Time and Calendars

A day is roughly twenty-four hours because that is how long Earth takes to rotate once.

A year lasts about 365 days because Earth needs that long to orbit the Sun once.

Leap years add an extra day in February to keep calendars aligned with Earth's orbit.

A leap year occurs nearly every four years, except at the turn of most centuries.

February is the shortest month, with 28 days in common years and 29 in leap years.

The months of September through December take their names from Latin words for seven through ten.

January is named after Janus, the Roman god of doorways and beginnings.

March was once the first month of the Roman year before January took that position.

July and August were renamed to honor Roman leaders long after the calendar was reformed.

Every month except February contains either 30 or 31 days in the modern calendar.

A week has seven days, a pattern inherited from ancient Babylonian and Jewish traditions.

The seven-day week is the only calendar cycle not tied to an astronomical event.

Time zones exist because the Sun reaches its highest point at different moments around the world.

Earth is divided into roughly 24 time zones, each one about 15 degrees of longitude wide.

Traveling east across a time zone means setting your watch forward by about one hour.

Coordinated Universal Time serves as the reference point from which all time zones are measured.

The prime meridian runs through Greenwich, England, and marks zero degrees of longitude.

Some regions use offsets of 30 or 45 minutes instead of whole hours from the reference time.

The International Date Line sits near the middle of the Pacific Ocean and shifts the calendar by a day.

Crossing the International Date Line westward moves you forward one calendar day.

Daylight saving time shifts clocks forward in spring and back again in autumn in many countries.

Not every country observes daylight saving time, and some regions near the equator never change clocks.

Sundials tell time by tracking the shadow cast by the Sun across a marked surface.

A sundial's shadow points roughly north at local noon in the Northern Hemisphere.

Hourglasses measure time by letting fine sand fall through a narrow opening at a steady rate.

Water clocks were among the earliest timekeepers, using a steady drip to mark passing hours.

Candle clocks marked time by burning evenly, with markings showing how much had melted.

The pendulum clock improved accuracy by using a swinging weight to regulate each tick.

Pendulums swing at a rate determined mostly by their length rather than by their weight.

Quartz clocks keep time by counting the vibrations of an electrically stimulated quartz crystal.

Atomic clocks measure time by counting the natural vibrations of atoms, making them extraordinarily precise.

Navigation satellites depend on extremely accurate onboard clocks to calculate positions on Earth's surface.

A second was once defined as a fraction of a day, but atoms now define it.

There are 60 seconds in a minute and 60 minutes in an hour.

Babylonian mathematicians favored the number sixty, which is why minutes and hours use it.

Ancient Egyptians divided daylight into twelve parts, which helped shape the twelve-hour clock.

Mechanical clocks first appeared in medieval Europe, often mounted in towers for public use.

Clock faces usually move clockwise because that is the direction a sundial's shadow travels in the north.

Early mechanical clocks had no faces and simply struck bells to announce each hour.

Wristwatches became common only in the twentieth century, replacing pocket watches for most people.

Digital clocks display numbers directly instead of using hands that rotate around a dial.

Earth's rotation is gradually slowing, so occasional leap seconds keep clocks matched to the planet.

Most countries use the Gregorian calendar for civil dates and official business.

The Gregorian calendar was introduced in 1582 to correct drifting dates in the older Julian calendar.

The Julian calendar added a leap day every four years, which slowly drifted against the seasons.

When a country switched calendars, people sometimes skipped several days to catch up with the seasons.

A leap day falls on February 29, giving that date birthdays only once every four years.

The Moon's cycle of roughly 29 days gave rise to the length of a month.

Many cultures began the new year in spring rather than in the middle of winter.

Chinese, Hebrew, and Islamic calendars follow lunar cycles and differ in length from the solar year.

A sidereal day is slightly shorter than 24 hours because Earth also moves along its orbit.

Standard time replaced thousands of local times, making railway schedules possible in the nineteenth century.