Basics
Tropical and sidereal zodiacs
Tropical zodiac, Western astrology
If you have seen two different Sun signs for the same birthday, you have met the difference between the tropical and sidereal zodiacs. Both divide the sky into twelve equal signs of 30 degrees. They differ in where the starting point is.
The tropical zodiac
The tropical zodiac is tied to the seasons. Aries begins at the March equinox, the moment day and night are equal in length and spring begins in the northern hemisphere. Cancer begins at the June solstice, Libra at the September equinox and Capricorn at the December solstice. Most Western astrologers use this system, and it is the default in Natalora.
The sidereal zodiac
The sidereal zodiac is tied to the fixed stars. Its signs are anchored to constellation backgrounds, as in traditional Indian and Thai astrology. If you are curious about the stars themselves, Natalora has a "Fixed stars" tab that shows 25 bright stars near the points in your chart; read more in the article "Fixed stars in your chart".
Why the two drift apart
Earth’s axis wobbles very slowly, like a spinning top, in a cycle of about 26,000 years. This is called precession. Because of it, the equinox point slides backward against the star background by about one degree every 72 years. The two zodiacs lined up roughly 1,700 years ago; today the sidereal signs sit about 24 degrees behind the tropical ones.
The exact gap is called the ayanamsa. Different traditions define it slightly differently.
What this means for your chart
With a 24-degree offset, most planets fall one sign earlier in the sidereal chart: any planet in the first 24 degrees of a sign moves back a sign. Someone with a tropical Sun at 10 degrees of Taurus would have a sidereal Sun at about 16 degrees of Aries. Only planets in the last six degrees of a sign stay in the same sign in both systems.
Choosing in the app
Natalora uses the tropical zodiac by default. In the chart options you can switch to sidereal and choose between two ayanamsas, Lahiri and Fagan-Bradley. Both give an offset of roughly 24 degrees from the tropical zodiac; they differ by a fraction of a degree.
Neither system is more true than the other; they are different frames of reference. Pick the one that matches the tradition you are learning, and stay consistent when you compare interpretations.