There are two astrologies. Movable astrology and Fixed astrology.
In Movable astrology, the beginning of the signs is the vernal point To get the planets longitude just look at a standard ephemeris and read directly the longitude of the planets.
But you must know that the vernal point is not fixed with regard to stars. And zodiac signs have their name coming from the fixed stars (Aries, Taurus etc..)... So to work with real constellation signs, as Hindus do it, you must correct movable longitudes to get fixed longitudes. The correction to apply is known as precessional distance or Ayanamsa.
For 1900, ayanamsa was about 24°, although the exact value is not known and is maybe 5° more. To translate a movable horoscope to fixed horoscope, you must subtract the ayanamsa to each longitude of planets and houses.
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