How to Calculate Lagna Manually: Local Sidereal Time

The lagna is the degree of the zodiac that was rising on the eastern horizon of the birthplace at the minute of birth, read in the sidereal zodiac. Software finds it instantly, but nothing in the calculation is hidden. It takes five inputs (the date, the clock time, the clock’s offset from UT, and the birthplace’s latitude and longitude) and six steps. The pivot of the whole method is local sidereal time, the clock that tells you which part of the sky is overhead.

The worked example below is an imaginary birth at 09:20 IST on 8 July 1992 in Delhi (28.6139° N, 77.209° E). Each figure is printed rounded the way you would write it down, and the hand arithmetic on those rounded figures is checked against the site’s own lagna code, the same code the free kundli runs.

Step 1: Clock Time to Universal Time

Sidereal time is counted from Greenwich, so the birth time has to be put on Greenwich’s clock first. IST is 5 hours 30 minutes ahead of UT, so 09:20 IST is 03:50 UT on the same date. A birth before 05:30 IST lands on the previous UT date, and the date in the next step must change with it.

Step 2: Days From the Standard Epoch

Astronomers count time in Julian days, a running day number. The standard formulas measure from noon UT on 1 January 2000, Julian day 2451545.0. For 03:50 UT on 8 July 1992 the Julian day is 2448811.6597, so the birth is d = −2733.3403 days from that epoch, negative because it came before it. Any astronomy textbook, or a Julian-day converter, gives the day number for a date.

Step 3: Greenwich Sidereal Time

The Earth turns once against the stars in about 1436.1 minutes, not 1,440, so a sky clock gains about 3.9 minutes a day on an ordinary one. The standard formula (Jean Meeus, Astronomical Algorithms, equation 12.4) gives Greenwich mean sidereal time in degrees:

GMST = 280.46061837 + 360.98564736629 × d + small terms in T² and T³, where T = d ÷ 36525

With d = −2733.3403 the formula gives −986,416.149°. Adding 2,741 whole turns of 360° brings it to 343.8510°. Divided by 15 to turn degrees into hours, that is 22h 55m 24s. The site’s code then adds the equation of the equinoxes, a nutation correction that comes to about 1.1 seconds of time here; by hand it can be left out.

Step 4: Local Sidereal Time

Local sidereal time is Greenwich sidereal time plus the birthplace’s longitude east, both in degrees: 343.8510° + 77.209° = 421.0600°, less 360° = 61.0600°, or 4h 04m 14s. It is the right ascension of the meridian, the line from north to south through the point overhead, and that is why it fixes the ascendant: once you know what is on the meridian, the latitude decides what is on the eastern horizon.

Each degree of longitude is four minutes of sidereal time. Delhi lies 5.3° west of 82.5° E, the meridian IST is set to, so its local sidereal time runs 21.2 minutes behind what the IST meridian would give at the same instant. The city pages under panchang by city print this local-time gap for each city.

Step 5: The Rising Degree

Write θ for the local sidereal time in degrees, φ for the latitude and ε for the tilt of the Earth’s axis to its orbit, the obliquity, which was 23.4402° in 1992. The tropical longitude L of the ascendant satisfies

tan L = −cos θ ÷ (cos ε × sin θ + sin ε × tan φ)

Here the top line is −0.4839 and the bottom line 1.0199. Taking the angle with a two-argument arctangent, which keeps track of the signs of both, gives −25.3817°, or 334.6183° once 360° is added. That is not yet the answer. The equation has two solutions 180° apart, the ascendant and the descendant, and this first one is the descendant: the degree setting in the west. Adding 180° gives the ascendant, 154.6183°. To tell the two apart without a diagram, the true ascendant is the root for which −cos L × sin θ + sin L × cos ε × cos θ comes out positive.

Step 6: Subtract the Ayanamsa

The longitude from step 5 is tropical, measured from the equinox. An Indian chart measures from a fixed star point, and the Lahiri ayanamsa for this birth is 23.7523°. 154.6183° − 23.7523° = 130.8660°. Thirty degrees to a sign, counting from Aries, puts that at 10°51′ of Leo: a Leo lagna, 10.9° from the nearest sign edge. The tropical figure alone would have said Virgo. What the ayanamsa is and why it grows each year is explained in the Lahiri ayanamsa and what it shifts.

Four Mistakes and What They Do to This Chart

Run the same birth through the same formula with one input wrong, and the cost of each slip is plain:

Skipping the conversion to UT is the costly one: five and a half hours of the sky’s turning moves the ascendant 72.1° here and lands it in Libra. Using the IST meridian instead of the birthplace shifted this lagna by 4.6°, which kept it in Leo; for a birthplace west of 82.5° E, as Delhi is, the error pushes the lagna later, so a birth within about 4.6° of the end of its sign would change sign. The descendant mistake always gives the opposite sign, and leaving out the ayanamsa gives the tropical rising sign, which Western astrology uses and an Indian chart does not.

A Lagna Table by Sidereal Time

Steps 5 and 6 can also be replaced by a table, a lagna sarani, drawn up for one latitude, giving the local sidereal time at which each sign begins to rise. With the local sidereal time of a birth from step 4, you read down the column to the last sign that has started. The table below is computed for 1 January 2026 at three cities in the site’s city list. Times are local sidereal time, hours and minutes, and each sign holds until the next row’s time; the clock wraps past 24:00 between Gemini and Cancer in all three columns.

The worked birth’s local sidereal time, 04:04, falls after Delhi’s Leo row and before the Virgo row, which is the answer step 6 gave. Two things in the table are worth noticing. Latitude matters: the three cities put the starts of Gemini and Sagittarius about half an hour of sidereal time apart, while the starts of Virgo and Pisces differ by no more than 3 minutes, because those two sidereal degrees lie close to the equinox points, where the zodiac crosses the celestial equator. And the table ages: the same Delhi column computed for 1976 runs 2.0 to 3.3 minutes earlier, because the ayanamsa has grown in the 50 years between. A printed table is only as good as its latitude and its year.

None of this applies to the hora, ghati and bhava lagnas. Those start from the Sun’s degree at sunrise and advance at a fixed rate with the time elapsed since, without any further horizon geometry; their arithmetic is on hora lagna, ghati lagna and bhava lagna.

How Exact the Inputs Must Be

The ascendant moves through all twelve signs in one sidereal day, about a degree every four minutes on average, faster in some signs and slower in others. So the birth time and the birthplace decide the precision, not the formulas: rounding to the arcminute anywhere in the six steps moves the lagna by no more than about an arcminute. How often the lagna and navamsa change measures how long the lagna and each divisional lagna hold before the next sign takes over. When a lagna comes out within a degree or two of a sign edge, check the birth time before relying on the sign.

To check a hand calculation, enter the same birth in the ascendant calculator, which runs these steps for whichever listed city you pick as the birthplace. How the lagna differs from the Moon sign is set out in rashi and lagna: the difference. In Hindi: लग्न कैसे निकालें: स्थानीय नाक्षत्र समय से.

In Brief

• Step 1: IST minus 5 h 30 m gives UT; watch the date for births before 05:30
• Steps 2–3: days from 1 January 2000, 12:00 UT, then Greenwich sidereal time, reduced to 0°–360°
• Step 4: add the east longitude; four minutes of sidereal time per degree
• Step 5: tan L = −cos θ ÷ (cos ε sin θ + sin ε tan φ), keeping the eastern root
• Step 6: subtract the Lahiri ayanamsa and count signs from Aries
• Table shortcut: sign starts by sidereal time, for one latitude and one epoch