Tropical astrology is based on the idea that early astrologers (mid-to-late first millennium BCE) defined the star signs according to the seasons in which the sun rose in them; it wishes to preserve the seasonal associations of those star signs by laying out new horoscopes against a first-millennium sky. For tropical astrologers therefore it is irrelevant that the solsticial points (tropics) have drifted from one constellation to another over the millennia, due to the precession of the equinoxes. The underlying philosophy remains unchanged in spite of precession, because it is based on the earth's (and therefore our) relationship to the sun, not to the stars. The names of the zodiacal constellations that became the star signs are supposed to suggest the characteristics of (the sun in) each segment of the year. Thus, Aries (House 1), representing the sun just returning to the northern hemisphere at the vernal equinox, symbolises unruly beginnings; Leo (House 5), representing the powerful sun of mid-summer, symbolises fertility and self-display; Sagittarius (House 9), representing the retreating or meditative sun close to the winter solstice, symbolises the search for understanding.
In its emphasis on the symbolic or metaphorical meaning of the star signs tropical astrology differs from sidereal astrology which claims intrinsic meaning for the star signs and wishes to preserve those meanings by laying out horoscopes against the actually occurring sky.
In the tropical zodiac the sun at the vernal equinox is considered to be in the first degree of the star sign Aries. This is because when astrology was being developed by the Babylonians and the Hellenistic Greeks the sun actually was in the constellation Aries at the vernal equinox (whereas now it is in Pisces). Even by the time the earliest horoscopes were written, however, in the fifth century BCE, the equinoctial point had drifted -- to about the 28th degree of Aries. The last time the vernal equinox was in the first degree of Aries was around 1900 BCE, give or take a hundred years, depending on how you read the ecliptic.
The tropical zodiac in astrology is largely exclusive to The Americas and Western European astrologers. Some Western astrologers choose to practice sidereal astrology, and criticise the tropical approach as erroneous because it does not align with the actual position of the stars. Sidereal astrologers also point out the absurdity of applying northern hemisphere seasons to the whole planet when there are now large populations within the southerm hemisphere who experience seasons six months apart from those in the north. Tropical astrologers counter that there may be many ways of interpreting the relationship of the current night sky to the pattern of star signs. This is a long running dispute within Western astrology.