating the Julian reform which confirms the accounts that the months were already irregular before Julius Caesar reformed them:
29, 28, 31, 29, 31, 29, 31, 29, 29, 31, 29, 29.

The Gregorian Calendar
The Julian calendar was in general use in Europe and Northern Africa from the times of the Roman Empire until 1582, when Pope Gregory XIII needed to create adjustments that were required because too many leap days had been added. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian year. As a result, the calculated date of Easter gradually moved out of phase with the March equinox. Consequently the Julian calendar gained a day about every 134 years. By 1582, it was ten days out of alignment from where it supposedly was in the year 325 during the Council of Nicaea, when the first effort to attain consensus in the church through an assembly representing all of Christendom included settling the calculation of the date of Easter.

The Gregorian calendar was soon adopted by most Catholic countries (e.g. Spain, Portugal, Poland, most of Italy). Protestant countries followed later, and the countries of Eastern Europe adopted the “new calendar” even later.

In Britain and the the British Empire (including the American colonies), Wednesday 2 September 1752 was followed by Thursday 14 September 1752. Crowds of angry people were shouting “Give us back our eleven days!”. Eleven days had been taken out of the calendar!

Detail from an “Election Entertainment” by William Hogarth, featuring the anti-Gregorian calendar banner “Give us our Eleven Days”, 1755.

The British calendar, still following the Julian Calendar devised by Julius Caesar, and because it had been in use for centuries, was 11 days ahead of the true solar year. The adjustment of the calendar would bring Britain into line with all the European countries that had adopted the reforms of 1582 instituted by Pope Gregory XIII, in what is now known as the Gregorian Calendar.

Russia remained on the Julian calendar until 1918 (1 February became 14 February), after the Russian Revolution (which is thus called the “October Revolution” though it occurred in Gregorian November)

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