# Mean sun moving along equator

1. Mar 9, 2012

### JeffOCA

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

2. Mar 9, 2012

3. Mar 9, 2012

### D H

Staff Emeritus
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. Mar 10, 2012

### JeffOCA

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. Mar 10, 2012

### D H

Staff Emeritus
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.