Calendars and Clocks
Solar and lunar calendars, leap years, time zones and the time the Sun tells.
- The Solar Calendar and the Tropical YearA solar calendar keeps its dates in step with the seasons.
- Lunar CalendarsA lunar calendar counts months from one cycle of Moon phases to the next.
- Leap YearsYears divisible by 4 are leap years, except century years that are not divisible by 400.
- Time ZonesEarth turns 15 degrees of longitude every hour, and time zones were built on that.
- The International Date LineThe International Date Line is the imaginary line where one calendar day ends and the next begins.
- Solar NoonSolar noon is the moment the Sun crosses the north-south line above your head and reaches its highest point of the day.
- The Sidereal Day and the Solar DayA sidereal day is one turn of Earth measured against the stars, about 23 hours 56 minutes 4 seconds.
- Two Lengths of a Lunar MonthThe sidereal month, about 27.32 days, is the Moon's return to the same star.
- The Equation of TimeThe equation of time is the difference between sundial time and clock time, and it reaches as much as about 16 minutes.