Most modern Western astrology uses the tropical zodiac. It divides the ecliptic into twelve equal 30-degree signs and anchors 0° Aries to the vernal equinox. The signs stay fixed to those seasonal turning points.
Jyotish is traditionally practiced with a sidereal zodiac, and the Vedic astrology I use is sidereal. It also uses twelve equal 30-degree signs, but its zodiac is referenced to the stellar background rather than being anchored to the equinox. The difference between the tropical and sidereal zero points is called the ayanamsha.
Because of a slow wobble in the Earth's axis called the precession of the equinoxes, those two reference frames have gradually separated. Using Lahiri ayanamsha today, the difference is roughly 24 degrees. To calculate sidereal longitude from tropical longitude, that ayanamsha value is subtracted.
This is not the same as saying Vedic astrology uses "the real constellations." Both systems divide the ecliptic into twelve equal 30-degree signs. The actual constellations in the sky are irregular in size and span. The distinction is reference frame, not constellation boundaries.
Whether the named sign changes depends on the planet's degree. With an ayanamsha of roughly 24°, most tropical placements move into the previous named sign sidereally, while placements in approximately the final six degrees of a tropical sign may remain in the same named sign sidereally.
The degree changes throughout regardless — only whether the named sign changes depends on where within the tropical sign the planet sits.
They are different zodiac reference frames embedded in different interpretive traditions. One is not simply more correct than the other — they are internally consistent frameworks with different reference points and different interpretive architectures built around those reference points.
Mixing them — using a sidereal rising sign with a tropical Moon sign, for example — tends to produce confused results because the frameworks carry different assumptions throughout. Each makes the most sense when used on its own terms.
Jyotish does not reduce you to one sign. The lagna is foundational because it establishes the houses and their rulers. The Moon is also foundational, including for Vimshottari timing. The Sun has its own role. So "which sign am I?" is already the wrong question if you are trying to understand a Vedic chart — it imports a Western framework where the Sun sign carries most of the identity weight, into a system where that is not how it works.
This is the part most explanations skip.
It is not just that your Sun moves from Scorpio to Libra. The entire house structure can shift. In the whole-sign Vedic framework I use, the rising sign establishes the first house and each sign becomes a house from there. If your rising sign changes, every planet may be in a different house. A planet that was in your 5th house may now be in your 4th or 6th. Its meaning changes significantly.
The planetary rulerships change with it. Which planets govern which life areas — career, relationships, money, health — depends on what those planets rule in the Vedic chart, which is determined by the rising sign. Different rising sign, different rulerships, different chart architecture.
It is also worth noting that Western astrology uses a variety of house systems — Placidus, Whole Sign, Koch, and others. When someone switches from a Western Placidus chart to a Vedic whole-sign chart, the changes can come from both the zodiac reference frame shift and the house system change, not solely from subtracting the ayanamsha. Both are happening simultaneously for many people.
Classical Vimshottari dasha is calculated from the Moon's sidereal nakshatra position. It belongs to the Jyotish framework, so I do not casually transplant it onto tropical placements and then mix the two systems. There are modern practitioners experimenting with tropical zodiac combined with sidereal nakshatra approaches — but that is a distinct methodological choice, not simply transferring a tool from one system to another.
For the same moment and the same ayanamsha model, the same offset is used. The ayanamsha itself changes slowly over time because of precession — so the value for someone born in 1970 differs slightly from someone born in 2000.
Different ayanamsha models also differ slightly from each other, sometimes enough to matter near sign or nakshatra boundaries. Lahiri is one of the most widely used in Indian Jyotish and is the default in most modern Vedic software.
You do not have to decide that one chart is fake and the other is real. But if you are reading a Vedic chart, read it using Vedic rules. A sidereal placement interpreted with tropical pop-astrology meanings is not really a Jyotish interpretation at all. The useful question is not "which version of me is correct?" It is "what does this chart mean inside the system that produced it?"