| |
 |
|
|
Science Forum Index » Geology Forum » When was Gregorian calendar right?
Page 1 of 1
|
| Author |
Message |
| Crown-Horned Snorkack |
 Posted: Wed Apr 30, 2008 10:05 pm |
|
|
|
Guest
|
Well, when was it?
The actual length of sideric day, and of tropical day, increases
slowly, as tidal friction irreversibly slows down the rotation of
Earth.
The actual length of sideric year can change - the energy of Earth´s
orbit may change as energy is exchanged between orbital movement of
Earth and other planets. But those changes are said to be minor. The
changes in tropical year are likewise minor.
The result is that the number of tropical days in a tropical year
decreases.
Sometime in palaeozoic, tidal rhytmites allegedly show that there were
400 days in a year.
But the rate of tidal slowing is not constant. It changes with changes
in the configuration of shelf seas and ocean basins. There would be
major changes during ice ages, for example.
The true duration of tropical year is around 365,2423 to 365,2424
tropical days. Gregorian calendar requires 365,2425 days.
The cumulative error of Gregorian calendar through recent is thus less
than two days.
But how valid was Gregorian calendar in ice age? |
|
|
| Back to top |
|
| oriel36 |
| Posted: Wed Apr 30, 2008 11:51 pm |
|
|
|
Guest
|
On May 1, 9:05 am, Crown-Horned Snorkack <chornedsnork...@hush.ai>
wrote:
Quote: Well, when was it?
The actual length of sideric day, and of tropical day, increases
slowly, as tidal friction irreversibly slows down the rotation of
Earth.
This assumption and conclusion can be filed under astrology with a
touch of a geostationary Earth attached to it even though it is the
basis for much of the empirical approach to structural and timekeeping
astronomy.
A few weeks ago I posted an analogy between the maximum depth of
crust compared to the entire structure of Earth in terms of a bowling
ball and it amounts to less than 1 mm,the purpose being to show that
geodynamics of crustal evolution,motion and deformation may be linked
to geodynamics of the rotating interior.The average depth of the ocean
is just over 2 miles while the Earth's diameter is over 7 900 miles
across the Equator,I will not even bother to reduce it to a bowling
ball analogy.The point is that this 'tidal friction' business is the
product of minds who have yet to discover what geodynamics actually do
and especially the motion and deformation of the crust through
differential rotation.
Quote: The actual length of sideric year can change - the energy of Earth´s
orbit may change as energy is exchanged between orbital movement of
Earth and other planets. But those changes are said to be minor. The
changes in tropical year are likewise minor.
The result is that the number of tropical days in a tropical year
decreases.
Sometime in palaeozoic, tidal rhytmites allegedly show that there were
400 days in a year.
But the rate of tidal slowing is not constant. It changes with changes
in the configuration of shelf seas and ocean basins. There would be
major changes during ice ages, for example.
The true duration of tropical year is around 365,2423 to 365,2424
tropical days. Gregorian calendar requires 365,2425 days.
The cumulative error of Gregorian calendar through recent is thus less
than two days.
But how valid was Gregorian calendar in ice age?
Timekeeping astronomy is fairly intricate and especially the means by
which cl;ocks are kept in sync with the axial cycle at 24 hours/360
degrees.The correlation was constructed as inviolate meaning that it
does not directly refer to axial rotation directly let alone tying the
Earth's rotation directly to celestial sphere geometry (hence the
sidereral day ect).
If anybody decides to get serious about any part of timekeeping and
structural astronomy or the strong possibility that the geodynamics of
crustal motion may involve rotational geodynamics then let me know. |
|
|
| Back to top |
|
| Crown-Horned Snorkack |
| Posted: Thu May 01, 2008 2:46 am |
|
|
|
Guest
|
On 1 mai, 15:03, "Androcles" <Headmas...@Hogwarts.physics> wrote:
Quote: This message is brought to you by Androcles
http://www.androcles01.pwp.blueyonder.co.uk/
"Crown-Horned Snorkack" <chornedsnork...@hush.ai> wrote in message
news:5b40436f-cb79-4966-807b-7b83b9eaacd8@25g2000hsx.googlegroups.com...
| Well, when was it?
| The actual length of sideric day, and of tropical day, increases
| slowly, as tidal friction irreversibly slows down the rotation of
| Earth.
| The actual length of sideric year can change - the energy of Earth´s
| orbit may change as energy is exchanged between orbital movement of
| Earth and other planets. But those changes are said to be minor. The
| changes in tropical year are likewise minor.
| The result is that the number of tropical days in a tropical year
| decreases.
| Sometime in palaeozoic, tidal rhytmites allegedly show that there were
| 400 days in a year.
| But the rate of tidal slowing is not constant. It changes with changes
| in the configuration of shelf seas and ocean basins. There would be
| major changes during ice ages, for example.
| The true duration of tropical year is around 365,2423 to 365,2424
| tropical days. Gregorian calendar requires 365,2425 days.
| The cumulative error of Gregorian calendar through recent is thus less
| than two days.
| But how valid was Gregorian calendar in ice age?
It was never "right", it is an approximation as you've pointed out.
For time spans in the order of 2000 years it is good enough to have
a leap day every four years (but not every century) simply because
society finds it convenient to use integers. In a human life span
of 70-80 years that's good enough, but if you want to be exact
you'll have a difficult task. Besides which nobody really cares
if the last ice cube melted at 3:05:45pm on July 23rd, 9,356 BC
by my watch, currently about 8 seconds slow, and I won't argue
if I'm proven wrong - which you cannot do anyway because you
don't know exactly when it was any more than I do.
The Gregorian calendar was adopted in favour of the Julian (still
used by astronomers)
How do astronomers define their Julian calendar?
Quote: because some leap days were missing and the
effect of the error is cumulative. With 25 leap days each century
we accumulate 20 * 25 days in 2 millennia and we do better with
20 * 24, but that undershoots so we add a leap day every 400
years. It is still not exact but it is close enough for everyday
purposes.
There are plenty of recent things which can be dated to a specific
year. Tree rings. Layered sediments in various water bodies. Snow
layers in Greenland and Antarctic ice sheets.
The year by year records of Greenland and Antarctic ice sheet clearly
go back to ice age.
Now, tidal sediments should clearly indicate phase of moon, neap/
spring tide. Right? |
|
|
| Back to top |
|
| Androcles |
| Posted: Thu May 01, 2008 7:03 am |
|
|
|
Guest
|
This message is brought to you by Androcles
http://www.androcles01.pwp.blueyonder.co.uk/
"Crown-Horned Snorkack" <chornedsnorkack@hush.ai> wrote in message
news:5b40436f-cb79-4966-807b-7b83b9eaacd8@25g2000hsx.googlegroups.com...
| Well, when was it?
| The actual length of sideric day, and of tropical day, increases
| slowly, as tidal friction irreversibly slows down the rotation of
| Earth.
| The actual length of sideric year can change - the energy of Earth´s
| orbit may change as energy is exchanged between orbital movement of
| Earth and other planets. But those changes are said to be minor. The
| changes in tropical year are likewise minor.
| The result is that the number of tropical days in a tropical year
| decreases.
| Sometime in palaeozoic, tidal rhytmites allegedly show that there were
| 400 days in a year.
| But the rate of tidal slowing is not constant. It changes with changes
| in the configuration of shelf seas and ocean basins. There would be
| major changes during ice ages, for example.
| The true duration of tropical year is around 365,2423 to 365,2424
| tropical days. Gregorian calendar requires 365,2425 days.
| The cumulative error of Gregorian calendar through recent is thus less
| than two days.
| But how valid was Gregorian calendar in ice age?
It was never "right", it is an approximation as you've pointed out.
For time spans in the order of 2000 years it is good enough to have
a leap day every four years (but not every century) simply because
society finds it convenient to use integers. In a human life span
of 70-80 years that's good enough, but if you want to be exact
you'll have a difficult task. Besides which nobody really cares
if the last ice cube melted at 3:05:45pm on July 23rd, 9,356 BC
by my watch, currently about 8 seconds slow, and I won't argue
if I'm proven wrong - which you cannot do anyway because you
don't know exactly when it was any more than I do.
The Gregorian calendar was adopted in favour of the Julian (still
used by astronomers) because some leap days were missing and the
effect of the error is cumulative. With 25 leap days each century
we accumulate 20 * 25 days in 2 millennia and we do better with
20 * 24, but that undershoots so we add a leap day every 400
years. It is still not exact but it is close enough for everyday
purposes. |
|
|
| Back to top |
|
| Crown-Horned Snorkack |
| Posted: Thu May 01, 2008 8:06 am |
|
|
|
Guest
|
On 1 mai, 19:53, "Mike Dworetsky" <platinum...@pants.btinternet.com>
wrote:
Quote: "Crown-Horned Snorkack" <chornedsnork...@hush.ai> wrote in message
news:5b40436f-cb79-4966-807b-7b83b9eaacd8@25g2000hsx.googlegroups.com...
Well, when was it?
The actual length of sideric day, and of tropical day, increases
slowly, as tidal friction irreversibly slows down the rotation of
Earth.
The actual length of sideric year can change - the energy of Earth´s
orbit may change as energy is exchanged between orbital movement of
Earth and other planets. But those changes are said to be minor. The
changes in tropical year are likewise minor.
The result is that the number of tropical days in a tropical year
decreases.
Sometime in palaeozoic, tidal rhytmites allegedly show that there were
400 days in a year.
But the rate of tidal slowing is not constant. It changes with changes
in the configuration of shelf seas and ocean basins. There would be
major changes during ice ages, for example.
The true duration of tropical year is around 365,2423 to 365,2424
tropical days. Gregorian calendar requires 365,2425 days.
The cumulative error of Gregorian calendar through recent is thus less
than two days.
But how valid was Gregorian calendar in ice age?
The question is, in the everyday sense, meaningless, because the Gregorian
calendar is not used for years prior to 1582. Not for any sensible purpose,
because the main event it was designed to regulate thenceforward--the date
of Easter--had already taken place in each of those previous years. (Later
in England--Newton was born on Christmas Day in the Julian calendar, so we
still call that his birthday.) Looking backwards for dates is what you get
using a "proleptic" calendar and usually the Julian calendar is used for
this.
This causes problems. Julian calendar is considerably farther from
true length of tropical year than the Gregorian.
Quote: Aside from the removal of 10 days in 1582, the difference between Julian
and Gregorian is in the number of days in an average year: 365.25 or
365.2425. So if you tried to decide the date in some very distant past year
(e.g., 10,000 BC) you would have to account for that difference, or about
.75 d per century or 7.5 days per millennium.
And 75 days over 10 millennia. Which means that a proleptic Julian
calendar is plainly out of alignment with seasons in early recent.
In which calendar month did Laach lake eruption take place?
Quote: Of course we also have a cumulative error in the sense that the actual
period length of a tropical year is 365.2422 days (to 4 decimals) so the
Gregorian calendar itself will gradually need some reforms; a leap year will
have to be omitted, perhaps the year 4000. I imagine the Catholic Church
will give thought to this in about 1700 years or so. (Could be an
interesting SciFi story theme?). The error accumulated is about 1 day every
3300 years rather than 4000 years, so in the very far future a further
reform might be needed.
The effect of changes in the rotation of the Earth would be smaller but
would accumulate more rapidly as you went back in time. The figure for c.
2000 years ago is of the order of three hours error. See:
http://www.blackwell-synergy.com/doi/pdf/10.1046/j.1468-4004.2003.442...
It is difficult to extrapolate rotational rates back accurately over many
millennia because more than tidal effects are involved.
Indeed. However, they could be measured directly.
Misalignment over a few weeks should show up in geologic record.
Quote: Astronomers use something called the Julian Day number which is a running
count. The Julian period starts on 1 January 4713 BC (Julian calendar) and
lasts for 7980 years. It starts on the day when the Roman Indiction, Golden
Number, and Solar number all had a value of 1. This allows them to avoid
having to use dates in one calendar or another for calculated events in the
distant past (like eclipses). For accurate timings they of course have to
use a dynamical time scale independent of variations of Earth rotation.
Indeed. Julian day is bound to rotation of Earth which, in long term,
is more variable than tropical year (bound to orbital movement of
Earth). Neither of which is constant flow of time as bound to inertial
laws.
Quote: When they define somenthing to be measured in years they usually use a unit
of Julian years of 365.25 days, e.g., if discussing the period of a visual
binary star, or the orbit of the Sun around the galaxy.
Of course, it may be a question what the most important year is:
tropical, sideric or anomalistic... |
|
|
| Back to top |
|
| Androcles |
| Posted: Thu May 01, 2008 9:06 am |
|
|
|
Guest
|
This message is brought to you by Androcles
http://www.androcles01.pwp.blueyonder.co.uk/
"Crown-Horned Snorkack" <chornedsnorkack@hush.ai> wrote in message
news:aca40425-08a7-445e-9fdc-e33a0847bfe0@m36g2000hse.googlegroups.com...
On 1 mai, 15:03, "Androcles" <Headmas...@Hogwarts.physics> wrote:
Quote: This message is brought to you by Androcles
http://www.androcles01.pwp.blueyonder.co.uk/
"Crown-Horned Snorkack" <chornedsnork...@hush.ai> wrote in message
news:5b40436f-cb79-4966-807b-7b83b9eaacd8@25g2000hsx.googlegroups.com...
| Well, when was it?
| The actual length of sideric day, and of tropical day, increases
| slowly, as tidal friction irreversibly slows down the rotation of
| Earth.
| The actual length of sideric year can change - the energy of Earth´s
| orbit may change as energy is exchanged between orbital movement of
| Earth and other planets. But those changes are said to be minor. The
| changes in tropical year are likewise minor.
| The result is that the number of tropical days in a tropical year
| decreases.
| Sometime in palaeozoic, tidal rhytmites allegedly show that there were
| 400 days in a year.
| But the rate of tidal slowing is not constant. It changes with changes
| in the configuration of shelf seas and ocean basins. There would be
| major changes during ice ages, for example.
| The true duration of tropical year is around 365,2423 to 365,2424
| tropical days. Gregorian calendar requires 365,2425 days.
| The cumulative error of Gregorian calendar through recent is thus less
| than two days.
| But how valid was Gregorian calendar in ice age?
It was never "right", it is an approximation as you've pointed out.
For time spans in the order of 2000 years it is good enough to have
a leap day every four years (but not every century) simply because
society finds it convenient to use integers. In a human life span
of 70-80 years that's good enough, but if you want to be exact
you'll have a difficult task. Besides which nobody really cares
if the last ice cube melted at 3:05:45pm on July 23rd, 9,356 BC
by my watch, currently about 8 seconds slow, and I won't argue
if I'm proven wrong - which you cannot do anyway because you
don't know exactly when it was any more than I do.
The Gregorian calendar was adopted in favour of the Julian (still
used by astronomers)
| How do astronomers define their Julian calendar?
It's a mythical date startiing at zero since the world supposedly began,
counted in days, according to some ancient religions. The reason for
retaining it is that there are records of astronomical importance
(particularly
planetary data) which nobody wanted to throw away, and so astronomers
retained the older system of counting days rather than convert to day,
month, year system which is awkward - if only because some months have 31
days and some have 30 (except February).
http://en.wikipedia.org/wiki/Julian_day
Quote: because some leap days were missing and the
effect of the error is cumulative. With 25 leap days each century
we accumulate 20 * 25 days in 2 millennia and we do better with
20 * 24, but that undershoots so we add a leap day every 400
years. It is still not exact but it is close enough for everyday
purposes.
| There are plenty of recent things which can be dated to a specific
| year. Tree rings. Layered sediments in various water bodies. Snow
| layers in Greenland and Antarctic ice sheets.
When Pope Gregory was alive those things didn't matter. The still
don't if you get a speeding ticket on Friday the 13th and can prove
you were in the Scottish Highlands on that date. Calendars can be
useful for all manner of things including alibis.
| The year by year records of Greenland and Antarctic ice sheet clearly
| go back to ice age.
So it may, but you are still locked into integer years. You cannot look
at one tree ring and pick out any particular day in that ring, the growth
rate is not linear. Some rings are thicker than others which may tell us
something about the climate that year when the growth was poor because
it was a dry summer and a wet spring or vice versa. What a good
scientist will do is look at both the tree ring and the ice record and
make some assessment based on all the data he can gather, but there
is no guarantee he's right.
| Now, tidal sediments should clearly indicate phase of moon, neap/
| spring tide. Right?
Not necessarily. A single tsunami can wipe the record clean, a glacier
can gouge a grove in sedimentary rock which in turn can be covered
by a volcanic eruption... which epoch are you trying to pin down? |
|
|
| Back to top |
|
| Mike Dworetsky |
| Posted: Thu May 01, 2008 11:53 am |
|
|
|
Guest
|
"Crown-Horned Snorkack" <chornedsnorkack@hush.ai> wrote in message
news:5b40436f-cb79-4966-807b-7b83b9eaacd8@25g2000hsx.googlegroups.com...
Quote: Well, when was it?
The actual length of sideric day, and of tropical day, increases
slowly, as tidal friction irreversibly slows down the rotation of
Earth.
The actual length of sideric year can change - the energy of Earth´s
orbit may change as energy is exchanged between orbital movement of
Earth and other planets. But those changes are said to be minor. The
changes in tropical year are likewise minor.
The result is that the number of tropical days in a tropical year
decreases.
Sometime in palaeozoic, tidal rhytmites allegedly show that there were
400 days in a year.
But the rate of tidal slowing is not constant. It changes with changes
in the configuration of shelf seas and ocean basins. There would be
major changes during ice ages, for example.
The true duration of tropical year is around 365,2423 to 365,2424
tropical days. Gregorian calendar requires 365,2425 days.
The cumulative error of Gregorian calendar through recent is thus less
than two days.
But how valid was Gregorian calendar in ice age?
The question is, in the everyday sense, meaningless, because the Gregorian
calendar is not used for years prior to 1582. Not for any sensible purpose,
because the main event it was designed to regulate thenceforward--the date
of Easter--had already taken place in each of those previous years. (Later
in England--Newton was born on Christmas Day in the Julian calendar, so we
still call that his birthday.) Looking backwards for dates is what you get
using a "proleptic" calendar and usually the Julian calendar is used for
this.
Aside from the removal of 10 days in 1582, the difference between Julian
and Gregorian is in the number of days in an average year: 365.25 or
365.2425. So if you tried to decide the date in some very distant past year
(e.g., 10,000 BC) you would have to account for that difference, or about
..75 d per century or 7.5 days per millennium.
Of course we also have a cumulative error in the sense that the actual
period length of a tropical year is 365.2422 days (to 4 decimals) so the
Gregorian calendar itself will gradually need some reforms; a leap year will
have to be omitted, perhaps the year 4000. I imagine the Catholic Church
will give thought to this in about 1700 years or so. (Could be an
interesting SciFi story theme?). The error accumulated is about 1 day every
3300 years rather than 4000 years, so in the very far future a further
reform might be needed.
The effect of changes in the rotation of the Earth would be smaller but
would accumulate more rapidly as you went back in time. The figure for c.
2000 years ago is of the order of three hours error. See:
http://www.blackwell-synergy.com/doi/pdf/10.1046/j.1468-4004.2003.44222.x
It is difficult to extrapolate rotational rates back accurately over many
millennia because more than tidal effects are involved.
Astronomers use something called the Julian Day number which is a running
count. The Julian period starts on 1 January 4713 BC (Julian calendar) and
lasts for 7980 years. It starts on the day when the Roman Indiction, Golden
Number, and Solar number all had a value of 1. This allows them to avoid
having to use dates in one calendar or another for calculated events in the
distant past (like eclipses). For accurate timings they of course have to
use a dynamical time scale independent of variations of Earth rotation.
When they define somenthing to be measured in years they usually use a unit
of Julian years of 365.25 days, e.g., if discussing the period of a visual
binary star, or the orbit of the Sun around the galaxy.
--
Mike Dworetsky
(Remove pants sp*mbl*ck to reply) |
|
|
| Back to top |
|
| Dr J R Stockton |
| Posted: Thu May 01, 2008 2:35 pm |
|
|
|
Guest
|
In sci.astro message <4uadnbgKKvbyaITVnZ2dnUVZ8q-rnZ2d@bt.com>, Thu, 1
May 2008 17:53:03, Mike Dworetsky <platinum198@pants.btinternet.com>
posted:
Quote:
Astronomers use something called the Julian Day number which is a
running count. The Julian period starts on 1 January 4713 BC (Julian
calendar) and lasts for 7980 years. It starts on the day when the
Roman Indiction, Golden Number, and Solar number all had a value of 1.
This allows them to avoid having to use dates in one calendar or
another for calculated events in the distant past (like eclipses). For
accurate timings they of course have to use a dynamical time scale
independent of variations of Earth rotation. When they define
somenthing to be measured in years they usually use a unit of Julian
years of 365.25 days, e.g., if discussing the period of a visual binary
star, or the orbit of the Sun around the galaxy.
Julian Day Number starts at noon GMT = 0.0 on that day, which was Monday
BC 4714 Nov 24 proleptic Gregorian, or -4713-11-24.
Chronological Julian Day Number can be used to count local days from the
local midnight which started that date.
--
(c) John Stockton, nr London, UK. ?@merlyn.demon.co.uk Turnpike v6.05 MIME.
Web <URL:http://www.merlyn.demon.co.uk/> - FAQish topics, acronyms, & links. |
|
|
| Back to top |
|
| Mike Dworetsky |
| Posted: Thu May 01, 2008 4:35 pm |
|
|
|
Guest
|
"Crown-Horned Snorkack" <chornedsnorkack@hush.ai> wrote in message
news:7c123121-f96a-4fa4-9e14-c9bb54a88e3b@24g2000hsh.googlegroups.com...
Quote: On 1 mai, 19:53, "Mike Dworetsky" <platinum...@pants.btinternet.com
wrote:
"Crown-Horned Snorkack" <chornedsnork...@hush.ai> wrote in message
news:5b40436f-cb79-4966-807b-7b83b9eaacd8@25g2000hsx.googlegroups.com...
Well, when was it?
The actual length of sideric day, and of tropical day, increases
slowly, as tidal friction irreversibly slows down the rotation of
Earth.
The actual length of sideric year can change - the energy of Earth´s
orbit may change as energy is exchanged between orbital movement of
Earth and other planets. But those changes are said to be minor. The
changes in tropical year are likewise minor.
The result is that the number of tropical days in a tropical year
decreases.
Sometime in palaeozoic, tidal rhytmites allegedly show that there were
400 days in a year.
But the rate of tidal slowing is not constant. It changes with changes
in the configuration of shelf seas and ocean basins. There would be
major changes during ice ages, for example.
The true duration of tropical year is around 365,2423 to 365,2424
tropical days. Gregorian calendar requires 365,2425 days.
The cumulative error of Gregorian calendar through recent is thus less
than two days.
But how valid was Gregorian calendar in ice age?
The question is, in the everyday sense, meaningless, because the
Gregorian
calendar is not used for years prior to 1582. Not for any sensible
purpose,
because the main event it was designed to regulate thenceforward--the
date
of Easter--had already taken place in each of those previous years.
(Later
in England--Newton was born on Christmas Day in the Julian calendar, so
we
still call that his birthday.) Looking backwards for dates is what you
get
using a "proleptic" calendar and usually the Julian calendar is used for
this.
This causes problems. Julian calendar is considerably farther from
true length of tropical year than the Gregorian.
Yes, but why is it of any importance to be able to put "modern" (Julian or
Gregorian)
dates to ancient events that took place in now-dead civilisations with their
own calendars, which were the only ones of importance for their purposes?
At the time of those events the Julian calendar had not even been invented.
(And of course the Gregorian neither.)
Quote:
Aside from the removal of 10 days in 1582, the difference between Julian
and Gregorian is in the number of days in an average year: 365.25 or
365.2425. So if you tried to decide the date in some very distant past
year
(e.g., 10,000 BC) you would have to account for that difference, or about
. .75 d per century or 7.5 days per millennium.
And 75 days over 10 millennia. Which means that a proleptic Julian
calendar is plainly out of alignment with seasons in early recent.
Absolutely. But unless you are organising Christian or other modern
religious festivals
for long dead ancient stone age peoples this does not really matter.
Quote:
In which calendar month did Laach lake eruption take place?
I'm not familiar with this specific event, but presumably one could judge
the season from fossil pollens, etc trapped in sediments. Though you would
need to take climatic differences into account. For example, the northern
summer solstice currently coincides with aphelion.
bout about 13,000 years ago it was near the perihelion, so northern summers
would have been considerably hotter, other things being equal.
Quote:
Of course we also have a cumulative error in the sense that the actual
period length of a tropical year is 365.2422 days (to 4 decimals) so the
Gregorian calendar itself will gradually need some reforms; a leap year
will
have to be omitted, perhaps the year 4000. I imagine the Catholic Church
will give thought to this in about 1700 years or so. (Could be an
interesting SciFi story theme?). The error accumulated is about 1 day
every
3300 years rather than 4000 years, so in the very far future a further
reform might be needed.
The effect of changes in the rotation of the Earth would be smaller but
would accumulate more rapidly as you went back in time. The figure for
c.
2000 years ago is of the order of three hours error. See:
http://www.blackwell-synergy.com/doi/pdf/10.1046/j.1468-4004.2003.442...
It is difficult to extrapolate rotational rates back accurately over many
millennia because more than tidal effects are involved.
Indeed. However, they could be measured directly.
Misalignment over a few weeks should show up in geologic record.
An interesting suggestion, but I doubt that anyone has been able to exploit
it yet with any confidence to establish say a three or four second per day
faster rotation tens or hundreds of thousands of years ago. There are
practical difficulties...such as the assumption that plants were similar
enough to today's plants to generate the pollen on the same seasonal dates.
Quote: Astronomers use something called the Julian Day number which is a running
count. The Julian period starts on 1 January 4713 BC (Julian calendar)
and
lasts for 7980 years. It starts on the day when the Roman Indiction,
Golden
Number, and Solar number all had a value of 1. This allows them to avoid
having to use dates in one calendar or another for calculated events in
the
distant past (like eclipses). For accurate timings they of course have
to
use a dynamical time scale independent of variations of Earth rotation.
Indeed. Julian day is bound to rotation of Earth which, in long term,
is more variable than tropical year (bound to orbital movement of
Earth). Neither of which is constant flow of time as bound to inertial
laws.
But the point is, there is no practical need to assign modern historical
dates to ancient events predating the invention of writing and calendrical
records. If you would like to do this for your own amusement, go ahead. No
historian needs to do this. The oldest dates (Ancient Egypt) are uncertain
by decades, so why worry about the exact date as described by *our*
calendar?
Quote: When they define somenthing to be measured in years they usually use a
unit
of Julian years of 365.25 days, e.g., if discussing the period of a visual
binary star, or the orbit of the Sun around the galaxy.
Of course, it may be a question what the most important year is:
tropical, sideric or anomalistic...
Yes, it depends on what you are discussing. Orbits? sidereal or
anomalistic. The date of the Vernal Equinox? Tropical.
--
Mike Dworetsky
(Remove pants sp*mbl*ck to reply) |
|
|
| Back to top |
|
| |
|
Page 1 of 1
All times are GMT - 5 Hours
The time now is Sat Sep 06, 2008 1:54 am
|
|