9

9/24, or 375 parts per thousand — well under half gold. A legal UK hallmarking standard, and not recognised as gold in some countries. The scale is karat with a K — a proportion of gold in an alloy — and is not the carat with a C used for gemstones, which is a unit of mass equal to 200 milligrams. A one-carat diamond and 24-karat gold have nothing in common but a homophone.

The rest mass of an electron is about 9 × 10⁻³¹ kilograms. Roughly 1/1836 of a proton's mass, which is why almost all of an atom's mass is in its nucleus.

There are about 9 months of human gestation, conventionally counted (humans; about 40 weeks from the last menstrual period).

The conventional acceleration of free fall at the Earth's surface is about 9 metres per second squared (worldwide, defined by the CGPM in 1901). Exactly 9.80665 by definition, and a convention rather than a measurement: real surface gravity varies by about half a per cent between the equator and the poles, and falls with altitude.

There are about 9 digits in the average digit span of Mandarin speakers (Mandarin Chinese; English averages nearer 7). Digit span depends partly on how long the number words take to say, so it varies with the language rather than with the person. Mandarin digit words are short, which is the usual explanation for the difference.

The distance light travels in a year is about 9 trillion kilometres. A light-year is a distance and not a time, which is the most common misunderstanding of the unit.

Comet 9P/Tempel 1 was hit by a 372-kilogram impactor from the Deep Impact mission in July 2005, excavating subsurface material for spectroscopic analysis.

Human pregnancy is conventionally described as nine months. Clinically it is measured as about 40 weeks from the last menstrual period, which is nearer ten lunar months — the nine-month figure is a calendar-month approximation rather than a medical one.

9 is the atomic number of fluorine, the most chemically reactive of all elements.

The VHF marine channel commonly used for hailing to keep channel 16 clear is 9 (North America and elsewhere by local rule).

The whole-number part of standard gravity is 9 (worldwide, CGPM). Standard gravity is exactly 9.80665 metres per second squared, adopted as a convention in 1901 rather than measured anywhere in particular — real surface gravity varies by about half a per cent between the equator and the poles, and falls with altitude.

There are about 9 terawatts of electricity generating capacity installed worldwide. Figure as of 2023. Against roughly 29,000 terawatt-hours actually generated in a year, which is a far smaller number than continuous full output would give — most capacity sits idle most of the time, and wind and solar are unavailable rather than idle.

A baseball team fields nine players and a regulation game is nine innings, which is why 'the ninth' names the last chance to score without needing further explanation to an American audience. Both counts date from the 1857 rules convention that also fixed the ninety-foot basepath; before it, games were played to a fixed number of runs rather than a fixed length.

In the history of imperial China, nine was reserved to the emperor, and the buildings and objects of the imperial court were laid out in nines and multiples of nine. The Forbidden City's door studs are set nine by nine, and the number's association is with the sound of the word for lasting rather than with counting. This is a scholarly account rather than the tradition's own framing.

English idioms including "dressed to the nines" and "cloud nine" use 9 to signify completeness or a high degree — a documented linguistic pattern, not a claim about the number's inherent meaning.

A superstition in Japanese-speaking contexts holds that nine is avoided because one of its readings, ku, is also the word for suffering (Japan). It works the same way as the avoidance of four, and for the same reason — a homophone rather than a doctrine. Hospitals commonly omit both room numbers. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

In Thai-speaking contexts, it is traditional that nine is sought after because its name resembles the word for progressing, and it is common in addresses, number plates and dates chosen for ceremonies. The opposite of the Japanese association with the same digit, which is avoided for sounding like the word for suffering. Both are about the sound of the word rather than the quantity. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

In ancient Greek religion, it is traditional that nine Muses are named as the patrons of the arts and sciences. The canonical nine and their separate domains are late; earlier sources give three, and the assignment of a particular art to a particular Muse is later still. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

Western numerology assigns nine the meaning of completion and endings, reached by summing the digits of a birth date until one digit remains. The digit-summing method dates from the early twentieth century. Often presented as Pythagorean. The historical Pythagoreans did assign qualities to numbers, but the birth-date reduction method is a modern construction and the attribution backdates it by two thousand years. This is a modern popular interpretation, not a historical or doctrinal claim.

Presented as belief or tradition, not as an established fact.

In Old Norse poetry, nine worlds are named as held by the world tree, and nine recurs through the poems — nine nights hanging, nine steps taken, nine days between. The tidy list of nine worlds familiar from modern retellings is largely a later reconstruction; the poems name the number without ever enumerating all nine. This is a scholarly account rather than the tradition's own framing.

In the Bahá'í Faith, nine is used as a symbol of completeness, and houses of worship are built with nine sides. The highest single digit, read as standing for the gathering of all faiths. Every Bahá'í house of worship has nine entrances and a dome, and the nine-pointed star is the most common emblem of the faith. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

There are 9 justices on the Supreme Court of the United States (United States; set by statute, not by the Constitution). The number has been six, seven, nine and ten at different times, and Congress could change it again by ordinary legislation.

The area of the Sahara is about 9 million square kilometres (North Africa). Comparable to the area of the United States or of China.

Every multiple of 9 has digits that themselves sum to a multiple of 9 — 27 gives 9, 144 gives 9, 8,514 gives 18 and then 9. The rule falls out of base 10 being one more than 9, and it underpins 'casting out nines', a checking method for hand arithmetic used for well over a thousand years.