The Tropical system, on which much of Western astrology is generally based, is no longer based on the actual observable position of stars in the zodiac or the night sky. This is because the Tropical zodiac is based upon the equinoxes and not on the fixed stars. One of the fixed stars is Antares, which can be found Sidereally in the heart of Scorpio.
The beginning of the Tropical zodiac is at the first degree of Aries, the vernal equinox. This is also the position of the Sun at the first day of Spring. The orientation of the equinoxes is a moveable system compared to the backdrop of fixed stars, due to the precession of the earth on its axis. This phenomenon of the precession, (which also involves the changing tilt of the earth), causes the Earth to move backwards in the zodiac, relative to the fixed stars. Over a period of around 25,000 years, a point on the earth relative to the fixed stars makes a full circle of the zodiac.
This means that for a Tropical astrologer, Jupiter in Sagittarius would very likely be actually found placed in Scorpio when gazing at the night sky. In fact all of the planets in the Tropical zodiac are actually found 23 degrees behind their actual Tropical placement when we convert the chart to a Fixed star or Sidereal Vedic chart. This means that some or all of the planets in a Tropical chart will move backwards from one sign/house to another, if we want to convert the Tropical placement of planets to the actual placement of stars in the sky (the Sidereal placement).
Around 2000 years ago when Western astrology was in its formative years the two systems happened to coincide. Since then, with the precession of the equinoxes, the two zodiacs have been moving apart by 50’ per year. There is now over a 23-degree difference between the two systems.
The Tropical zodiac is also based on the orientation of the earth to the Sun, not on stars. The Vedic system is based on the fixed stars and the Moon.
For this reason the Sun is very important to Western astrologers, whereas the Vedic astrologers have made the Moon their most important planet.
Reference : Astrology of the Seers by David Frawley