28 is, like all natural numbers1, interesting. Let’s look at a few facts about it.
- Mathematically speaking, 28 is a “perfect number” – one which is equal to the sum of its proper divisors (i.e. 1 + 2 + 4 + 7 + 14 = 28). It is the second perfect number, the first being 6 and the next being 496.
- It’s a triangular number, as seen in the header image – it is the sum of the first seven natural numbers.
- Additionally, it is the sum of the first five primes (2 + 3 + 5 + 7 + 11) and the sum of the first five non-primes (1 + 4 + 6 + 8 + 9). No other number is thought to have all three of these properties (triangularity, sum of first n primes, sum of first m non-primes).
- It’s also a hexagonal number, just to show off.
- It’s the atomic number of Nickel.
- It’s the gravity, in Gs, on the surface of the sun.
- It’s the number of days in a lunar cycle.
- It’s also the length of the shortest month in the Gregorian calendar in days.
- It’s the length of every month in the never-used International Fixed Calendar
- It’s the number of Earth years in an astrological orbit of Saturn (though the actual astronomical figure is 29.46 years).
- It’s the number of dominoes in a standard domino set.
- It’s the age I turned today.