Do people in the past and people today have the same difference in earth clocks?
If you want to check the chronology of ΔT, you cannot apply one number from today or an old book to all history.
The Earth's rotation is not completely uniform. If you want to convert the universal time in ancient records to the time used to calculate the motion of celestial bodies, you must estimate the difference in that era. First, look through the history table in the original book, and then see in which years each formula can only be used.
Guess first
When converting ancient observations to time, can we always use the same ΔT in modern times?
Show answer and explanation
cannot — The Earth's rotation changes over time, and the difference also varies.
Then connect experience and science
Let’s make it clear first
The same ΔT cannot be used for all eras. Table 10.A of the original book lists historical values starting from several years.
ΔT = TT − UT1
Only within the given range when looking up the table
There are values every two years in the table. This lesson linearly interpolates between adjacent table values and does not treat them as new actual measurements.
Original table range 1620–1998
When choosing a formula, you also need to look at the valid year.
The high-order empirical formula is only suitable for the specified era, and may be seriously distorted when used to calculate other eras.
1800–1997 Monolithic and segmented types were restricted respectively
Understand the difference between old predictions and current measurements
The period after 2000 in the book is an extrapolation from that time. It is part of the history of algorithms and cannot be directly regarded as today's latest ΔT.
Ancient or future estimates with far less accuracy than modern observations
First select the original table interpolation, integral or segmented according to the year and mode, and then find ΔT seconds. Different segments use different independent variable origins and coefficients; one segment of polynomial cannot be extended to all years.
give it a try
Compare the negative value for 1900, the table value for 1962 and the ancient estimate of 333 years. When out of bounds, the page will explicitly reject that route.
No need to memorize symbols
- ΔT
- TT−UT1, seconds
- t
- Number of centuries since 2000
- θ
- Page 80 Number of centuries since 1900 in empirical expressions
Algorithm, accuracy and scope of application
Completely enter 190 two-year interval values in Table 10.A; linear interpolation between two years is the clear query method for this lesson. Three high-order empirical polynomials are realized in Equations 10.1/10.2 and page 80, and the applicable age is strictly limited. It is not intended to replace modern IERS or new historical Earth rotation studies.
my understanding check
Outside the range of the table, can we continue to extrapolate the table values as actual measurements?
cannot — Extrapolations are additional assumptions, not original observations.
Can the original book's estimate of the future be called today's latest observation?
Can't — A distinction must be made between the age of the data and current actual measurements.
The empirical formula contains a lot of decimals. Can it be guaranteed to be accurate in any era?
cannot — The effective range and error are more important than the number of display digits.
Time differences also have dates and origins. Check the data range first, and then convert the time scale.
Jean Meeus · Astronomical Algorithms, 2nd ed. · No. 10 chapter