It’s about time – 13th Nov 2014

In November we scheduled an extra meeting “It’s About Time” presented by Professor Donald Kurtz. Don Kurtz is Professor of Astrophysics at the University of Central Lancashire. His research is in asteroseismology and magnetic stars and he teaches the undergraduate introductory astrophysics course, as well as other courses.

Don’s opening question was “What is time? Is there a past and a future or just a continuously moving instantaneous time?” Rather than enter into a theological or scientific debate about this question, he presented a very interesting, entertaining and, occasionally historically provocative, talk about the measurement of time through the ages, including the surprising scientific variability of it even in modern times. Some of the themes explained were:

As we know, measurement of time today is commonly expressed in terms of seconds, minutes, hours, days, weeks, months and years. However:

Days: We could use Solar days or Sidereal days. When does a day begin? Civil (Midnight); Astronomical (Noon); Swahili (Sunrise), Jewish Sabbath (Sunset). What is the length of a day? There are 86,400 seconds in a day. However, the length of a day is not constant throughout the year. It is affected both by the Moon (monthly effects plus long term tidal locking effect between the Earth and Moon slowing rotation of the Earth around its axis over time), our path around the Sun (being non-circular), effect of seasons (weather/winds and ocean/land tidal friction varying the rotation of the Earth at the millisecond scale). ‘Leap seconds’ are used to keep the correct UTC time of day close to the mean solar time.

Days of the week: These have varied from 5 to 10 days historically and by culture. The Romans used 8 days. Our current 7 day week pre-dates the Old Testament. This was based upon “7 Planets”. However, those were not the planets as we understand them today because the modern solar system was not understood. Instead they were the “7 Wanderers” in the sky, namely Mercury, Venus, (not Earth which wasn’t known as a planet), Mars, Jupiter, Saturn, the Sun and the Moon! Each day was named after one of the “Gods” i.e. Saturn’s day, Sun’s day, Moon’s day, Tiu’s day, Woden’s day, Thor’s day, Freya’s day.

Months: Keeping time with the Moon is awkward but not as awkward as the Roman Republican Calendar which originally had 10 named months followed by a 2 month gap (January / February)!

The Year: Sidereal vs Tropical (solar) year. Impact of Precession over thousands of years.

Don also gave us a historical tour through the history of calendars through the ages up to the modern calendar we commonly use today. This isn’t repeated in this meeting report. However, one example you may recognise is the Julian Calendar introduced by Julius Caesar in 46BC to more closely approximate the Tropical (solar) year. It had a regular year of 365.25 days divided into 12 months with alternating 30 / 31 day durations (apart from February), with a leap day every 4 years. The month of Quintillis was renamed to Julius upon the death of Julius Caesar and given 31 days duration. Subsequently the month of Sextillis was assigned to Julius’ successor Augustus following his death and of course it also had to have 31 days. The transition from the Julian calendar to the Gregorian calendar, again to more closely approximate the Tropical (solar) year, started in 1582 in Italy, Spain and Portugal but took until 1752 to be adopted by other previously Julian calendar countries, including England and Scotland in September of that year.

Don showed us that the calendar has a many interesting and sometimes provocative stories to it and we certainly know more about those than we did before.

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