The International Astronomical Union (IAU) has redefined the astronomical unit (AU), a fundamental measurement in astronomy, to be a fixed constant value of exactly 149,597,870,700 meters[1][2]. This change, which took effect in 2012, represents a significant update to astronomical standards.
Key Aspects of the Redefinition
Simplification: The new definition is much simpler than the old one, which was based on a complex mathematical formula involving the mass of the Sun, the length of a day, and the Gaussian gravitational constant[1][2].
Fixed Value: The AU is no longer a variable measurement dependent on changing factors. It is now a precise, constant distance[1].
Consistency with Relativity: The new definition aligns better with Einstein’s theory of general relativity, avoiding issues with the previous definition that could yield different values depending on the observer’s location in the solar system[3].
Benefits for Astronomers
The simplified AU definition offers several advantages:
– Easier explanation to students and the public
– Improved precision for calculations in solar system dynamics
– Compatibility with all astronomical time scales (TCB, TDB, TCG, TT, etc.)[4]
– Allows direct experimental determination of the solar mass parameter in SI units[4]
Historical Context
The AU concept dates back to early astronomers like Giovanni Cassini, who used trigonometry to estimate the Earth-Sun distance. The previous definition, while useful for many years, became outdated as measurement technologies improved[1].
Impact on Astronomy
While this change is significant for the field of astronomy, its effects are primarily relevant to high-precision solar system dynamics[2]. The relative difference between the old and new definitions is extremely small (about one ten-billionth), ensuring continuity in astronomical calculations[2].
The Reality of Earth-Sun Distance
- The actual distance between Earth and Sun varies throughout the year.
- At perihelion (closest approach), Earth is about 147.1 million km from the Sun.
- At aphelion (farthest point), Earth is about 152.1 million km from the Sun.
- This variation is due to Earth’s elliptical orbit and cannot be changed by definition.
Purpose of the AU Definition
- The fixed AU provides a consistent reference for astronomical calculations.
- It simplifies certain types of computations in celestial mechanics.
- It does not alter or negate the actual, variable distance between Earth and Sun.
The Earth-Sun distance remains variable, regardless of how we define our units of measurement.
In conclusion, the IAU’s redefinition of the astronomical unit modernizes this fundamental measurement, aligning it with current scientific understanding and technological capabilities. It simplifies calculations for astronomers while maintaining the precision necessary for advanced research in solar system dynamics.
Read More
[1] https://www.space.com/17733-earth-sun-distance-astronomical-unit.html
[2] https://www.observatoiredeparis.psl.eu/the-new-definition-of-the-astronomical-unit.html?lang=en
[3] https://phys.org/news/2012-09-iau-votes-redefine-astronomical-constant.html
[4] https://syrte.obspm.fr/iau/IAU_resolutions/Redefua2012.pdf
[5] https://www.nature.com/articles/nature.2012.11416
[6] https://www.tidjma.tn/en/astro/astronomical-unit-distance/
[7] https://www.newscientist.com/article/dn13286-astronomical-unit-may-need-to-be-redefined/
[8] https://www.iau.org/public/themes/measuring/
[9] https://en.wikipedia.org/wiki/Astronomical_unit
[10] https://www.iau.org/static/resolutions/IAU2012_English.pdf
[11] https://ui.adsabs.harvard.edu/abs/2012IAUJD…7E..40C/abstract
[12] https://www.scientificamerican.com/article/astronomical-unit-or-earth-sun-distance-gets-an-overhaul/