<P> The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the year in most calendars, but the astronomer's value changes over time, so William Herschel's suggested correction to the Gregorian calendar may become unnecessary by the year 4000 . </P> <P> The calendar year can be divided into four quarters, often abbreviated as Q1, Q2, Q3, and Q4 . </P> <Ul> <Li> First quarter, Q1: 1 January--31 March (90 days or 91 days in leap years) </Li> <Li> Second quarter, Q2: 1 April--30 June (91 days) </Li> <Li> Third quarter, Q3: 1 July--30 September (92 days) </Li> <Li> Fourth quarter, Q4: 1 October--31 December (92 days) </Li> </Ul> <Li> First quarter, Q1: 1 January--31 March (90 days or 91 days in leap years) </Li>

What are the four quarters of the year