1531
1531 is the year Halley's Comet appeared and was later used to establish its period.
- Odd
- Prime
Where 1531 fits
Why 1531 is special
Halley compared records of 1531, 1607 and 1682 and recognised one returning object. Historical observation, not new measurement, is what made the prediction possible.
Same number, different meanings
How 1531 is understood across different contexts — never implying one meaning because of another.
- Mathematics
Halley compared records of 1531, 1607 and 1682 and recognised one returning object. Historical observation, not new measurement, is what made the prediction possible.
- History
In 1531, an earthquake devastated Lisbon, killing thousands (Portugal).
- Science
Halley identified his comet by comparing three separate historical sightings rather than by observing anything new. The method — mining existing records for a pattern — is as important as the result.
1531 is part of
Factors & digit properties
- Divisors (2)
- 1, 1531
- Prime factorisation
- 1531
- Digit sum
- 10
- Digital root
- 1
Representations
- Decimal
- 1531
- Scientific notation
- 1.531 × 10^3
Roman numerals
M D X X X I
Binary
1 0 1 1 1 1 1 1 0 1 1
1 × 1,024 + 0 × 512 + 1 × 256 + 1 × 128 + 1 × 64 + 1 × 32 + 1 × 16 + 1 × 8 + 0 × 4 + 1 × 2 + 1 × 1 = 1,531
Octal
2 7 7 3
2 × 512 + 7 × 64 + 7 × 8 + 3 × 1 = 1,531
Hexadecimal
5 f b
5 × 256 + f × 16 + b × 1 = 1,531
Read as powers of two: 1,024 + 256 + 128 + 64 + 32 + 16 + 8 + 2 + 1 = 1,531.
In historical numeral systems
Egyptian
1 lotus, 5 coils of rope, 3 hobbles, 1 stroke
1 × 1,000 + 5 × 100 + 3 × 10 + 1 × 1 = 1,531. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
2 corner wedges + 5 vertical wedges (sixties place), 3 corner wedges + 1 vertical wedge (units place)
25 × 60 + 31 = 1531. Places are separated by a space on the tablet, with the higher place written first.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
Other mathematical patterns
- Deficient number
Related numbers & connections
1531 is a year in the 16th century, in the 1530s.
Halley identified his comet by matching records from 1531, 1607 and 1682 as one returning object rather than three separate ones.
View 1682 →Numbers Around Us
- appears inHistorical years
why?
Strength 0.50, confidence "high".
Other connections
- developed from1682
why?
Strength 0.50, confidence "high".
History
In 1531, an earthquake devastated Lisbon, killing thousands (Portugal).
DocumentedHistoryGeography: PortugalPeriod: 1531
Science
Halley identified his comet by comparing three separate historical sightings rather than by observing anything new. The method — mining existing records for a pattern — is as important as the result.
DocumentedSciencePeriod: 1531
Common questions
Is 1531 a prime number?
Yes, 1531 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 1531?
The factors of 1531 are 1, 1531.
What is 1531 in Roman numerals?
1531 in Roman numerals is MDXXXI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 2 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026