Date: October 4, 2009
Title: Measuring Time
Podcaster: Nik Whitehead
Organization: None
Description: Most people think of astronomy as being completely unrelated to their daily lives. They couldn’t be more wrong. Our day-to-day lives are governed by clocks and calendars, devices that only exist because ancient man looked at and measured the movements of objects in the sky. This podcast will look at how those early observations became today’s clocks and calendars.
Bio: Nik is a lecturer in computer science at the University of Akureyri in northern Iceland… but computer science is not her passion. She has a Bachelor’s degree in astronomy and astrophysics then took her Masters and Doctoral degrees in computer science when she realised that there are not enough jobs in astronomy to go around. What she’d really like to be when she grows up is either the navigator of the starship Enterprise or maybe a space traffic controller.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Palomar Observatory, a world-class center of astronomical research that is owned and operated by the California Institute of Technology. Learn more at www.astro.caltech.edu/palomar/.
Transcript:
Measuring Time
Podcast 4th October 2009
Hello! Welcome to the International Year of Astronomy podcast for October 4th. I’m Nik Whitehead, talking to you once more from Akureyri in Iceland, where I teach computer science at the university. Although computing is my day job, astronomy is my first love, and I’m delighted to be able to contribute again to the 365 Days of Astronomy series.
What time is it right now? If I ask you that question I assume that you’ll look at your watch, a clock, or maybe your mobile phone to provide the answer. But although we’ve had clocks of some sort for over five and a half thousand years, human beings have measured time for far longer than that, and most of their measurements are based upon astronomical observations of some sort. So today I’m going introduce you to some of the astronomy we take for granted – the time and the date.
Human beings and most other higher life forms respond in some way to the cycles of light and darkness caused by daily the rotation of the Earth on its axis. As a result, it was natural for the day to be the first period defined by mankind. Today we define a day as starting at midnight, but early civilisations defined their days based upon two astronomical events – the rising and the setting of the sun. Which of these events was chosen depended upon the culture involved. The ancient Egyptians measured the day from sunrise to sunrise, while in Jewish tradition the day starts at sunset or at the point when you can see three second-magnitude stars in the sky.
If sunrise marks the beginning of the day then how much of the day has passed can be worked out by measuring the position of the sun in the sky. This apparent motion was used by the ancient Egyptians and Babylonians about five and a half thousand years ago to measure time using sundials. It allowed the day to be split into hours, each of which was defined as either a twelfth of the time between sunrise and sunset or as a twenty-fourth of an entire day. But why divide it into twelve hours? This is probably because the Babylonians based their counting system on twelve rather than ten. The first hour started at sunrise, the seventh hour started at noon and the twelfth hour ended at sunset. It was the Egyptians who first divided the night into hours, by identifying the start of each hour with the rising of a different star.
There is a problem with dividing the day into twelve equal parts. In countries close to the equator days and nights are of similar length throughout the year, but as you get closer to the poles the variation between day length and night length through the year becomes greater and greater. This means that the length of an hour will also vary throughout the year. In fact, these unequal hours were used throughout Europe by the Church, who defined the times for prayer based upon the hours of the sundial and then rang a bell to call people to services.
In many places, however, it can be difficult to define the moment of sunrise or sunset because of either local geography or local weather conditions. It is difficult to see the sun if it is behind a hill or if it is raining. It therefore became the practice to use not the rising or setting sun as the reference point, but to use instead the point at which the sun reaches the local meridian, its highest point in the sky, as the time at which the day started. The day thus began at noon, and this system is still used by astronomers today, partly because it means that when they’re recording observations at night the date does not change between pre-midnight and post-midnight observations. Observations are recorded using the Julian Date system, which allows astronomers to date an observation as a decimal number of days from the starting point of noon at Greenwich in four thousand seven hundred and thirteen BC. This date was chosen because it was the first point at which the three cycles of the Julian calendar were in their first year together. It also conveniently pre-dated all historical records of astronomical observations known at the time.
Apart from not changing date in the middle of the night, the Julian Date system also has the advantage that it provides a standard method for recording days no matter what sort of civil calendar is used. A calendar is simply a method of counting numbers of days and organising these into distinct periods. The smallest calendar period in common use is the week, with our current seven-day week being the invention of the ancient Babylonians. Many cultures have linked these seven days with the names of the seven non-fixed celestial objects – the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn.
Seven days is not, however, the only calendar available. The Etruscans and the Romans started off using an eight-day week. The ancient Egyptians used a ten-day week, with the start of each week being marked by the first sighting just before dawn of a different star. The Chinese also used a ten-day week. These ten-day weeks were grouped in threes to make up months of thirty days each.
Now thirty days is very close to the twenty-nine and a half days it takes to go from full moon to full moon – a period that’s known as a synodic month. The word month in English is closely related to the word moon for just that reason. Unfortunately this means that there is not a whole number of months in a solar year. The Islamic world still uses a true lunar calendar of twelve months, with the result that the months drift by eleven days each year until after a period of thirty three years the lunar calendar comes back into synch with the solar calendar. In this system the start of each month is based upon the first sighting of the crescent moon at Mecca.
Another lunar calendar that is still widely used is the Chinese calendar. This is actually a lunisolar calendar, one that takes into account the motion of both the moon and the sun. It has twelve months to a year, numbered one to twelve, each of which starts at midnight on the night of the new moon. Every second or third year an extra intercalary or leap month has to be added somewhere in the calendar so that the winter solstice always falls within month eleven of the year. Chinese New Year is therefore normally the second new moon after the winter solstice.
In most parts of the world, however, the normal calendar is the Gregorian calendar, a solar calendar of three hundred and sixty five days. Leap days are added approximately every four years to keep the calendar in synch with the sidereal year, the time taken for the Earth to complete one orbit around the sun relative to the fixed stars.
End of podcast:
365 Days of Astronomy
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The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.
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