You enter the same date, time and place into two Kundli calculators and get two different charts. One shows a Cancer Lagna, the other Gemini; one starts the Venus Mahadasha in August 1997, the other in May. Usually neither tool is broken. Each was given slightly different inputs or follows a different convention, and a chart is sensitive to both.
This post works through the causes in the order worth checking them, using a fictional birth: 20 July 1995, 06:10, Pune. Every chart position and date below is AstroMystra's own calculation. With these inputs our Kundli calculator gives a Cancer Lagna at 2°42′, the Moon in Aries at 9°26′ in Ashwini nakshatra (pada 3), and a Ketu Mahadasha balance that ends on 7 August 1997.
The aim is to compare calculations, not to rank which reading is better. Two charts have to agree on the numbers before their interpretations can be compared at all.
1. Western or Vedic zodiac
Check this first, because it causes the largest differences. Western calculators use the tropical zodiac; a Kundli uses the sidereal zodiac, which in July 1995 sat 23°47′ behind it under the Lahiri ayanamsa. For the example birth:
- Moon: tropical Taurus 3°13′, sidereal Aries 9°26′
- Lagna: tropical Cancer 26°30′, sidereal Cancer 2°42′
- Sun: tropical Cancer 26°52′, sidereal Cancer 3°04′
A Western app would call this a Taurus Moon and a Kundli an Aries (Mesha) Moon. Each is correct for its own zodiac. Tropical vs Sidereal Zodiac shows the conversion step by step.
2. Birth time
The Lagna turns through the whole zodiac in a day, about one sign every two hours on average, so it is the most time-sensitive point in a chart. In this example it moves about 13 arcminutes every clock minute:
- 05:40: Gemini 26°01′
- 05:58: Cancer 0°02′, the first minute with a Cancer Lagna
- 06:10: Cancer 2°42′
- 07:10: Cancer 16°09′
A birth remembered as "around six" could have a Gemini or a Cancer Lagna, and in a whole-sign Kundli every house is counted from the Lagna sign. If one relative remembers 05:50 and another 06:10, two calculators will disagree while each does exactly what it was asked.
The Moon is slower, about half a degree an hour. Between 05:40 and 07:10 it moves only from Aries 9°10′ to 9°56′ and stays in Ashwini pada 3. The Vimshottari dasha balance, though, is worked out from the Moon's exact position inside its nakshatra, so that small movement still moves dates: the Ketu balance ends on 7 August 1997 for 06:10 and on 2 May 1997 for 07:10. One hour of birth time shifts every later Mahadasha date by about three months. The Lagna calculator and Dasha calculator let you try neighbouring times yourself.
3. Birth place
The place gives the calculator two things: coordinates and a time zone. Clocks across India show IST, but the sky differs from east to west. At 06:10 IST the Lagna is Cancer 2°42′ in Pune and Cancer 6°25′ in Hyderabad, about 4½° of longitude further east. Near a sign boundary, entering the nearest large city instead of the actual town can be enough to change the Lagna sign.
Place names also repeat. Entered as "Hyderabad" alone, AstroMystra's calculator picks Hyderabad in India, the most populous match. Someone born at 06:10 local time in Hyderabad, Sindh, was born half an hour later in absolute terms, because Pakistan's clocks run 30 minutes behind IST. With the right place, their Lagna is Cancer 7°08′ and the Ketu balance ends on 19 June 1997, not 7 August. Add the state or country to the place name, and where a calculator shows the place it matched, check it.
4. Historical clock changes
Local clock time has to be converted to Universal Time, and the offset depends on the date as well as the place. According to the IANA time zone database, which most software uses, Indian clocks were set an hour ahead of IST, at UTC+6:30, from October 1941 to May 1942 and again from September 1942 to October 1945.
Take a birth at 08:00 on 14 March 1944 in Kolkata. With the wartime offset, that is 01:30 UTC and the Lagna is Pisces 23°15′. A calculator that applies today's +5:30 places the birth at 02:30 UTC and gets Aries 12°15′, a different sign. The Moon barely moves (Libra 15°20′ against 15°51′, both in Swati). Daylight saving time causes the same problem for births in Europe, North America and elsewhere, and AstroMystra applies each zone's historical rules for the birth date.
5. Ayanamsa
Vedic calculators do not all use the same ayanamsa. Lahiri, used by India's Rashtriya Panchang and by AstroMystra, is the most common. Raman's is about 1½° smaller, while KP (Krishnamurti) differs from Lahiri by only a few arcminutes. Changing the ayanamsa moves every planet and the Lagna by the same amount, so it changes a sign only near a boundary.
In the example, Saturn is at Pisces 0°48′ under Lahiri. Any ayanamsa more than 0°48′ larger would put it in Aquarius; a smaller one, such as Raman's, keeps it in Pisces. Nakshatra and pada boundaries are much closer together, every 13°20′ and 3°20′, so they change more often than signs. Ashwini pada 3 ends at Aries 10°00′; with an ayanamsa about 1½° smaller, the Moon at Aries 9°26′ moves past that point, into pada 4, and the dasha balance changes with it.
6. House system
AstroMystra's Kundli uses whole-sign houses: the Lagna sign, Cancer, is the first house, Leo the second, and so on. Some Vedic software also offers a Bhava Chalit chart, which builds the houses around the Lagna degree instead of the Lagna sign. In its simplest equal-house form, each house is 30° wide with the Lagna degree at its middle.
For the example, that Chalit first house runs from Gemini 17°42′ to Cancer 17°42′. Mercury (Gemini 23°46′) and Venus (Gemini 24°17′) are in the twelfth house of the whole-sign chart and in the first house of the Chalit chart. Neither chart is a calculation error; they answer "which house?" with different rules. Western calculators usually use quadrant systems such as Placidus, whose houses vary in size.
7. Rahu, Ketu and dasha conventions
Rahu and Ketu are the lunar nodes. Calculators use either the mean node, which moves at an average rate, or the true node, which wobbles around it, and the two can be up to about 1½° apart. AstroMystra's charts do not yet calculate node positions at all, as the Methodology page explains, so if another Kundli places Rahu in a house, ours has no figure to compare it with.
Dasha dates also depend on the length of year. AstroMystra uses a 365.25-day year. Software that uses a 360-day year makes each dasha year 5¼ days shorter, so a 20-year Venus Mahadasha comes out about 105 days shorter, and the gap grows with every later period.
8. Rounding and display
Some tools show decimal degrees and others degrees and minutes: 2.71° and 2°42′ are the same position. Near a boundary, rounding can show 0°00′ of the next sign in one tool and 29°59′ in another, so check the unrounded value before deciding which sign a planet is in.
A comparison checklist
Before comparing two Kundlis, write down for each one:
- Zodiac: tropical or sidereal.
- The time zone and offset used for the birth date, or the UTC time.
- The place matched, with its latitude and longitude.
- The birth time entered, and how sure the family is of it.
- The ayanamsa.
- Whole-sign (Rashi) or Bhava Chalit houses.
- Mean or true node, and the dasha year length.
If every item matches and the positions still differ by more than a few arcminutes, or the Moon's sign or nakshatra differs, one of the tools probably has a fault. If you think it is ours, report it through our Corrections page with both sets of inputs.
Sources
- Positions, Lagna and dasha dates: AstroMystra's calculation with the open-source astronomy-engine library, the Lahiri ayanamsa and a 365.25-day dasha year; method on our Methodology page.
- Historical time-zone offsets: the IANA time zone database, as built into modern browsers and Node.js.
- Ayanamsa and node conventions: the Swiss Ephemeris documentation describes the sidereal modes and the mean and true nodes. The Raman and node figures above are approximate.