From an astronomical perspective it just can't be.
We're talking about astrology, not astronomy. They are different and use different laws and concepts.
Constellations are an illusion driven by relativity and pattern recognition.
Astrology uses zodiac signs, not constellations. They are related to them, but they're not the same thing.
For example, the constellations have variable sizes, where all signs in astrology are all 30 degrees each. There'd also the constellation of Ophiuchus between Scorpio and Sagittarius, but it's not used in astrology.
If you were to leave our solar system and go to another, you would be faced with a "different" set of constellations due to shifting your position, and even our own sun would appear to be part of one.
Exactly. If we were on another planet far away we would have a different type of astrology, with different signs. Again, I do not see how this supposedly invalidates it.
Nobody said that this type of astrology we use on earth works for the entire universe. It only works for us from our perspective, obviously.
How can astrology be real when we can demonstrate that it is an illusion?
You only demonstrated that different solar systems and planets can have different types of astrology.
And another thing, forget the constellations for a moment. Do you not agree that from the Earth's perspective the sun is in different parts of the Ecliptic during different times and seasons, yes? Well, then people can perceive with their intuition and higher senses these parts, their characteristics and how planetary positions can affect the earth. And the same with the planets and their characteristics.
The signs are based on the constellations perceived intuitively and are used as a convenient way for these parts of the sky,
Which is why there are remarkable similarities, even between completely different cultures with different systems of astrology, like the Vedic and Western.
As others said, we use astrology because we get results. It works. And that's what mostly matters. You can't just philosophize or theorize why it might not work, you must practice it for yourself.