CHAPTER01
Dividing the day
Did an Edo-period “hour” change length between summer and winter?
First, set today’s clock aside for a moment. Discover a way of keeping time that divides daylight and night separately.
FEATURE 01 · CLOCKS & CALENDARS
Keeping Time
What time is it? Who are we keeping time with?
“Let’s meet at twelve.” Unpack that everyday agreement through the Sun, clocks, calendars and language.
Start by comparingThe same time, in different places.
Between measuring the passage of a day and agreeing on a shared time lie a series of choices. Explore them through five starting points.
Your observation station / Compare on equal terms
The clock says noon. What does the Sun say?
Hold Japan Standard Time at 12:00. Using 135° east as the reference, change only longitude to compare mean solar time.
Japan Standard Time display
12:00Our shared clock timeModeled local mean solar time
12:20Model at 140° EModeled offset due to longitude
20 min ahead(140 − 135)° × 4 min/° = +20 min
Move east and local time advances; move west and it falls behind. The shared Japan Standard Time display remains at 12:00 in this diagram.
This model does not predict the actual Sun
This explanatory model adds only the time offset due to longitude relative to 135° E to Japan Standard Time. It does not account for the equation of time, date, latitude, terrain, or small differences between UTC and Earth’s rotation. It does not show actual sunrise, sunset, or meridian-transit times.
Japan Standard Time is now generated using atomic clocks. Even at 135° E, the actual Sun’s meridian transit does not always occur at 12:00.
The model’s basis and links to original sources
- National Institute of Information and Communications Technology (NICT) — Japan Standard Time ↗
The reference meridian of 135° E, and the generation of present-day Japan Standard Time by advancing UTC by 9 hours.
- National Astronomical Observatory of Japan — How can I find the time of the Sun’s meridian transit? ↗
A 1° difference in longitude corresponds to a 4-minute time difference. Actual meridian-transit times depend on place and date.
- NAOJ Ephemeris Computation Office — Calendar Wiki: Solar time ↗
The distinction between mean and apparent solar time, and their difference: the equation of time.
After comparing, connect the ideas through reading
A reading path through 5 chapters.
Each article gives you a reason to continue. You can also begin with whichever chapter interests you.
The next questionAfter dividing a day in one place, try bringing the clocks of distant cities into agreement.
- CHAPTER02
Clocks in distant cities
When all of Japan says noon, why doesn't the Sun?
Carry the longitude model into the question of shared time. Distinguish the local progress of the Sun from the clocks used by society.
The next questionAfter clock time, consider the labels we give dates. What happens at the boundary between calendars?
- CHAPTER03
The day the calendar changed
Month 12, day 2—then New Year’s Day. Where did the missing dates go?
Place one day beside the next. Even when their date labels change dramatically, count elapsed time separately.
The next questionEven after choosing a calendar, there is still work to do to keep it aligned with the seasons.
- CHAPTER04
Rules that keep a calendar aligned
Leap years come every four years. So what about 2100?
Start with “once every four years,” then follow the exceptions beyond it. Read the rules that keep a calendar usable over long spans of time.
The next questionFinally, move from clocks and calendars to conversation. Even when we know the rules, will our meeting plans be understood?
- CHAPTER05
Words for making plans
If you agree to meet “shiasatte,” will you arrive on the same day?
Compare cases in which the same word brings different days to mind. Consider what information makes a plan easier to confirm.
Take one question with you before you leave
Clock times, calendars, and the words we use to make plans all need a shared reference.
- Which divisions of the day are being used?
- Does the reference stay the same from place to place?
- Do both people mean the same day and time?
The next time you look at something familiar, try recalling one of these three questions.