Why does Ramadan move through the seasons?

Ramadan moves because the Islamic calendar follows only the Moon. Its year is twelve lunar months, 354 or 355 days, which is 11 to 12 days shorter than the solar year that sets our seasons. And unlike many other calendars, it never adds a catch-up month. So each year Ramadan begins roughly 11 days earlier by the Gregorian calendar, and over about 33 years it slides all the way through the seasons.

That has very concrete effects. Fasting runs from dawn to sunset, and the length of that day depends on place and season: a Ramadan in midsummer means far longer fasting days than one in winter.

Each month traditionally begins when the first thin crescent of the new moon is sighted. Astronomers can predict it, but many Muslims prefer to see it with their own eyes, which is why Ramadan can start a day apart in different countries.

Other traditions made a different choice. The Hebrew calendar is also built on lunar months, but it adds a thirteenth month in 7 years out of every 19, a second month of Adar, so Passover stays in spring. Chinese, Hindu and Buddhist calendars do something similar. The Islamic calendar's refusal to correct for the Sun is deliberate: Muslims believe a Quranic verse forbade an older practice called Nasī', which adjusted the months.