Here are some mathematical facts all about the number 28 (twenty eight).

The number 28 is the number between 27 and 29.

The number 28 is an even number, as the number ends in 8.

Historical Facts

The First World War was triggered by the assassination of Archduke Franz Ferdinand on June 28, 1914

It was formally ended by the Treaty of Versailles which was signed on June 28 1919.

The Three Mile Island incident took place on March 28 1979.

The most common isotope of Silicon has an atomic weight of 28

Title 28 of the U.S. Code deals with the judiciary and judicial procedure

The Space Shuttle Challenger expired on January 28, 1986.

28 is the the first triangular number to be the sum of two cubes 13+33.

The longest known sociable chain (http://thesaurus.maths.org/dictionary/map/word/3405) has 28 links, starting with 12,496.

A license plate on the cover to the Beatles album “Abbey Road” reads “IF28”, which spun the theories that Paul McCartney would die at that age.

The tweny-eighth result of an Altavista search of “twenty-eight” is a page entitled “Twenty-Eight Common Mistakes,” which contains a list of 29 common mistakes in job hunting.

The Moon’s sidereal period is about 28 days whereas its synodic period is 29.5 days so you are partially right.
The sidereal period is the time it takes the Moon to reach the same position in the sky (the background of the stars) or the time it takes the Moon to return to the same celestial longitude.

The 28-letter word anticostituzionalissimamente (‘in very anticostitutional way’) is the longest word in the Italian language.
The synodic period which is much more widely known is the time for the Moon to go through 1 complete phase cycle. For example, the time from one Full Moon to the next averages 29.5 days.

The factor pairs of 28 are:

1 × 28,  2 × 14,  and  4 × 7

Therefore there are 6 divisors of 28, which are 1,2 4, 7, 14 and 28.

The number 28 has two prime factors which are 2nd 7.

The product of prime factors for the number 28 is 2 × 2 × 7.

The number 28 is also a perfect number since it can be written as the sum of its divisors excluding itself (1 + 2 + 4 + 7 + 14)

The first ten multiples of 28 are 28, 56, 84, 112, 140, 168, 196, 224,252 and 280. All of these multiples are even numbers.

If you double 28 you get 56 and half of 28 is 14.

28 squared is 784 (since 28 multiplied by 28 is 784).

Also, the cube of 28 is 21952 (28 × 28 × 28)

The square root of 28 is 5.29 rounded to 3 significant figures.

28% written as a decimal is 0.28 and 7/25 as a fraction.

28 miles is about the same length as 44.8 kilometres (28 × 1.6).

28 gallons is about the same volume as 126 litres. (28 × 4.5).

28 pounds is a about the same weight as 13 kilograms (28 ÷ 2.2).

A 2D shape made up of 28 sides is called a icosikaioctagon.

Other facts about the number 28

In Chemistry, 28 is the atomic number of nickel.

In Bingo, ball number 28 is called overweight.

In Roman numerals 28 is written as XXVIII.

In the Swedish alphabet there are 28 characters.

Saturn takes approximately 28 years to orbit the sun.