Within the Sidereal system the consideration of the starting point is SUBJECTIVE (thus there are a lot of ayanamshas), BUT the 12 segments/signs can be EXACTLY MEASURED along the Ecliptic (each of them is exactly 30 arc degrees too.) 
    Within the system of the true (or real) Constellations the consideration of a starting point is SUBJECTIVE AND the determination of the angular size of the 12 sectors is SUBJECTIVE TOO. To determine the starting (and ending) points of 
    the real Constellations along the Ecliptic (and thus to consider the Zodiac of the true Constellations) we must choose some Marker Stars.