Why is my Sun sign different in Vedic astrology?
Because Vedic astrology measures the zodiac from the stars and Western astrology measures it from the seasons, and the two have drifted about 24° apart. A sign is 30° wide, so roughly four people in five land one sign earlier in their Vedic chart, which tends to come as a surprise.
Two starting points
Western astrology uses the tropical zodiac, which starts Aries at the March equinox wherever the stars happen to be, so it’s essentially a calendar of the seasons and Aries always begins in spring. Vedic astrology (Jyotish) uses the sidereal zodiac, which ties each sign to the stars behind it.
The two lined up roughly 1,700 years ago. Since then the equinox has slipped backwards against the stars by about a degree every 72 years, which is precession, the slow wobble of the Earth’s axis. That gap, about 24° today, is called the ayanamsa.
Who changes sign, and who doesn’t
Take 24° off your Western Sun and you have your Vedic one. If your Sun sits in the first 24° or so of its sign, it moves back into the sign before; if it’s in the last six degrees, it stays put. That’s why about four in five people find a different Sun sign, and one in five don’t. Your exact degree is on your chart.
The same 24° applies to everything, so your Moon sign and rising sign often move too. The houses are drawn differently as well: Vedic charts use whole signs, one sign to one house. More on house systems.
Neither one is a mistake
The usual objection is “so one of them has to be wrong”, and it’s a fair one, albeit based on the idea that they’re measuring the same thing. They answer different questions, one about the seasons and one about the stars, and each tradition has read its own for well over a thousand years. What goes wrong is mixing them, like reading a Vedic placement with Western meanings, which is a bit like following a recipe in Celsius with a Fahrenheit oven, except that here both settings are correct for their own kitchen.
Which ayanamsa we use
Schools differ on the exact size of the gap by a degree or so. We use Lahiri, India’s official ayanamsa and the default in most Jyotish software, at about 24°13′ for a birth today. If your Sun is within a degree of a sign’s edge, a different ayanamsa could put it in the next sign, so it’s worth knowing which one a chart uses before comparing two.
Third House draws both charts, and reads each by its own tradition’s rules. See one chart four ways.