46
4 Time
marked candles. The year was often, but not always, broken into 365 days, with
subdivisions, now known as months, based on the cycle of the Moon.
In 46 BCE, the introduction of the Julian calendar by Julius Caesar as the official
Roman calendar resolved many of the problems of earlier systems of timekeeping,
and the span and influence of the Roman Empire spread the Julian calendar throughout Europe and North Africa.
By 1582, it was becoming clear that small discrepancies in the Julian calendar were
causing the seasons to drift. This was resolved by the introduction of the Gregorian
calendar, named after the pope at the time, which resulted in a ten-day offset. The
change encountered considerable resistance, with some people believing they were
losing ten days of their lives! Surprisingly, it was not until the 1920s that the last
country using the Julian calendar finally abandoned it.
It is worth stopping for a brief while before we move from date to time to discuss
BCE and CE (also known as BC and AD). The move to a CE dating system occurred
before the introduction of the concept of zero in the Western world. The consequence
of this is that 1 BCE is followed by 1 CE. In general, this should not cause you any
problems unless you are using dates that predate the common era. Likewise, you
should be aware that there is a ten-day difference between Julian and Gregorian
dates.
1 If you are using a pre-Gregorian date for some reason, you will need to
convert.
By the fourteenth century, mechanical clocks, used mainly for religious reasons and important cultural gatherings, were becoming increasingly common. The
introduction of the pendulum by the Dutch astronomer, mathematician, and inventor Christiaan Huygens and the hairspring by Robert Hooke, who coincidently
also coined the word “cell” in relation to biology, led to clocks becoming more
widespread, portable, and affordable.
All clocks have inaccuracies and require resetting to a known time on occasions.
Up until 1847, the vast majority of clocks were set at noon. However, the timing of
noon is a function of local longitude, and hence noon in London occurs approximately
ten minutes before noon in Bristol, which lies 2
◦ 3
west of London. It was not until the
coming of the railways, requiring well-coordinated timetables over a large network,
that the need for a centrally set time system become apparent. Ultimately, this led to
the introduction of time zones and Greenwich Mean Time (GMT) as the de facto
standard.
Typically, time zones are approximagely one hour wide and are measured from the
time in Greenwich, in the United Kingdom, Greenwich Mean Time (GMT), with
each hour representing 15
◦ of latitude. Hence Western European Time, one hour
ahead of GMT, is notated GMT+0100, whilst Honduras is at GMT-0600. Although
since 1972 the standard has been Coordinated Universal Time One (UT1), the
difference between GMT and UT1 is small enough (0.9 s) that for most purposes
in astronomy it can be ignored. The introduction of daylight saving time (British
summer time in the UK) can somewhat complicate matters. Beware if you are not
1 The two calendars continue to diverge. The difference increased to 11 days in 1700, and today it
is 13 days.
4 Time
marked candles. The year was often, but not always, broken into 365 days, with
subdivisions, now known as months, based on the cycle of the Moon.
In 46 BCE, the introduction of the Julian calendar by Julius Caesar as the official
Roman calendar resolved many of the problems of earlier systems of timekeeping,
and the span and influence of the Roman Empire spread the Julian calendar throughout Europe and North Africa.
By 1582, it was becoming clear that small discrepancies in the Julian calendar were
causing the seasons to drift. This was resolved by the introduction of the Gregorian
calendar, named after the pope at the time, which resulted in a ten-day offset. The
change encountered considerable resistance, with some people believing they were
losing ten days of their lives! Surprisingly, it was not until the 1920s that the last
country using the Julian calendar finally abandoned it.
It is worth stopping for a brief while before we move from date to time to discuss
BCE and CE (also known as BC and AD). The move to a CE dating system occurred
before the introduction of the concept of zero in the Western world. The consequence
of this is that 1 BCE is followed by 1 CE. In general, this should not cause you any
problems unless you are using dates that predate the common era. Likewise, you
should be aware that there is a ten-day difference between Julian and Gregorian
dates.
1 If you are using a pre-Gregorian date for some reason, you will need to
convert.
By the fourteenth century, mechanical clocks, used mainly for religious reasons and important cultural gatherings, were becoming increasingly common. The
introduction of the pendulum by the Dutch astronomer, mathematician, and inventor Christiaan Huygens and the hairspring by Robert Hooke, who coincidently
also coined the word “cell” in relation to biology, led to clocks becoming more
widespread, portable, and affordable.
All clocks have inaccuracies and require resetting to a known time on occasions.
Up until 1847, the vast majority of clocks were set at noon. However, the timing of
noon is a function of local longitude, and hence noon in London occurs approximately
ten minutes before noon in Bristol, which lies 2
◦ 3
west of London. It was not until the
coming of the railways, requiring well-coordinated timetables over a large network,
that the need for a centrally set time system become apparent. Ultimately, this led to
the introduction of time zones and Greenwich Mean Time (GMT) as the de facto
standard.
Typically, time zones are approximagely one hour wide and are measured from the
time in Greenwich, in the United Kingdom, Greenwich Mean Time (GMT), with
each hour representing 15
◦ of latitude. Hence Western European Time, one hour
ahead of GMT, is notated GMT+0100, whilst Honduras is at GMT-0600. Although
since 1972 the standard has been Coordinated Universal Time One (UT1), the
difference between GMT and UT1 is small enough (0.9 s) that for most purposes
in astronomy it can be ignored. The introduction of daylight saving time (British
summer time in the UK) can somewhat complicate matters. Beware if you are not
1 The two calendars continue to diverge. The difference increased to 11 days in 1700, and today it
is 13 days.