Ancient books recorded that Sirius was red. Why is it white now?

Sirius is the brightest star in the night sky. The Sirius we see now is beyond doubt. It must be blue and white. In some earlier records of China, Sirius was also regarded as the standard of "white".

In the observation records in Europe almost 2000 years ago, the observation results about Sirius showed a different red color! Could Ptolemy have made a particularly elementary mistake? If so, he is not the only astronomer who has made mistakes. Because in the European literature more than 2000 years ago, Horace, Seneca and Artaz all described Sirius as red, and it was 200 years earlier than Ptolemy.

Is it possible that Sirius was really red and changed from red to blue in 2000? Or is there an ancient signal source about Sirius that people including Ptolemy can't specifically believe? And it is not consistent with the description of ancient books in China. This is actually an interesting question. The following are the results of Hubble observation!

According to what we know about stars now, stars are very, very constant for most of their life cycles, and the year 2000 is too short to make the colors of stars remarkable.

You may ask whether this is related to the correlation between sunset and sunrise and the density of the atmosphere! This is also the first guess of scientists: when the moon or the sun is close to the horizon, it looks red. For example: the moon. Is this statement reasonable? But the poor problem is that there is no specific way to determine the answer given. Why? First of all, many other bright stars we see will appear in the same position near the horizon like Sirius, and their colors have been accurately reported, except for the color difference of Sirius. In addition, the special phenomenon that objects near the horizon turn red is obvious to all, and it is hard to believe that people who are proficient in astronomy like Ptolemy are not aware of this. Also, Ptolemy lived in Egypt, where Sirius was in a very different position in the sky, and would not appear near the horizon many times a year. It has also been pointed out that some different phenomena may have occurred in Ptolemy, such as the eruption of ultra-hot volcanoes, and the particles in the upper atmosphere changed the color of the celestial surface. The probability of this happening is too small, and Horace in Europe in BC 1 century is different from Ptolemy in Africa in the 2nd century. ..