It's not entirely accurate to say there's "never" an exact measurement for the Sun. Scientists have incredibly precise measurements for many of its properties, such as its:

* Diameter: 1,392,684 km (865,000 miles)

* Mass: 1.989 × 10^30 kg (4.385 × 10^30 lbs)

* Surface temperature: 5,505°C (9,939°F)

* Luminosity: 3.846 × 10^26 watts

However, you're right to point out that there are inherent challenges in measuring the Sun that lead to some inherent uncertainty:

* Dynamic nature: The Sun is a constantly changing star, with fluctuating activity like sunspots and solar flares. This makes it difficult to get a perfectly "snapshot" measurement.

* Distance: Even though we have precise measurements of the Earth-Sun distance, there's still a tiny margin of error.

* Limitations of measurement tools: Our instruments, even the most sophisticated ones, have inherent limitations in accuracy and precision.

* The Sun's core: We cannot directly measure the Sun's core, so our understanding of its properties relies on complex models and indirect measurements.

Therefore, while we have incredibly accurate measurements of the Sun, there will always be a small margin of error due to the dynamic nature of the star, the limitations of our tools, and the difficulty in measuring its core.

It's important to remember that science is constantly evolving, and as technology improves, our measurements of the Sun will become even more precise.