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DAYLIGHT SAVING TIME

Why do we have Daylight Saving Time?

By moving our clocks forward one hour between spring and fall, we more effectively utilize the light of the sun to keep homes and businesses lit, saving electricity.

When did Daylight Saving Time move from the end to the beginning of April in the United States?

The shift of the beginning of Daylight Saving Time from the last Sunday in April to the first Sunday in April took place in 1987, when the Uniform Time Act was amended. In 2007, Daylight Saving Time was moved again to the second Sunday in March and the first Sunday in November.

When do countries in the Southern Hemisphere observe Daylight Saving Time?

Because Daylight Saving Time is an effort to save daylight during the summer months, Daylight Saving Time in the Southern Hemisphere occurs from October through March.

KEEPING TIME

What is a sundial?

A sundial is an instrument that uses the sun to measure time. A sundial consists of an angled marker, called a gnomon, that casts a shadow on a plate, called a dial plane. On the dial plane there are marks indicating the hours of the day. During the day, as the sun moves across the sky, the shadow from the gnomon moves across the dial plane, indicating the hour. Sundials were used to measure time before clocks and watches were invented.

When were the first clocks made?

Around 3500 b.c.e. clocks began to appear in a variety of cultures, most notably in Sumeria in what is now the Middle East.

What is a water clock?

Water clocks were the first clocks that didn't rely upon sunlight (as with sundials) to tell time. They operated by dripping water from containers at measured intervals. There were two key types of water clocks: those that measured time by the amount of water remaining in the clock and those that measured time by how much water dripped from the clock and filled a measuring device.

When was the first watch made?

In the early sixteenth century, German locksmith Peter Kenlein began to produce portable clocks called Nurenberg eggs. Advances in later centuries led to timepieces that could be worn on the wrist.

What is an atomic clock?

An atomic clock uses measurements of energy released from atoms to precisely measure time. The current model of the atomic clock, created in 1957 by Norman Ramsey, uses measurements from cesium atoms. Atomic clocks are used by NASA, physicists, astronomers, and other scientists who need extremely precise time.

CALENDARS

What do b.c. and a.d. stand for?

In our modern calendar, the year 0 represents the year of the birth of Jesus Christ. Years before his birth are known as B.C., or "before Christ." Years since his birth are known as a.d. or "Anno Domini," the "Year of Our Lord."

One of the first instruments to measure time was the sundial.

One of the first instruments to measure time was the sundial.

What was the longest year in history?

The year 46 b.c.e. was decreed by the Roman Emperor Julius Caesar to be 445 days long in order to correct the calendar, which was 80 days off, based on the seasons.

What do b.c.e. and c.e. stand for?

To secularize the calendar, the terms B.C.E. and C.E. have come into use to replace B.C. and A.D. respectively. B.C.E. means "before common era" and C.E. stands for "common era."

What is the problem with a calendar based on the cycles of the moon?

The time between two new moons is 29 and one half days. After 12 lunar months, a calendar based on the cycles of the moon falls short of a solar year—and thus the cycle of seasons—by 11.25 days. To compensate, the Hebrew Calendar, which is based on the moon cycle, has a regulated 19-year cycle in which an extra month is added every two or three years.

How did Julius Caesar fix the calendar?

For years, the Romans had been using a calendar based on lunar cycles. Since each lunar month is 29.5 days, 12 months only adds up to 354 days. But seasons do not follow a lunar cycle, they follow a solar one. A solar year lasts 365 days, 5 hours, 49 minutes. Julius Caesar implemented a solar calendar so that the seasons would occur at the same times every year. Additionally, Caesar made each year 365 days long, with every fourth year being 366 days long (a "leap" year). Unfortunately, each calendar year was still 11 minutes longer than a solar year, a problem that Caesar did not feel was a big concern at the time.

 
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