Chapter 1 – Time (Measuring time, different calendars)

Measuring time, different calendars
Spectrum Calendar, prints a calendar for any year after 1582
Julian Date, works out the Julian Day number for any date
Julian Calendar, prints a complete Julian Day calendar for any month of any year
Day of the Week, identifies the day of the week for any date
Interval Days, the interval between any two dates from a few minutes to centuries apart
Local Sidereal Time, calculates ‘star time’ for any date and hour
Reaction Timer, find out your reaction time to get accurate readings.

Time and the calendar

Time is not just the ticking of a clock. If time did not exist, then neither would the universe. Time is fundamentally a measure of change in the position of an object (ie movement) — be it an electron in orbit about the atomic nucleus or the separation of galaxies since the Big Bang.

Fortunately, time seems so natural it’s just like breathing. The ticking clock is a nice homely reminder of passing events measured in seconds, minutes and hours. And the calendar marks some order in events beyond a 24-hour period.

Astronomers use all the familiar systems to measure time, but variations are needed.

The Earth’s timekeeping
The Earth, once thought to be a perfect timekeeper in its 24-hour rotation period, has been found to have small ‘glitches’ that can be detected with modern super-accurate clocks. Not only do we have leap years to correct the calendar (because there are 365.2422 days in a year), but ‘leap seconds’ to correct the grinding rotation of Earth. Leap seconds are only applied at very infrequent intervals, of perhaps a year or more, and their necessity cannot be predicted with any certainty.

The Earth can be compared to a spinning ice skater. Extending the arms will slow a skater down — the spin is converted into ‘angular momentum’. In the case of the Earth, it appears that climatic changes (like a series of North Atlantic depressions over several months) can contribute to slight changes in the Earth’s angular movements, resulting in the need for leap seconds. It has not been proved conclusively that this happens, but it is worth remembering that a single summer thunderstorm over Britain releases more energy than would a single H-bomb — such is the power of weather.

The calendar
The 365 days in the year (366 in a leap year) are allocated in seven days to a week, between 28 and 31 days to a month, and twelve months to the year. The word month is probably a corruption of ‘moonth’ for this is the approximate length of time which the Moon takes to orbit the Earth The four quarters of the Moon, ie between New Moon and First Quarter and so on to Full Moon, Last Quarter and New Moon again, occupy about seven days each.

The Gregorian or Reform Calendar
Julius Caesar is recognised for the introduction of the calendar which allowed for leap years every fourth year, so losing the quarter of a day (about 6 hours) gained each year. This assumed that the year was 365.25 days long whereas it proved to be 0.0078 days (11.23 minutes) less than this. This may seem trivial in a year, but over the centuries it can amount to an error of several days to the start of the year. In the sixteenth century the matter was considered serious.

In October 1582, Pope Gregory introduced the calendar which bears his name, designed to improve on the Julian Calendar. The days of October 5th to October 14th, inclusive, of that year were deleted completely, so that October 4th was immediately followed by October 15th. This brought the calendar back into line with that started by Julius Caesar.

The second measure was to ensure that such a discrepancy did not occur again for several thousand years. If the full century was divisible by 400 then it would be a leap year: otherwise it would not. This means that 1700 1800, and 1900 (leap years in the Julian Calendar) would not be, whereas 1600 and 2000 would remain leap years. Simple but effective. The error of 11.23 minutes is now reduced to about 26 seconds between the civil year (as accepted by governments) and the tropical year, or true year This 26 seconds error can be corrected in the calendar by omitting a leap year every 3334 years but we can let the future generations worry about that! You can see that Pope Gregory (and his calendar advisor Clavius) were far-sighted men.

Because the Gregorian Calendar was instigated by the Vatican, it did not meet with approval from Protestant countries like England. England and the North American colonies did not adopt this Calendar until September 1752. This is why reference is rarely made to the events of October 1582 in American (computing) books. One of the last states to adopt the Gregorian Calendar was the USSR in 1917.

The message here is important for the historian whether of political or astronomical subjects. The calendar used by a witness must be carefully researched before it is possible to know to which date he refers.

With this background information we can now proceed with some pro­grams relevant to the calendar and time.



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