Ephemeris time facts for kids

The term ephemeris time (often shortened to ET) describes a special way of measuring time. Astronomers use it to track how planets, moons, and stars move in space.

Historically, ET was a standard time scale adopted in 1952. Scientists created it because Earth's rotation, used for timekeeping, wasn't perfectly steady. They needed a more uniform, predictable time. This new time was based on the steady movements of celestial bodies, like Earth orbiting the Sun. It was a big step in defining time using physics.

Today, the idea of ephemeris time continues in modern, very precise time scales. These are used by places like the Jet Propulsion Laboratory (JPL) for tracking spacecraft. These modern versions are even more accurate. They also consider the complex rules of relativity.

The original 1952 standard for ephemeris time left an important legacy. Its unit, the ephemeris second, became the basis for the modern second we use today in our clocks!

Understanding Ephemeris Time

Why We Needed a New Way to Tell Time

For a long time, people thought Earth spun at a perfectly steady rate. This daily spin measured our time. But by the late 1800s, astronomers found Earth's rotation wasn't always smooth. It had tiny, irregular changes, and was even slowing down a little over long periods.

Imagine timing a race with a clock that speeds up or slows down! This wasn't good for precise astronomy, where tiny differences matter.

The Idea of a "Uniform" Time

Because of these issues, scientists like Willem de Sitter and Gerald Maurice Clemence realized a "uniform" time was needed. This new time wouldn't depend on Earth's wobbly spin. Instead, it would use the predictable, steady movements of planets and moons, following gravity's laws.

In 1948, American astronomer G. M. Clemence proposed this new time scale. He suggested it was "for the convenience of astronomers and other scientists only." Daily life would still use time based on Earth's rotation.

Ephemeris Time is Born

In 1950, an astronomy conference recommended this new time scale. Its unit would be based on the length of a year in 1900. They named it ephemeris time, a suggestion from Dirk Brouwer.

The International Astronomical Union (IAU) officially approved ephemeris time in 1952. ET became the main time scale for many astronomical calculations until the 1970s.

Over the years, the "ephemeris second" (ET's unit) was fine-tuned. It helped define the modern second we use today, based on atomic clocks.

Defining Ephemeris Time (1952)

Ephemeris time was defined using Earth's orbit around the Sun. Scientists used old, detailed tables from Simon Newcomb (1895). These tables predicted the Sun's position.

They noticed the Sun's actual observed positions didn't quite match Newcomb's predictions. This was when using time based on Earth's slightly wobbly rotation.

So, they defined a *new* time: Ephemeris Time. This new time made Newcomb's original formulas perfectly match the observed Sun's position. This created a very steady and predictable way to measure time.

The unit of this new time was the "ephemeris second." By comparing Ephemeris Time with Universal Time (UT), scientists could see the difference. This difference, called ΔT, showed how much Earth's rotation varied.

Measuring Ephemeris Time with the Moon

Ephemeris time was defined by Earth's orbit around the Sun. But it was often measured by observing the Moon's orbit. The Moon moves much faster across the sky than the Sun. This made it easier to get more accurate time measurements, like using a faster clock hand.

Atomic Clocks and the Future of Time

A few years after ephemeris time, the atomic clock appeared. These clocks are incredibly accurate. They use atom vibrations (like caesium) to keep time.

In 1958, scientists compared atomic clocks with ephemeris time. They found the "ephemeris second" almost exactly equaled a specific number of cesium atom vibrations. This was a huge discovery!

Atomic clocks offered a more convenient and uniform way to keep time. They ran continuously and precisely. These clocks led to International Atomic Time (TAI) and Terrestrial Time (TT), the super-accurate time scales we use today. They were designed to be very similar to ephemeris time.

The amazing accuracy of atomic clocks, plus better observations, meant ephemeris time was eventually replaced. It was a great step, but new scales were more refined.

Newer, More Precise Time Scales

In 1976, the International Astronomical Union (IAU) decided ephemeris time's definition wasn't perfect. It didn't fully account for relativity effects. These are tiny but important for extreme precision.

So, starting in 1984, ephemeris time was replaced. Two new, more advanced time scales took its place: Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB). These were designed to be more accurate. They included relativistic effects.

Later, in the 1990s, these were further refined. Today we use time scales like Terrestrial Time (TT), Geocentric Coordinate Time (TCG), and Barycentric Coordinate Time (TCB). Ephemeris time was an important early version of these modern, super-accurate systems.

Time for Space Missions

The Jet Propulsion Laboratory (JPL), famous for space missions, developed very precise calculations. These predict the positions of the Sun, Moon, and planets. These calculations are called "ephemerides."

Since the 1960s, JPL has used its own special time scale called T_eph (pronounced "T-eff"). This is for its calculations. T_eph is a relativistic time scale. It considers how gravity and speed affect time, as Albert Einstein described.

T_eph is very closely related to modern time scales from the IAU, like TCB. For practical uses, especially for clocks near Earth, T_eph is an excellent approximation of Terrestrial Time. JPL's ephemerides are widely used, making T_eph important in astronomy and space exploration.

In 2006, the IAU recognized T_eph's importance. They stated it is practically the same as the re-defined TDB. This means T_eph is a very precise continuation of the original ephemeris time concept.

Ephemeris Time in Astronomy Books

From 1960 to 1983, official astronomy books used ephemeris time. These included the "Astronomical Ephemeris" (UK) and the "American Ephemeris and Nautical Almanac." They used ET for their main predictions of celestial body positions. Before that, they used Universal Time (UT).

However, the "Nautical Almanac," a separate book for sailors, kept using UT. This was more practical for navigation based on Earth's rotation.

After 1983, these official almanacs switched. They began using the even more accurate ephemerides developed by the Jet Propulsion Laboratory (JPL).

How the Ephemeris Second Became Our Modern Second

One of ephemeris time's lasting impacts is how it helped define the second we use daily.

In 1956, the "ephemeris second" was officially defined. It was based on a specific fraction of the tropical year in 1900.

When caesium atomic clocks became available in 1955, they showed Earth's rotation was irregular. This proved the old "mean solar second" wasn't precise enough for science.

After careful comparisons, scientists like William Markowitz in 1958 found one ephemeris second equaled 9,192,631,770 cycles of radiation from a cesium-133 atom.

This discovery was so important. In 1967/68, the General Conference on Weights and Measures (CGPM) officially redefined the SI second. This is the standard second used worldwide. The new definition was based on this atomic measurement:

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

This new definition uses the exact number of cesium cycles scientists measured. These equaled the ephemeris second. So, ephemeris time directly influenced the length of our standard second today!

The difference between ephemeris time (ET) and Universal Time (UT) is called ΔT. This difference changes over time, showing how much Earth's rotation varies.

Modern time scales like Terrestrial Time (TT) and International Atomic Time (TAI) are very stable. They have a fixed relationship. TT is always 32.184 seconds ahead of TAI. This ensures a smooth connection to ephemeris time's legacy.