When all of Japan says noon, why doesn't the Sun?
East of the standard meridian, solar noon comes earlier. Yet even in Akashi, it does not always fall at 12:00. Separate the difference due to place from the difference due to date to untangle the relationship between clocks and the Sun.
Estimated time for the article text only. Allow extra time for figures, comparisons and interactive exhibits.
12:00 in Japan Standard Time, 12:00 in local mean solar time, and the actual Sun's meridian transit use different references. One degree of longitude corresponds to 4 minutes; the actual transit also depends on the equation of time, which changes with the date.
When both clocks say noon, is the Sun in the same place?
Imagine arranging to meet at noon in two distant cities, with both clocks set to the same time. If they use Japan Standard Time, both will show 12:00. But at that instant, will the Sun be due south in both cities? Sharing a clock does not mean that everything in the sky happens at the same moment.
The Sun's passage due south is called its meridian transit. In this article, we will consider places around Honshu, Japan's main island, where the Sun transits to the south. The National Astronomical Observatory of Japan explains that even at a location on the standard meridian of 135° east, the Sun does not transit at exactly 12:00 every day. There must be more to the answer than simply being some distance from 135°.
Let us divide the puzzle into two parts. First, what changes when we move east or west on the same day? Second, what changes when we stay in one place but change the date? Following these separately reveals that the differences between our clocks and the sky have different causes.
READ THE COMPARISON / Make a discovery in the diagram
Clocks can agree on 12:00 while solar time differs.
A simplified model that varies only longitude, using 135°E as the reference.
11:20
Approximate local mean solar time
10° west of the reference
12:00
Approximate local mean solar time
Reference meridian
12:40
Approximate local mean solar time
10° east of the reference
Longitude difference × 4 min/degree = time difference
Comparison conditionsAn explanatory model using mean solar time. For simplicity, it omits precise time-scale offsets relative to Japan Standard Time and does not apply the date-dependent equation of time. This diagram cannot give today’s actual solar noon or sunrise.
Change the longitude and compare →See the evidence behind the diagram and the article’s sources
- National Institute of Information and Communications Technology (NICT) — Japan Standard Time ↗ (opens in a new tab)
- National Astronomical Observatory of Japan — How can I find the time of the Sun's meridian transit? ↗ (opens in a new tab)
- National Astronomical Observatory of Japan — Does the time of the Sun's meridian transit change from day to day? ↗ (opens in a new tab)
- Ephemeris Computation Office, National Astronomical Observatory of Japan — About sidereal time ↗ (opens in a new tab)
- National Astronomical Observatory of Japan — Do sunrise and sunset times differ from place to place? ↗ (opens in a new tab)
One degree of longitude is four minutes on the Sun's clock
Start by reading longitude, which describes a place's east–west position on a map, as marks on a clock. If the Sun's average daily circuit takes 24 hours and covers 360°, the time corresponding to 1° is 24 × 60 ÷ 360, or 4 minutes. That gives 1 hour for 15°, and 20 minutes for 5°. The National Astronomical Observatory of Japan also converts 1° of longitude into 4 minutes of time when calculating how transit times differ between places.
The farther east a place lies, the earlier it experiences the Sun's transit on a given day. If two places differ in longitude by 5°, for example, the Sun transits roughly 20 minutes earlier at the eastern one. This is not because a clock in the east runs faster. It is because two places sharing the same clock time face different directions on Earth.
The 24 hours used here is the average length of a day measured relative to the Sun. It does not mean that Earth's rotation period relative to distant stars is exactly 24 hours. Agreeing on what a complete turn is measured against is the starting point for this conversion.
Place three clocks at 130°, 135°, and 140° east
To isolate the effect of longitude, imagine locations at 130°, 135°, and 140° east. These are not observations for particular cities or dates. Picture each location with a clock showing the same Japan Standard Time and another showing the local mean solar time. We will leave differences of less than a second between time systems outside this discussion.
When Japan Standard Time is 12:00, the mean solar time at 135° east is approximately 12:00. At 140° east, 5° farther east, it is 12:20; at 130° east, 5° farther west, it is 11:40. We are looking at all three places at the same instant, yet the clocks based on the Sun show different times.
Now reverse the question: at what Japan Standard Time does the local mean solar time reach 12:00? At 140° east it is 11:40, at 135° east it is 12:00, and at 130° east it is 12:20. Saying that the Sun's clock is ahead in the east and saying that noon arrives earlier there are two ways of describing the same relationship. To use this table to predict the actual Sun's transit, however, we still need the date.
The mean Sun is not in the sky
The slightly curious name "mean solar time" contains a useful device for keeping time. The Sun we actually see is called the apparent Sun, and its apparent motion is not uniform throughout the year. The interval from one day's transit to the next cannot always be treated as having the same length.
Astronomers therefore imagine a mean Sun whose motion has had these irregularities smoothed out. Mean solar time is time measured against this mean Sun. There is no second Sun in the sky, nor can we simply average the positions we see each day. It is a reference that connects a celestial body's changing motion with evenly spaced units of time.
The table of three locations above used this average. On some days the actual Sun is ahead of the mean Sun; on others it is behind. Even at 135° east, where the longitude difference is zero, the difference between these two Suns remains. That is why standing on the standard meridian does not guarantee that the Sun will be due south at noon every day.
Two motions combine to create the difference from one date to another
Why is the Sun's apparent motion uneven? One reason is that Earth's orbit around the Sun is an ellipse, so its orbital speed is not constant. The other is that Earth's rotation axis is tilted relative to its orbital plane. The direction in which the Sun travels through the starry sky does not align with the east–west scale that corresponds to clock time. Projecting its motion onto that scale also introduces unevenness.
The difference in position between the apparent Sun and the mean Sun, including both of these effects and converted into time, is called the equation of time. The longitude difference stays the same if we keep our location fixed. The equation of time, by contrast, changes with the date. Thinking of the difference due to place and the difference due to date as two separate corrections helps make the relationship clear.
In the National Astronomical Observatory of Japan's explanation, a positive equation of time means that the actual Sun transits earlier than the mean Sun; a negative value means that it transits later. Some sources use the opposite sign convention, however. Before adding or subtracting a number, check which quantity has been subtracted from which.
The 135° reference and the way time is generated today
The meridian at 135° east is an important reference for understanding Japan's time system. Divide the angle of 135° east of Greenwich by 15° per hour, and the result is 9 hours. NICT's explanation connects the origin of the system based on this meridian with the fact that today's Japan Standard Time is 9 hours ahead of Coordinated Universal Time, UTC.
Japan Standard Time is not, however, reset each day by observing the Sun at 135° east. NICT uses atomic clocks to generate UTC(NICT), its realization of UTC, and advances that time by 9 hours to generate Japan Standard Time, JST. We can distinguish the geographical reference from the mechanism that continually produces a stable time signal.
It would therefore go too far to infer from the everyday explanation "Japan Standard Time is the time at 135° east" that the actual Sun must transit at 12:00 every day. Sharing a time standard makes it convenient to coordinate plans across distant places. Removing local differences in the sky's motion is not part of that agreement.
To find today's transit time, specify both place and date
To move from our three-location model back to the actual sky, we need both longitude and date. Looking up a transit time for a particular place and date, through a service such as the National Astronomical Observatory of Japan's Ephemeris Computation Office, lets us account for changes that the mean Sun alone cannot represent. The times of 11:40 and 12:20 calculated from longitude alone must not be treated as a timetable that applies throughout the year.
Nor can the relationship of 4 minutes per 1° of longitude be extended unconditionally to differences in sunrise and sunset times. Sunrise and sunset also involve latitude and the relationship to the horizon. What we have compared here is the correspondence between longitude and time around the Sun's transit, not the length of daylight.
The labels attached to the numbers are your guide. "Japan Standard Time," "local mean solar time," and "the Sun's transit time on a particular date" may all be shown on similar clock faces, but they are different quantities. Does a table list only locations, or does it also include dates? That distinction is the first clue to what its numbers actually calculate.
From noon on a clock to the world behind a map
Clocks in distant cities show 12:00 at the same instant because people have chosen to use a shared time for daily life. Behind that agreement, differences in the Sun's clock between eastern and western locations continue, as do changes in the Sun's motion through the year. Reading a common time and reading the local sky are two perspectives we can use together.
When finding a destination on a map, look at how many degrees east or west of the standard meridian it lies. That tells you whether mean solar noon comes earlier or later there. But to determine today's actual transit time, you need the date. There is a clear boundary between what a longitude calculation can tell us and what we need to check in an almanac.
When we say "noon," do we mean 12:00 on the clocks society shares, or are we thinking of the midpoint of the Sun's daily journey? Adding just that one question takes us from a familiar clock to a map, and from the map to Earth's motion. There is only one number 12 on the clock, but there are several layers of agreement behind it.
Sources and verification notes
This article is an original synthesis of primary sources and explanations from specialist institutions.
Generative AI assists with research, structure and English translation. Claims are compared with sources and editorial checks are performed; independent specialist and native-English reviews have not yet been completed.
- National Institute of Information and Communications Technology (NICT)Japan Standard Time ↗ (opens in a new tab)
- National Astronomical Observatory of JapanHow can I find the time of the Sun's meridian transit? ↗ (opens in a new tab)
- National Astronomical Observatory of JapanDoes the time of the Sun's meridian transit change from day to day? ↗ (opens in a new tab)
- Ephemeris Computation Office, National Astronomical Observatory of JapanAbout sidereal time ↗ (opens in a new tab)
- National Astronomical Observatory of JapanDo sunrise and sunset times differ from place to place? ↗ (opens in a new tab)
Sources checked:
Official explanations from NICT and the National Astronomical Observatory of Japan were opened and checked for how JST is generated, the relationship between 135° and 9 hours, the conversion of 1° of longitude to 4 minutes, the mean and apparent Suns, and the causes and sign convention of the equation of time. The locations at 130°, 135°, and 140° east are explanatory examples; no transit calculations for specific dates or shadow observations were carried out. This model, expressed in minutes, does not address differences of less than a second between UTC-based time and mean solar time. Two additional NAOJ explanations were also consulted to check solar days and sidereal time, and the limits of applying this relationship to sunrise and sunset.
Rights, status, and revision history
- Status
- Free public edition. Preparation and revision history is retained below.
- 0.1.1
- Rights to the text and diagrams
- The original Japanese text is an independently written summary. The SVG illustrations and conceptual diagrams were newly created. No external images, original source text, or audio recordings are reproduced.
- How sources are used
- Sources are referenced by links only. All rights to each source remain with its respective rights holders.
- · v0.1.0 — Checked primary sources and explanations from specialist institutions to create a private preview draft with original comparisons and diagrams. Connected it to a flagship feature. Formal publication has not been approved.
- · v0.1.1 — Added a reasoned detour to the v17 Earth & Humanity feature. Existing prose, comparison conditions, sources and source-check dates are unchanged.
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