Scientists have long known that our Sun rotates about once a month.
There may be cause to believe that the Sun had earlier rotated
more rapidly. There are two phenomena that may have that consequence. It's axis of rotation is also NOT lined up with where all the planets are, as it is TILTED by quite an angle! This is a problem regarding both the Conservation of Angular Momentum and the Conservation of Energy. The Sun has existed for around five billion years. At the beginning of that time, the Sun had some specific amount of angular momentum of rotation. Our traditional understanding of Physics says that angular momentum must be conserved. A commonly seen example is when an ice skater is slowly spinning with arms extended, draws her arms in and suddenly spins very rapidly.
The Sun creates a LOT of energy. Every second, about 3.86 * 10^{26} joules (wattseconds) of energy is radiated outward from the Sun's surface. That energy was all created from the conversion of mass to energy in accordance with Einstein's E=mc^{2}. Knowing that the speed of light is 3.0 * 10^{8} meters per second, we can easily determine that 4.3 * 10^{9} kg of mass disappears every second from the Sun. Isn't that something? 4.3 BILLION kilograms of the Sun disappear every second! (It will still last for many billions of years to come!)


Continuing this logic, the Sun has existed for around 1.6 * 10^{17} seconds, so, if its radiation has been relatively constant during that time, a total of about 7 * 10^{26} kg has already disappeared in this way. The present mass of the Sun is 1.99 * 10^{30}, so only about one part in 2500 has already disappeared.
When the mass disappears, it disappears near the very center of the Sun, where it is hottest. In other words, the atoms of the Sun that disappear in the conversion to energy, have minimal rotational angular momentum. This suggests that this effect would have a tiny increase in the rotation rate of the Sun, about one part in two million. Pretty much irrelevant!
The other effect that may be affecting the rotation rate of the Sun has to do with a lot of turbulence that exists on and in the Sun. We can see the churning motions of the surface. We assume that the heat driven effects continue throughout the Sun, where the rising heat from the center (where the energy is created) is moving upward and outward to get to the outer surface where it can be radiated into space. Convection cells are certainly one of the main processes inside the Sun in accomplishing this heat transfer.
That churning motion must necessarily create frictional heat losses. Some of the kinetic energy of rotational motion must be converted into frictional heat, which means there would be less remaining kinetic energy of rotation. This would imply a gradual slowing of the rotation rate. In the process, the amount of rotational angular momentum would also decrease, but that would go into the turbulence eddies.
For more information on this effect, please see my similar article on Planetary Rotation of Jupiter, Saturn, and the Earth.
This effect in the Sun appears to be much more substantial than the 'disappearance effect' mentioned above. That suggests that the Sun may have significantly slowed down in its rotation during the past five billion years. Trying to quantify this effect is difficult, because there are so many variables inside the Sun that are unknown.
Conservation of Angular Momentum  An Exception or Violation (Sept 2006)
Galaxy Spiral Arms Stability and Dynamics A purely Newtonian gravitational explanation (Nov 1997, Aug 1998)
Twins Paradox of Relativity Is Absolutely Wrong (research 19972004, published Aug 2004)
Perturbation Theory. Gravitational Theory and Resonance (Aug 2001, Dec 2001)
Origin of the Earth. Planetary Gravitational Resonances (Dec 2001)
Rotation of the Sun (Jan 2000)
Origin of the Universe. Cosmogony  Cosmology (more logical than the Big Bang) (devised 1960, internet 1998)
Time Passes Faster Here on Earth than on the Moon (but only a fraction of a second per year!) (Jan 2009)
Globular Clusters. All Globulars Must Regularly Pass Through the cluttered Galaxy Plane, which would be very disruptive to their pristine form. (Nov 1997, Aug 1998)
Existence of Photons. A Hubble Experiment to Confirm the Existence of Individual Photons (experimental proof of quanta) (Feb 2000)
Origin of the Moon  A New Theory (June 2000)
Planetary Rotation of Jupiter, Saturn, and the Earth (Jupiter has a lot of gaseous turbulence which should have slowed down its rapid rotation over billions of years) (March 1998)
Cepheid Variable Stars. Velocity Graph Analysis (Feb 2003)
Compton Effect of Astrophysics. A Possible New Compton Effect (Mar 2003)
Olbers Paradox Regarding Neutrinos (Oct 2004)
Kepler and Newton. Calculations (2006)
Pulsars. Pulsars May Be Quite Different than we have Assumed (June 2008)
Sun and Stars  How the Sun Works  Nuclear Fusion in Creating Light and Heat (Aug 2006)
Stars  How They Work  Nuclear Fusion. Lives of Stars and You (Aug 2004)
Sundial Time Correction  Equation of Time. Sundial to ClockTime Correction Factor (Jan 2009)
General Relativity  A Moon Experiment to Confirm It. Confirming General Relativity with a simple experiment. (Jan 2009)
General Relativity and Time Dilation. Does Time Dilation Result? (Jan 2009)
Geysers on Io. Source of Driving Energy (June 1998)
Mass Extinction, a New Explanation. A New Explanation for Apparent Periodicity of Mass Extinctions (May 1998, August 2001)
Precession of Gyroscopes and of the Earth. Gyroscope Precession and Precession of the Earth's Equinoxes (Apr 1998)
Ocean Tides  The Physics and Logic. Mathematical Explanation of Tides (Jan 2002)
Earth's Spinning  Perfect Energy Source (1990, Dec. 2009)
Earth's Magnetic Field  Source and Logic. Complex nature of the magnetic field and its source (March 1996)
Earth Spinning Energy  Perfect Energy Source From the Earth's Spinning (1990, Nov. 2002)
Nuclear or Atomic Physics Related Subjects:
Nuclear Physics  Statistical Analysis of Isotope Masses Nuclear Structure. (research 19962003, published Nov 2003)
Quantum Defect is NOT a Mathematical Defect It Can Be Calculated The Quantum Defect is a Physical Quantity and not a Fudge Factor(July 2007)
Atomic Physics  NIST Atomic Ionization Data Patterns Surprising Patterns in the NIST Data Regarding Atomic Ionization (June 2007)
Nuclear Physics  Logical Inconsistencies (August 2007)
Neutrinos  Where Did they all Come From? (August 2004)
Neutrinos  Olbers Paradox Means Neutrinos from Everywhere (Oct 2004)
Quantum Nuclear Physics. A Possible Alternative (Aug 2001, Dec 2001, Jan 2004)
Quantum Physics  Quantum Dynamics. A Potential Improvement (2006)
Quantum Physics is Compatible with the Standard Model (2002, Sept 2006, Oct 2010)
Quantum Dynamics (March 2008)
Ionization Potential  NIST Data Patterns. Surprising patterns among different elements (March 2003)
Mass Defect Chart. (calculation, formula) (research 19962003, published Nov 2003)
Assorted other Physics Subjects:
Precession of Gyroscopes and of the Earth. Gyroscope Precession and Precession of the Earth's Equinoxes (Apr 1998)
Earth's Magnetic Field  Source and Logic. Complex nature of the magnetic field and its source (March 1996)
Earth Spinning Energy  Perfect Energy Source (1990, Nov. 2002)
Earth Energy Flow Rates due to Precessional Effects (63,000 MegaWatts) (Sept 2006)
Accurate Mass of the Earth. Gravitational Constant  An Important Gravitation Experiment. (Feb 2004)
Tornadoes  The Physics of How They Operate, including How they Form. Solar Energy, an Immense Source of Energy, Far Greater than all Fossil Fuels (Feb 2000, Feb 2006, May 2009)
Radiometric Age Dating  Carbon14 Age Determination. Carbon14, C14 (Dec 1998)
Mass Extinction, an Old Explanation. An Old Explanation for Apparent Periodicity of Mass Extinctions (Aug 2003)
Hurricanes, the Physics and Analysis A Credible Approach to Hurricane Reduction (Feb 2001)
Sundial Time Correction  Equation of Time. Sundial to ClockTime Correction Factor (Jan 2009)
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago