I only pressed the shutter once, why are there two times in the record?
The same moment can be marked by two sets of timing scales, and different readings do not mean that it happened twice.
Astronomical observations left a record. If you want to know where the earth was at that time, you will use UT1; if you want to give this moment to calculations that require a uniform time scale, you will use TT. Don’t rush into the conversion yet: see how the same record can get two time tags at the same time.
Guess first
The same observation record is marked with UT1 and TT at the same time, and the readings differ by one minute. Does this mean that it was taken twice, one minute apart?
Show answer and explanation
No, they are two sets of readings at the same moment. — Two sets of time scales mark different readings for the same physical instant, and the difference in readings cannot be directly understood as the time interval between two events.
Then connect experience and science
First distinguish between time zones and time scales
The time zone conversion between Beijing time and UTC is not equal to the conversion between TT and UT1. No specific historical dates are entered in this lesson.
UT1: reflects the time of the Earth's rotation
Post two readings for the same instant
Blue is the TT reading, gray is the UT1 reading. Set ΔT and observe the difference between the two scales.
ΔT = TT − UT1
Convert seconds to days
When calculating using Julian days, you cannot directly add 60 seconds to 60 days; you must divide by 86,400 seconds per day.
JD(TT) = JD(UT1) + ΔT / 86400
Know where the difference comes from
ΔT here is the assumed value you set. Actual calculations must use observations or models of corresponding eras, and today's values cannot be used for all eras.
Readings on different time scales at the same instant
Expand the calculation
ΔTSecond = TT − UT1 JD(TT) = JD(UT1) + ΔTSecond/86400 TT = TAI + 32.184Second
First confirm the units of seconds and days, and then add them together. Both sets of readings mark the same instant; UT1 cannot be directly replaced with UTC.
give it a try
First select "One minute difference", then select "The date has passed". The two labels are always attached to the same record, not two photos taken.
No need to memorize symbols
- UT1
- Universal time related to Earth's rotation
- TT
- Earth time; this lesson is used to illustrate the relationship with UT1
- ΔT
- TT − UT1, unit: seconds
- JD
- Number of consecutive days; the same instant has different values on different time scales
Algorithm, accuracy and scope of application
Chapter 10 of the original book discusses TD and UT; here, TT and UT1 are explicitly used to illustrate the difference, and TT and TDB are not regarded as strictly the same. The ΔT slider is a hypothetical experiment and does not predict current or future values. No leap second watch is used, and no UTC conversion is done. The TT−TAI=32.184 second fixed relationship in the code is also compared with the TAI2TT of AA+; the experiment of TT−UT1 is as defined and checked across days with integer seconds. Scenario description: Record A and the screen are schematics of events and do not simulate the actual starry sky or the operation of real clocks. ΔT is an assumed value set by the user, not the current measured value or future prediction; this lesson does not do UTC or time zone conversion. UT1 is related to the rotation of the earth, and TT is a uniform time scale; the actual conversion must use the difference of the corresponding era.
my understanding check
When we change the hypothesis ΔT in this lesson, what changes?
Time conversion tag for the same record — The slider setting converts the experimental hypothetical difference and does not control the actual Earth's rotation or the speed of the atomic clock.
The UT1 label is 23:59:40 of the current day. Assuming ΔT=+60 seconds, what should the TT label be?
Next day 00:00:40 — The reading plus one minute spans midnight; there is still only one record for A.
Can it be directly regarded as precision UT1 by subtracting 8 hours from Beijing time?
Can't — The usual time zone conversion gives UTC. UTC and UT1 are different time scales; the conversion of TT/UT1 is not a simple time zone difference.
The same instant can have different time readings. Recognize the time scale first, then interpret the difference; don’t treat label changes as a repeat of the event.
Jean Meeus · Astronomical Algorithms, 2nd ed. · No. 10 chapter