Why Do We Use a Mean Sun That Moves Along the Equator Instead of the Ecliptic?

In summary: The second was defined in terms of the Earth's orbit for a brief period of time, from 1960 to 1967. While a system based on the Earth's orbit is better than a system based on the Earth's daily rotation, there are still problems with such a system.Eventually, in 1967, the International Astronomical Union (IAU) introduced the mean sun system, which defined time in terms of the mean sun's position in the sky. This system eliminated the need for a rotating Earth and various nutations, leading to a more accurate timekeeping mechanism.
  • #1
JeffOCA
49
0
Hi,

Due to Earth's axial tilt, the Sun's annual motion is along the ecliptic that is tilted to Earth's celestial equator.
When the Sun crosses the equator at both equinoxes or solstices, the Sun's daily shift is at an angle to the equator, so we have to do the projection of this shift onto the equator.

We like our clocks to run at a constant rate, so we cannot set them to follow the actual sun—instead they will follow a nonexistent object called the "mean sun" that moves along the celestial equator at a constant rate that matches the real sun's average rate over the year.

Question is : why introducing a mean sun moving along the equator instead of a mean sun moving along the ecliptic ? Moving along the ecliptic needs to do a projection.
With a mean sun moving along the ecliptic, no need for projection, only one effect remains in the computation of the equation of time : Earth's elliptical orbit.

In this way, it would be easier, no ?

Thanks
 
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  • #3
JeffOCA said:
We like our clocks to run at a constant rate, so we cannot set them to follow the actual sun—instead they will follow a nonexistent object called the "mean sun" that moves along the celestial equator at a constant rate that matches the real sun's average rate over the year.

Question is : why introducing a mean sun moving along the equator instead of a mean sun moving along the ecliptic ? Moving along the ecliptic needs to do a projection.
With a mean sun moving along the ecliptic, no need for projection, only one effect remains in the computation of the equation of time : Earth's elliptical orbit.

In this way, it would be easier, no ?
The second was defined in terms of the Earth's orbit for a brief period of time, from 1960 to 1967. While a system based on the Earth's orbit is better than a system based on the Earth's daily rotation, there are still problems with such a system.

Far easier, and far more accurate, is to use atomic clocks. Atomic clocks have been the mechanism used to define time since 1967.
 
  • #4
D H said:
The second was defined in terms of the Earth's orbit for a brief period of time, from 1960 to 1967. While a system based on the Earth's orbit is better than a system based on the Earth's daily rotation, there are still problems with such a system.

You mean that mean sun was (by definition) moving along the ecliptic from 1960 to 1967 : i didn't know, thanks !
So, why we have changed the definition to make a mean sun moving along equator in 1967 ?

Thanks
 
  • #5
JeffOCA said:
You mean that mean sun was (by definition) moving along the ecliptic from 1960 to 1967
I'm trying to get a mental handle on what this mean sun moving along the ecliptic could even mean. I can't. Perhaps averaging out the Earth's orbit about the Earth-Moon barycenter?

The fictitious mean sun is a mechanism that removes the daily rotation and various nutations from the Earth's angular orientation, leaving only the long-term precession terms. It is inherently based on the rotating rather than orbiting Earth. Astronomers need to know the Earth's orientation because almost all telescopes are on the rotating Earth.

This fictitious mean sun, and other fictitious mean concepts, led to a confusing number of coordinate systems and time keeping mechanisms. Fortunately, much of that baggage has been abandoned. Every once in a while I still run across people who prefer to use true of date coordinates, mean of date coordinates, mean of 50 coordinates, ecliptic coordinates, etc. That has all pretty much gone by the wayside with the development of the International Celestial Reference Frame. Those who insist on using those older standards are now quite old themselves. They are retiring in droves. The weird stew of coordinates employed from 1895 to 1984 will soon be just a historical curiosity.

Time, too, has been simplified. Atomic clocks are a much more stable timekeeping mechanism than are either the Earth's rotation about its axis or its orbit about the Sun.
 

FAQ: Why Do We Use a Mean Sun That Moves Along the Equator Instead of the Ecliptic?

1. What is the "mean sun moving along equator"?

The mean sun moving along the equator refers to the average position of the sun in the sky as it appears to move along the equator over the course of a year. This is due to the tilt of the Earth's axis and its orbit around the sun.

2. How does the mean sun moving along the equator affect the length of day and night?

As the mean sun moves along the equator, it causes the length of day and night to vary throughout the year. This is because the tilt of the Earth's axis causes different parts of the Earth to receive more or less sunlight, resulting in longer or shorter days.

3. Is the mean sun's movement along the equator consistent every year?

No, the mean sun's movement along the equator can vary slightly from year to year due to factors such as the Earth's orbit, the tilt of its axis, and other astronomical influences. However, over a long period of time, it follows a predictable pattern.

4. What is the significance of the mean sun moving along the equator?

The mean sun's movement along the equator has significant effects on Earth's climate and weather patterns. It is also used in astronomy and navigation to determine the position of the sun and the seasons.

5. How is the mean sun moving along the equator calculated?

The mean sun's position along the equator is calculated by taking into account the tilt of the Earth's axis, its orbit around the sun, and other astronomical factors. This is done using complex mathematical equations and precise measurements from observatories around the world.

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