What is sidereal astrology?
Look Up!
Sidereal astrology takes as its premise the actual fixed stars in the sky above us, and their apparent relation to the surrounding space. Rather than being tied to seasonal time (the equinoxes and solstices, calculated according to the procession) sidereal astrology observes the apparent path of the planets and luminaries in the space marked by the actual stars above. This shifts the birth Sun sign, as well as most other signs by 23 degrees backwards from Western Tropical Zodiac.
Your Natal chart is a Map: The birth chart is a snapshot of the visible (and invisible-below the horizon or not visible because it is daytime) night sky at the exact time and location on Earth of a birth.
This snapshot is meant to be read as a map- both of the sky, and of one's inherited karmas at the time of birth. It shows us what we have arrived into, or what we have brought with us, depending on your spiritual/religious perspective on the nature of the soul and karma. Astrologers (or the lay person who wishes to study enough to do so) interpret these maps to determine many aspects of a person's personality or life path. The time and location matter a great deal, because the orientation of the map will change rather quickly depending on these details, leading to potentially very different interpretations.
As all maps do, however, there is some distortion from a three dimensional reality onto a two dimensional representation. The chart is divided into 12 equally sized zodiac signs (30degrees each) whereas the actual constellations which make up the zodiac are not of equal size (for example, Cancer is a larger constellation and Libra is the smallest constellation.) Nonetheless, the majority of the placements of planets you see in a birth chart would appear in the exact same starry constellations were you to look up in the night sky (or to look on a star mapping app.)