28 (number)

← 27 28 29 →
← 20 21 22 23 24 25 26 27 28 29 → List of numbers Integers ← 0 10 20 30 40 50 60 70 80 90 →
Cardinal	twenty-eight
Ordinal	th
Factorization	$2^{2}\cdot 7$
Divisors	1, 2, 4, 7, 14, 28
Greek numeral	ΚΗ´
Roman numeral	XXVIII
Binary	111002
Ternary	10013
Senary	446
Octal	348
Duodecimal	2412
Hexadecimal	1C16

28 (twenty-eight) is the natural number following 27 and preceding 29.

In mathematics

It is a composite number, its proper divisors being 1, 2, 4, 7, and 14.

Twenty-eight is the second perfect number. As a perfect number, it is related to the Mersenne prime 7, since 2²(2³ - 1) = 28. The next perfect number is 496, the previous being 6.

Twenty-eight is not the aliquot sum of any number other than itself and is therefore not a component in an aliquot sequence.

Since the greatest prime factor of 28² + 1 = 785 is 157, which is more than 28 twice, 28 is a Størmer number.

Twenty-eight is a harmonic divisor number, a happy number, a triangular number, a hexagonal number,and a centered nonagonal number. Twenty-eight is the sum of the first five prime numbers, and the sum of the totient function for the first nine integers.

It appears in the Padovan sequence, preceded by the terms 12, 16, 21 (it is the sum of the first two of these).

It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 2, 8, 10, 18, 28...

Twenty-eight is the third positive integer with a prime factorization of the form $2^{2}q$ where q is an odd prime.

Twenty-eight is the ninth and last number in early Indian magic square of order 3.

There are twenty-eight convex uniform honeycombs.

In science

• The atomic number of nickel
• The fourth magic number in physics

Astronomy

• The lunar month is around twenty-eight days or so.

• Messier object M28, a magnitude 8.5 globular cluster in the constellation Sagittarius

• The New General Catalogue object NGC 28, an elliptical galaxy in the constellation Phoenix

• The Saros number of the solar eclipse series which began on -1910 March 22 and ended on -630 April 28. The duration of Saros series 28 was 1280.1 years, and it contained 72 solar eclipses.

• The Saros number of the lunar eclipse series which began on -1915 June 27 and ended on -581 September 6. The duration of Saros series 28 was 1334.2 years, and it contained 75 lunar eclipses.

• The planet Saturn revolves around the Sun in approximately 28 years. This is acknowledged in Astrology and in cultures or religions that base partly on it (see below, other fields)

In other fields

Twenty-eight is:

• An abbreviation for such years as 1928 and 2028.
• In Gematriya, the system of Hebrew Numerology, the number 28 corresponds to the word koakh, meaning "power," "energy."
• The number of days in the shortest month of the Gregorian calendar, February (except in leap years, when there are twenty-nine). All twelve months of the Gregorian calendar have at least 28 days, regardless of the year.
• The Gregorian Calendar follows a 28-year cycle, since there are seven days in a week and leap year occurs every four years; a calendar from 28 years ago is the same as this year's (i.e. 1980 = 2008). However, this is only true if there have been exactly seven leap days in the time between the two years. As such, 1900 (as well as 2100, 2200, etc.) is not the same as 1872 for the simple reason that 1900 is not a leap year.
• The common name for the parrot 'Barnardius zonarius semitorquatus', widely distributed in Western Australia and South Australia. Its call sounds like "wenniate".
• The number of letters in the Danish and Swedish alphabets (not counting W), and also in the Arabic and Esperanto alphabets.
• In neo-Nazi circles, twenty-eight indicates Blood and Honour (28 = BH - B - second letter of the alphabet and H - the eight letter).
• The number of Chinese constellations, "Xiu" or "mansions" (a literal translation), equivalent to the 12 western zodiac constellations.
• The number of normal human teeth, not including the third molars (wisdom teeth).
• The number of dominoes in standard domino sets.
• Deriving from the 28 year cycle of Saturn's revolution around the Sun, the 28-year cycle as well as its subdivisions by 14 and 7 are supposed in Astrology to mark significant turning points or sections in the course of a persons development in life. Thus, the number 28 has special significance in the culture of religious sects such as the Kadiri and the Mevlevi dervishes. The 28-beat metric pattern often used in the music compositions accompanying the main part of the Mevlevi sema ritual is called the "Devri kebir", meaning the "Big Circle" and is a reference to above astronomical facts about the year and the Saturn year.
• In Québec, François Pérusse, in one of his best-selling Album du peuple made a parody of Wheel of fortune in which all of the letters picked by the contestant were present 28 times. As a result, 28 became an almost Mythical number used by many Québec youths, the phrase "Y'en a 28" (There are 28 [Letters]) became a running gag still used and recognized more than 15 years later.
• The Preludes, Opus 28 consists of Frédéric Chopin's 24 preludes for piano, ordinarily but not necessarily played together in concert.
• The postal code of the province of Madrid, in Spain.
• Twenty Eight is a popular game played in Kerala India.
• The number of days in 28 days later.
• The number of weeks in 28 weeks later.
• The number of the French department Eure-et-Loir

Historical years

28 A.D., 28 B.C., 1928, 2028, etc.

References

• Prime Curios! 28 from the Prime Pages