Time standard

We use clocks to tell time. Clocks help us know the day.

Long ago, people used the Earth to tell time. They watched the Earth spin. This helped them know the day.

But the Earth does not spin at the same speed. It can slow down or change. This makes it hard to be exact.

Now we use very special machines. These are called atomic clocks. They are much better than old clocks.

These new clocks use tiny parts of an atom. They help us stay on time everywhere. It is a big discovery!

How do we measure time? A time standard is a set of rules for measuring it. It tells us how fast time passes. It can also tell us specific points in time.

Long ago, people used the Earth to tell time. They watched how fast the Earth spins. Most people thought the Earth spun at the same speed every day. But scientists found that the Earth's spin changes. It can slow down or act in strange ways. This makes it hard to use the Earth as a perfect clock.

To fix this, we use new tools. In 1955, people made the caesium atomic clock. An atomic clock is a very precise machine. It uses the tiny parts of an atom to count time. It is much more steady than the Earth's spin.

Today, we use these clocks for many things. They help run the Global Positioning System, or GPS. This system helps us find our way using satellites. We also use them to keep a standard called UTC. This helps everyone in the world stay on the same schedule.

A time standard is a set of rules for measuring time. It tells us how fast time passes and marks specific moments. We use these rules to stay on the same schedule. In the past, people used custom and practice to track time. Now, we use official specifications to keep things accurate. We can measure time using a clock. This clock counts changes in a natural event or a machine. Some standards show us the time of day. Others show us how long a period of time lasts.

Time works in different ways depending on what we need. An instant is just one single point on a timeline. A time interval is the space between two instants. You can measure the duration of that interval in minutes or hours. Scientists also use chronology to study the order of past events. This helps us understand history. One big system is the geologic time scale. It organizes the many events that shaped the Earth and its life.

For a long time, people looked to the Earth for time. They used the Earth's rotation to set their clocks. In the 1800s, people thought the Earth spun at a constant rate. However, astronomers noticed something strange. They studied eclipses and other observations. They found that the Earth's spin is actually slowing down. It also has small irregularities that make it an unsteady clock. Because of this, scientists started using the Earth's orbit around the Sun. This is called ephemeris time.

New technology changed everything in the middle of the last century. In 1955, scientists invented the caesium atomic clock. This was a huge step for accuracy. In 1967, the SI second became the official base unit for time. This second is defined by the caesium-133 atom. It counts exactly 9,192,631,770 periods of radiation from that atom. Today, atomic clocks are used for many important tasks. They help create International Atomic Time, which is known as TAI.

These precise standards connect to the tools you use every day. The Global Positioning System, or GPS, relies on very steady time. GPS time is kept separate but stays in sync with UTC. UTC is a standard that stays very close to the Earth's rotation. To keep them close, scientists sometimes add a "leap second." This keeps our clocks from drifting too far from the Sun. Even your local time zone uses these rules. It uses a fixed number of hours to stay near Universal Time.

A time standard is a formal specification used to measure the rate at which time passes. It can also define specific points in time, known as instants. These standards are essential because they provide a universal way to coordinate activities across the world. Without them, we would rely on local customs rather than precise, shared measurements. Modern time standards are officially recognized to ensure accuracy in science and technology.

Measuring time involves counting the changes in a specific phenomenon. This could be a natural event, like the Earth's rotation, or an artificial machine, like a clock. To understand time, we must distinguish between different concepts. An instant is a single point on a time axis and has no value on its own. A date is a mark attributed to an instant using a specific time scale, such as a formal ISO format. A time interval is the space between two instants. Finally, duration is the actual quantity of time within an interval, such as a number of minutes.

Historically, time standards were based on the Earth's rotation. This is often called solar time. Apparent solar time is based on the solar day, which is the period between one solar noon and the next. However, the Earth's orbit is elliptical and its axis is tilted. These factors cause the apparent solar day to vary by a few dozen seconds. This creates differences of up to 16 minutes between apparent solar time and mean solar time. Astronomers also use sidereal time, which is measured by the stars. A sidereal day is about 3 minutes and 56 seconds shorter than a mean solar day.

In the 19th century, astronomers began to doubt that the Earth's rotation was constant. By studying eclipse records and other observations, they found evidence of irregularities. They discovered that the Earth's rotation is actually slowing down over time. Because of this instability, scientists sought more reliable methods. In 1952, they began using ephemeris time for astronomical work. This standard is based on the Earth's orbital period and the motion of the Moon. In 1960, the ephemeris second was officially adopted as part of the International System of Units.

The invention of the caesium atomic clock in 1955 revolutionized timekeeping. In 1967, the SI second was defined using atomic properties. The SI second is the duration of exactly 9,192,631,770 periods of radiation. This radiation comes from the transition between two hyperfine levels of the caesium-133 atom. This definition is extremely precise and serves as the basis for all atomic timescales. These include International Atomic Time (TAI), which is produced by the International Bureau of Weights and Measures (BIPM). TAI uses the combined input of many atomic clocks around the world.

Today, several different time scales work together to keep our world synchronized. Coordinated Universal Time (UTC) is an atomic time scale designed to approximate Universal Time (UT1). Because Earth's rotation is irregular, UTC must be adjusted to stay close to UT1. Scientists do this by adding leap seconds. These one-second steps keep UTC within 0.9 seconds of UT1. The Global Positioning System (GPS) also uses a very precise time signal. GPS time (GPST) is maintained independently but is synchronized with UTC. It is defined with a constant offset of 19 seconds from TAI.

These complex systems connect to the civil time we use every day. Standard time in a time zone is usually a fixed number of hours away from UTC. This offset ensures that a new day begins when the Sun is near the nadir meridian. Some regions also use Daylight saving time, where the difference changes by one hour twice a year. For astronomers, the Julian day number is a useful tool. It counts the days elapsed since noon on January 1, 4713 B.C. This prevents the confusion of date skips during a single night of observation.

Time standards also exist for different physical perspectives in space. Geocentric Coordinate Time (TCG) uses the center of the Earth's mass as its origin. Barycentric Coordinate Time (TCB) uses the center of mass of the entire Solar System, called the barycenter. These coordinate times are necessary for high-level physics and space navigation. They allow scientists to calculate time accurately even when accounting for gravity and speed. By using these various scales, we can navigate the Earth and the solar system with incredible precision.