Comparing two figures, and reading the result
Compare takes any two of the 153 figures and returns four numbers - lifespan overlap, birth gap, era distance, shared cities - plus the roles the two share, the roles only one of them has, and, if the atlas records one, the direct relationship between them. Pick the two from the dropdowns; everything is computed from the same records the figure pages render.
Nubala · Published · 8 min read
Each of the four numbers answers a narrower question than it looks like it does. Lifespan overlap says the two were alive at the same time; it says nothing about contact. Shared cities says the atlas associates both with a place; it does not say they were there together. Era distance is a distance in the atlas's own list of twelve eras, not in years - two figures fourteen years apart can be eight eras apart. This guide works through four real pairs to show where each metric helps and where it misleads.
The four metrics, and what each one is actually measuring
Three of the four are arithmetic on the stored birth and death years. The fourth is arithmetic on the era list.
Lifespan overlap is the number of years both were alive, with the range; when there is none, the page shows the gap instead. Birth gap is the difference between the two birth years - useful for generational distance, and often more informative than the overlap, because two people can overlap for decades while one is a child. Shared cities is the intersection of the two figures' city lists, with the unique ones shown on each side. Era distance is how far apart the two eras sit in the atlas's own ordering of twelve, which is editorial rather than chronological.
Because the eras are not ordered strictly by date - they are a mix of dynasties, regions and themes - a large era distance can mean almost nothing about time. The worked examples below include a pair fourteen years apart with an era distance of eight.
| Metric | Computed from | Answers | Does not answer |
|---|---|---|---|
| Lifespan overlap | Both stored birth and death years | Were they alive at the same time, and for how long | Whether they ever met or knew of each other |
| Birth gap | The two birth years | How many years separate the generations | Which of them was active first |
| Era distance | Position in the atlas's list of 12 eras | How far apart the atlas files them | How far apart they lived |
| Shared cities | The two city lists | Which places the atlas links to both | Whether they were there at the same time |
Pair one: 70 years of overlap and a documented meeting
Compare Timur, 1336 to 1405, with Ibn Khaldun, 1332 to 1406. The page returns a lifespan overlap of 70 years, a birth gap of 4, an era distance of 1 - Central Asia against Thinkers & Poets - and one shared city, Damascus.
This is the case where every metric points the same way, and the atlas confirms it: the direct relationship on Timur's page records that Ibn Khaldun met him during the siege of Damascus in 1401. The one shared city is the city where the meeting happened. A conqueror and the historian of civilisational cycles, exact contemporaries, in the same place, once.
Note what made the inference safe. It was not the 70-year overlap and it was not the shared city; it was the recorded relationship. The other three numbers only told you the meeting was possible.
Pair two: three shared cities and no meeting
Now compare Mansa Musa, about 1280 to about 1337, with Ibn Battuta, 1304 to about 1369. The page returns 34 years of overlap, a birth gap of 24, an era distance of 1, and three shared cities: Timbuktu, Mecca and Cairo. On the metrics alone this looks like a stronger case than Timur and Ibn Khaldun.
It is not. The relationship the atlas records says that Ibn Battuta visited Mali shortly after Musa's reign. The three shared cities are the three places where the two men's itineraries happen to coincide on paper: Musa passed through Cairo and Mecca on the pilgrimage his page dates to 1324, and Ibn Battuta reached the Mali empire decades later. Same cities, different years.
This pair is the clearest warning the atlas offers about its own comparison tool. Shared cities is a set intersection over a whole lifetime. It is excellent for finding out which places connect two careers, and it is no evidence at all that two people were ever in a room.
Pair three: fourteen years apart and eight eras apart
Compare Al-Ghazali, 1058 to 1111, with Ibn Rushd, 1126 to 1198. The overlap is nil - a 14-year gap - and the era distance is 8, the largest in any of these examples. Al-Ghazali is filed under Thinkers & Poets and Ibn Rushd under The Golden Age, which sit at opposite ends of the atlas's era list.
Those two figures are intellectually adjacent. Ibn Rushd's own page records the connection: he wrote Tahafut al-Tahafut - the Incoherence of the Incoherence - in reply to al-Ghazali's attack on the philosophers. The reply came about 85 years after the attack, in a different part of the Islamic world, and it is one of the most direct exchanges in the corpus.
The lesson is to ignore era distance when you want chronology. It is a measure of how the atlas files people, and because Thinkers & Poets is a theme spanning about 700 to 1800 CE rather than a period, anybody filed under it will show a large era distance from nearly everyone. Use the birth gap - 68 years - for the generational question. See what the Golden Age label covers for why the twelve eras are not twelve periods.
Pair four: the roles are the comparison
Compare Al-Khwarizmi, about 780 to about 850, with Omar Khayyam, 1048 to 1131. There is no overlap - a 197-year gap - the era distance is 0, and there is exactly one shared city, Samarkand.
Here the useful panel is the roles. Both carry mathematician, scientist and scholar; Khayyam adds poet, and al-Khwarizmi adds nothing the other lacks. That is the comparison in one line: the same mathematical tradition, two and a half centuries apart, with a second career attached at the later end. The atlas's recorded relationship makes the continuity explicit - Khayyam is noted as the later mathematician who extended al-Khwarizmi's algebraic work, and Khayyam's own page names him as the earlier algebraist he built on.
The role panel is the part of the compare page worth reading first when the two figures are not contemporaries, because it is the only one of the metrics that does not depend on dates at all.
- Use lifespan overlap to rule a meeting out, never to rule one in.
- Use birth gap for the generational question - it survives approximate dates better than the overlap does.
- Ignore era distance for chronology; it measures the atlas's filing, not time.
- Read shared cities as which places connect two careers, not as where two people met.
- If the atlas records a direct relationship, that single sentence outranks all four numbers.
Where the comparison runs out
Two limits are worth knowing before you build an argument on a comparison.
First, 62 of the 153 figures carry at least one approximate date, so an overlap can be a year or two out in either direction. For Mansa Musa, both ends are approximate; the 34-year figure is a reasonable estimate, not a measurement. Hijri and Gregorian dates sets out which forms of date the atlas uses.
Second, 14 of the 153 figures are on no city page at all - mostly the Sahelian and Southeast Asian figures, including Nana Asma'u, Usman dan Fodio and Hamka. Comparing two of those returns a shared-cities count of zero that means no data rather than no connection. Who the atlas counts as a figure explains where that gap comes from.
Used with those two caveats, the comparison is at its best as a preparation step: pick two figures, note what the atlas does and does not record about the pair, and then read both profiles knowing what you are looking for.
Questions readers ask
What does lifespan overlap mean on the compare page?
The number of years in which both figures were alive, with the range shown; when they never overlapped, the page shows the gap instead. It establishes that a meeting was possible and nothing more - Mansa Musa and Ibn Battuta overlap by 34 years and never met.
Why are two figures only 14 years apart shown as 8 eras apart?
Because era distance counts positions in the atlas's list of twelve eras, which mixes dynasties, regions and themes rather than running strictly by date. Al-Ghazali and Ibn Rushd are the example: an era distance of 8, a birth gap of 68, and a direct intellectual exchange between them.
Does a shared city mean the two figures met there?
No. Shared cities is the intersection of two lifetime city lists. Mansa Musa and Ibn Battuta share three cities and were in them decades apart. Only a recorded relationship on one of the two pages is evidence of contact.
Can I link to a particular comparison?
Not at the moment - the two figures are chosen in the dropdowns on the compare page rather than in the URL, so a comparison cannot be bookmarked or shared as a link. Both figure pages can be, and they carry the same underlying records.
Which metric is most useful?
The direct relationship, when the atlas records one, because it is the only field written by a human about that specific pair. Of the four numbers, the birth gap is the most robust: it needs only two dates and is unaffected by how long either figure lived.
Sources
- Robert Irwin, Ibn Khaldun: An Intellectual Biography (Princeton University Press, 2018) - cited on the Ibn Khaldun page.
- Beatrice Forbes Manz, The Rise and Rule of Tamerlane (Cambridge University Press, 1989) - cited on the Timur page.
- Ross E. Dunn, The Adventures of Ibn Battuta: A Muslim Traveler of the Fourteenth Century (University of California Press, 2005) - cited on the Ibn Battuta page.
- Nehemia Levtzion, Ancient Ghana and Mali (Methuen, 1973) - cited on the Mansa Musa page.
- Majid Fakhry, Averroes: His Life, Works and Influence (Oneworld, 2001) - cited on the Ibn Rushd page.
Counts in this guide are derived from the atlas data itself, so the prose cannot drift from the corpus. How to read the sources a figure page cites explains how the bibliographies are built and what each kind of work can carry.