28

There are about 28 grams in an ounce (the international avoirdupois ounce). Exactly 28.349523125 grams, because the pound was defined as exactly 0.45359237 kilograms in 1959. A troy ounce, used for precious metals, is a different unit at about 31.1 grams.

The orbital speed of the International Space Station is about 28 thousand kilometres per hour (low Earth orbit). Fast enough to go round the Earth every ninety minutes, which is why the crew see sixteen sunrises a day.

The Moon's orbital (synodic) period is approximately 29.5 days; 28 is a commonly cited rounded approximation of a lunar month, and February's 28-day (29 in leap years) length is often loosely associated with it.

There are 28 tiles in a standard set of dominoes (worldwide, the double-six set). Every unordered pair of values from 0 to 6, including doubles — which is the seventh triangular number, and the same arithmetic gives 55 tiles for a double-nine set and 91 for double-twelve.

28 days is exactly four weeks, which makes February in a common year the only month whose length is a whole number of weeks. One practical consequence: every date in that February falls on the same weekday as the date seven, fourteen and twenty-one days before it, so the 1st, 8th, 15th and 22nd all share a weekday. The same arithmetic is why 28-day cycles are convenient for medication packs and pay periods.

The Roman calendar originally ran ten months from March, leaving winter uncounted. The traditional account credits Numa Pompilius with adding January and February, and with preferring odd-numbered month lengths — leaving one month to absorb the remainder. February, then the last month of the year, took it and has kept 28 days since. The attribution to Numa is traditional rather than documented contemporaneously.

28 is the second perfect number: its proper divisors (1, 2, 4, 7 and 14) sum to exactly 28. Perfect numbers are rare — 6 is the first, and only a few dozen are known at all.