Moon Phase and Tithi

What this page adds to the calculator above

The calculator above gives five values for a date: a tithi from 1 to 30, a paksha, an elongation in degrees, one of eight phase names and an illuminated percentage. The notes under it already explain the arithmetic in brief and three limits. This section goes further. It sets out where the tithi and the paksha sit in the classical texts this app is built on, follows the calculation through the real code with the actual intermediate numbers, shows how every possible result should be read, and maps which other parts of the app use the same lunar day. Everything classical is cited to Brihat Parashara Hora Shastra (BPHS) or Saravali, and every number comes from running the engine, as described on the methodology page.

One idea runs through the whole page. The five values are not five measurements. They are one measurement, the angle between the Moon and the Sun, reported five ways. Once that is clear, results that look contradictory at first, such as a Full Moon label on a Krishna paksha day, stop being puzzling.

The tithi and the paksha in the classical texts

A month of thirty lunar days

Saravali Ch. 7 Sl. 4 states the frame the engine uses: each lunar month begins with the first day of the bright half and ends with the thirtieth tithi, Amavasya. That is exactly the numbering this calculator returns. Tithi 1 is the first lunar day after the Sun and Moon meet, tithi 15 closes the bright half at the full Moon, tithi 16 opens the dark half, and tithi 30 is Amavasya. BPHS treats the paksha and the tithi as part of the basic identity of a birth: in its chapter on lost horoscopes (Ch. 78 Sl. 3-4) Parashara lists the year, the half-year, the season, the month, the Paksha, the Tithi, the Nakshatra, the Lagna and the Rashi together as the things one needs to know about a nativity.

Waxing and waning as benefic and malefic

The most used classical consequence of the paksha is the nature of the Moon itself. Saravali Ch. 4 Sl. 9 names Jupiter, Mercury and Venus as natural benefics, Saturn, Mars and the Sun as natural malefics, and then adds that the waning Moon is also counted with the malefics. Santhanam's note in BPHS Ch. 3 puts it the same way: the waning Moon of Krishna Paksha is a malefic and the waxing Moon of Shukla Paksha a benefic. The same note adds an important softening - a Moon joined with a benefic, or aspected by one, turns benefic even when waning. So the paksha is a starting condition, not a final verdict. The wider picture of the Moon as a graha is on the Moon in Vedic astrology page.

BPHS Ch. 3 Sl. 35-38 extends the idea beyond the Moon: during the dark half the malefics are strong, and the benefics acquire strength in the bright half. Ch. 10 Sl. 5 uses the paksha together with day or night birth, saying that a benefic's aspect on the Lagna protects one born at night in the bright half, and a malefic's aspect does the same for one born by day in the dark half. In both verses the paksha is a switch that decides which class of graha is working at its best.

Paksha Bala: the same angle as a strength

BPHS Ch. 27 Sl. 10-11 turns the Sun-Moon angle into a number. Deduct the Sun's longitude from the Moon's; if the result exceeds six signs, deduct it from twelve; convert to degrees and divide by three. That is the Paksh Bal of each benefic, and sixty minus it goes to each malefic. Sl. 18 adds that the Moon's Paksh Bal is itself her Chesht Bal, her motional strength. The first two steps are the elongation this calculator prints, folded so that it never exceeds 180. For the worked example below, elongation 90.28 divided by three is 30.09 on a scale whose top is 60 - a hand calculation from the verse, not a figure the app returns. The app's Shad Bala calculator does not compute it: the code comment in its Kala bala step says the paksha half is held at a fixed 0.5 baseline. The elongation on this page is therefore the place to see the raw quantity.

Where the texts disagree: two halves or three thirds

The sources do not all cut the month the same way, and the difference matters for how a waning Moon is read. Saravali Ch. 5 Sl. 16-17 reports the view of the elder Yavanas: the Moon is of medium strength for ten days from the first of the bright half; exceedingly strong for the next ten, from the eleventh of the bright half through the fifth of the dark half; and of very little strength from the sixth of the dark half to Amavasya. Santhanam's note in BPHS Ch. 3 gives the same view in degrees - within 120 of the Sun medium, 120 to 240 very auspicious, 240 to 0 bereft of strength - and says plainly that this concerns the Moon's strength, while the waxing and waning rule concerns whether she is benefic or malefic.

The third row is where the two schemes part. A Moon at tithi 17 is waning, so by Saravali Ch. 4 she is counted with the malefics, yet by the Yavana view in the next chapter of the same book she is in her strongest ten days. Both statements are in the sources. Neither is an error; they answer different questions, and a reading that quotes only one of them is leaving something out.

The dark end of the month

BPHS Ch. 85 Sl. 1-4 lists circumstances in which a birth is called inauspicious even with a well-placed Lagna, and the list includes Amavasya, the fourteenth tithi (Chaturdasi), Krishna Paksha and Tithikshaya. The same passage says there are remedial measures, and Ch. 86 is devoted to them. This page reports that the list exists because it is in the primary source; it does not treat it as a verdict on anyone. The calculator raises no flag for these tithis, the app prescribes no remedies, and the softening rule quoted above from Ch. 3 - a waning Moon with a benefic turns benefic - comes from the same book.

BPHS Ch. 92 Sl. 1-2 names a narrower junction, Tithi Gandanta: the last two ghatikas of the Purna tithis (the 5th, 10th and 15th) and the first two ghatikas of the Nanda tithis (the 1st, 6th and 11th). A ghatika is a sixtieth of a day. The calculator does not test for it. A birth time and the elongation shown above are enough to see how near a boundary a birth fell, which is discussed further down.

How the engine computes it, step by step

The handler does very little: it reads the date, turns it into one instant, and calls a single engine function, lunarDay. That function takes the Moon's longitude and the Sun's longitude from the same astronomy module that drives the birth chart, the Moon sign and the Sun sign calculators. The module header states its method and accuracy: standard Meeus algorithms, the Sun to about 0.01 of a degree and the Moon to about 0.1 of a degree.

For 14 August 1994 with no time, the instant is 12:00 India time, which is 06:30 UTC and Julian Day 2449578.7708. At that instant the engine places the Moon at tropical longitude 231.5354 and the Sun at 141.2543. The Lahiri ayanamsa for the day is 23.7779 (the ayanamsa calculator returns it for any date), giving sidereal longitudes of 207.7575 and 117.4764. Subtracting, the elongation is 90.28. The same subtraction on the tropical pair gives the same 90.28, because one ayanamsa is removed from both. This is why the tithi, unlike a sign or a nakshatra, does not depend on which ayanamsa an app uses.

1. 1.Tithi: 90.28 divided by 12 is 7.52; rounded down and increased by one, tithi 8.
2. 2.Paksha: 8 is not above 15, so Shukla.
3. 3.Phase name: 90.28 plus 22.5 is 112.78; divided by 45 and rounded down, 2; the third name in the list of eight, First Quarter.
4. 4.Illumination: the cosine of 90.28 degrees is about -0.005; one minus that, halved, is 0.502, shown as 50.2 percent.

Reading each of the five values

Elongation

This is the measurement; the other four are labels cut from it. It runs from 0 to 360 and only ever increases until it wraps. At 0 the Moon and Sun share a longitude, at 180 they are opposite. Below 180 the Moon is waxing, above it waning. If you keep one number from this page, keep this one, because it tells you how far into its tithi and its phase band the reading is, and it lets you redo any classical rule that is stated in degrees.

Tithi

The tithi is the twelve-degree slice the elongation falls in, numbered 1 to 30 from conjunction. The calculator returns the number only, not a name. To convert to the usual count within a fortnight, tithis 16 to 30 are the first to fifteenth of the dark half: subtract 15. Tithi 23 is the eighth of Krishna Paksha, tithi 29 its fourteenth (the Chaturdasi of BPHS Ch. 85), tithi 30 Amavasya.

Paksha

Shukla for tithis 1 to 15, Krishna for 16 to 30. The switch from Shukla to Krishna happens at exactly 180 degrees and from Krishna back to Shukla at 0. This is the value the classical rules above use: benefic or malefic Moon, which class of graha is stronger, and the Shodshottari condition described later.

Phase name

The eight names are the familiar Western ones, each covering a 45-degree band centred on 0, 45, 90 and so on. They are a convenience for readers who think in quarters and crescents; no classical rule in this app reads them. Because a band is centred on its angle while a paksha begins at it, the two do not line up. The Full Moon label covers 157.5 to 202.5, so it includes the end of the bright half and the start of the dark half. A result of Full Moon with Krishna paksha is correct, and means the Moon is just past opposition. In the same way New Moon covers 337.5 through 0 to 22.5 and can appear with either paksha.

Illumination

The percentage is one minus the cosine of the elongation, halved. It is symmetric: 50.2 percent appears at 90.28 on the waxing side and again near 269.72 on the waning side, so the percentage alone never tells you the paksha. It also moves unevenly. Near new and full Moon a whole tithi changes it by about one point; around the quarters a tithi changes it by more than ten. A reading of 99 percent therefore spans several days, while 50 percent pins the moment down closely.

The five classical tithi groups

Inside each fortnight the fifteen tithis are sorted into five named groups of three. The engine holds this grouping in a table called TITHI_TYPES; its own comment attributes it to Muhurta Chintamani and standard panchang convention, a text that is not among the books in this app's source folder. Two of the groups are independently confirmed by BPHS Ch. 92, which names the 1st, 6th and 11th as Nanda tithis and the 5th, 10th and 15th as Purna tithis, and BPHS Ch. 89 speaks of a day free of Rikta-Bhadra Dosha, which presumes the Rikta and Bhadra groups without listing them.

This calculator does not print the group, but you can read it from the table. Inside the app, a saved profile's reading passes the birth tithi through a function called buildPanchangMeaning, which returns the group, a one-line paksha reading and the weekday lord. For the example date it returns Jaya for tithi 8, describes Shukla Paksha as classically read as a naturally building, forward-reaching mindset, and names the Sun as lord of the birth weekday, a Sunday. For a Krishna Paksha birth the wording is a naturally consolidating, inward-reaching mindset. These are descriptions of temperament offered as classical reading, not predictions.

A companion function, comparePanchang, sets today's tithi group against the birth one. Its comment is worth quoting in spirit: the classical sources give avoid-lists for particular activities, not a general conflict verdict between two tithis, so the function only reports a match of group or weekday and otherwise says the day is neutral. It does not invent tension between a birth tithi and a current one.

A worked day: 14 August 1994, hour by hour

The example under the calculator shows the noon reading and a day range of tithi 7 to tithi 9. Running the same date through the real handler with a birth time fills in what happened between.

The elongation grew by 3.29 degrees every six hours, a little under 0.55 of a degree an hour. Scanning lunarDay minute by minute places the boundaries: tithi 8 began at 00:33 India time on the 14th and tithi 9 began at 22:27 the same evening. Tithi 8 lasted 21.90 hours and fitted entirely inside one civil date, with half an hour of tithi 7 before it and an hour and a half of tithi 9 after it.

Three practical readings follow. First, the day range shows only the two ends of the day, 7 and 9; the tithi that covered almost the whole date, 8, appears only as the headline value. When the range skips a number, the skipped one is usually the answer for most birth times. Second, the calculator's flag that the tithi changed during the day is true here even though 22 of the 24 hours were tithi 8, so a changed flag is a reason to supply a time, not a sign that the headline is likely wrong. Third, the illumination moved from 44.5 to 56.0 percent across the date, so a percentage from a date-only reading is good to roughly six points either way near the quarters.

How long a tithi really lasts

A tithi is twelve degrees of angle, not a fixed number of hours, because the Moon's speed changes through its orbit. In the engine's scan from 1 August to 13 September 1994 the shortest tithi ran 21.18 hours (tithi 3 in early September) and the longest 26.58 hours (tithi 22 in late August). From the start of one tithi 1 on 7 August to the start of the next on 5 September was 29 days 9 hours 48 minutes, an average of about 23.5 hours per tithi for that month.

• A tithi shorter than a day can begin and end inside one civil date, as tithi 8 did above. A date-only reading then spans three tithis.
• A tithi longer than a day can cover a whole civil date. Only then does the day range report no change, and the date alone is enough.
• BPHS Ch. 85 mentions Tithikshaya among special birth conditions without defining it; the calculator does not detect it or name it.

Why a printed panchang may show a different tithi

This calculator reads the tithi at one instant: the birth time, or noon when there is none. A panchang names the tithi that is running at local sunrise and keeps that name for the whole day. The app's Daily Panchang follows the panchang rule whenever a place is given. For 14 August 1994 at Delhi it finds sunrise at 5:50 AM, reads the elongation there as 86.90 and returns tithi 8. With no place it has no sunrise to use; it reads a typed date at 12:00 UTC, gets 93.29, and says in its own output which basis it used.

On that date all the readings agree on tithi 8. They would not agree for a birth late that night: at 23:59 this calculator returns tithi 9, the tithi actually running at the moment of birth, while a panchang for the 14th still says tithi 8 because that was the tithi at sunrise. Neither is wrong. The panchang convention answers which lunar day the civil day belongs to, and is what festival dates and the day's windows such as choghadiya and Rahu Kaal hang from. The instant reading answers where the Moon stood relative to the Sun when you were born, which is what the Paksha Bala verse and the paksha rules need.

Limits and sensitivity to birth time

• Time. Of the calculators that work from a date alone, this is among the most time-sensitive. A sign lasts the Moon about two and a quarter days and a nakshatra about one, but a tithi lasts less than a day about as often as more. Expect the day range to report a change on most dates.
• Time zone. With no zone the engine assumes India time and flags it. A birth outside India read on the India clock is read several hours away from the true local noon, which moved the angle by about five degrees in the runs above.
• Accuracy near a boundary. The module header gives the Moon to about 0.1 of a degree. At the example's rate of 0.55 of a degree an hour, a tenth of a degree is about eleven minutes. A birth within a quarter of an hour of a tithi boundary, or of 180 degrees where the paksha flips, should be treated as undecided by this engine.
• Paksha is sturdier than tithi. It changes only twice a month. Unless the elongation is within a few degrees of 0 or 180, a date-only paksha is safe.
• Illumination is geometric. It comes from the longitude difference alone, as the note under the calculator says, so it is an idealised percentage.
• No naming and no flags. The result carries no tithi name, no tithi group, no gandanta test and no judgement of the tithi as good or bad.

If the birth time is unknown, the honest result is the pair of tithis in the day range plus the headline between them. The article on what still works without a birth time covers the same situation for the rest of the chart.

Where the rest of the app uses the same lunar day

The function behind this page is shared. Several features call lunarDay for the birth instant or for the present moment, and each uses only part of what it returns. Knowing which part helps you read those features.

Shodshottari dasha

BPHS Ch. 46 Sl. 23 says the Shodshottari dasha may be adopted for a birth by day in Krishna Paksha or by night in Shukla Paksha, and Sl. 24-26 add a second case: Lagna in the Moon's Hora with a Krishna Paksha birth, or in the Sun's Hora with a Shukla Paksha birth. The alternate dashas calculator tests both sentences, and the paksha it uses is the one on this page, taken at the birth instant. Day or night comes from the Sun's position at the birth place, so without a place the first case cannot be decided and the engine says so. The main timeline remains Vimshottari, which BPHS itself calls the most appropriate.

The Moon's dasha and antardasha

BPHS conditions several dasha results on the Moon's light. Ch. 47 Sl. 16-22 describes the good results of the Moon's own dasha when she is, among other things, fully powerful, and Sl. 23-26 say that a waning or debilitated Moon brings loss of wealth in her dasha. Ch. 54, on the Moon's antardasha within Mars's dasha, says the good effects are realised in full if the Moon is waxing and that a waning Moon reduces them to some extent. The phrase to some extent is the text's own, and it is a fair guide to weight: paksha modifies a reading, it does not reverse it. The Moon Mahadasha page takes this further.

Muhurat scoring

The muhurat calculator and the Muhurat feature read the tithi of the candidate moment, not of the birth. A window loses 15 points if its tithi is one of 4, 8, 9, 14, 19, 23, 24 or 29 - the Rikta tithis and the eighth of each half - and gains 5 otherwise; Shukla Paksha adds 7 and Krishna Paksha subtracts 4. The engine's own audit comment is candid that this avoid-list cites no text among the app's source books, and that Amavasya (30) is not on it because adding it would be a new rule without a source. So the example's tithi 8 is a Jaya tithi as a birth tithi and an avoided tithi for scheduling. The two uses are separate: one describes a nativity, the other grades a moment for starting something.

The daily reading

The daily feature calls lunarDay for today. The paksha nudges the day's energy score, which runs from 1 to 10, by plus 0.5 in Shukla Paksha and minus 0.3 in Krishna Paksha, on the stated reasoning that waxing supports action and waning supports completion. When the paksha is Krishna and the tithi is 27 or higher it adds a recommendation that frames the days up to Amavasya as a window for finishing rather than starting, and quotes the tithi and illumination as its evidence. That is a small, declared weight, not a forecast.

The Mental Clarity index

One of the app's composite indices gives 15 of its 100 points to the natal Moon's light, on the stated ground that a dark Moon is classically a weak Moon and that the app's Shad Bala leaves this component out. It measures light differently from this page: one minus the distance from 180, divided by 180, which rises in a straight line. That is the shape of the BPHS Paksha Bala formula, not of the visible disc. The two agree at the quarters and at new and full Moon and differ between them: at an elongation of 36 the straight-line figure is 20.0 percent (hand arithmetic from the formula) while this calculator shows 9.5. If the two percentages for your chart do not match, that is the reason.

The Moon's other coordinates

The tithi says nothing about where in the zodiac the Moon is. Two people born on the same tithi a month apart have Moons in different signs. For the sign use the Moon sign calculator, and for the star the nakshatra calculator; how those two fit together is explained in Moon sign and nakshatra. Because the tithi is free of the ayanamsa and the nakshatra is not, two apps can agree on your tithi and still differ on your star, a case covered in why apps disagree on your nakshatra.

Questions people ask about the Moon phase

My result says Full Moon but Krishna Paksha. Is that a mistake?

No. The Full Moon label covers elongations from 157.5 to 202.5, centred on opposition, while Krishna Paksha begins exactly at 180. Between 180 and 202.5 - all of tithi 16 and most of tithi 17 - both are true: the disc is still 96 percent or more lit and the Moon has started to wane.

The day range shows tithi 7 and tithi 9. Which one am I?

Probably neither. The range reports the tithi at 00:00 and at 23:59 local time. When it skips a number, a short tithi began and ended inside the date, and that middle tithi - the headline value - covered most of the day. On 14 August 1994 tithi 8 ran from 00:33 to 22:27 India time. Only a birth time settles it.

Does being born in Krishna Paksha mean a weak or harmful Moon?

The texts do not say that so simply. Saravali Ch. 4 counts the waning Moon with the malefics, but the Yavana view in Saravali Ch. 5 calls the Moon exceedingly strong up to the fifth of the dark half, and the note in BPHS Ch. 3 says a waning Moon joined or aspected by a benefic turns benefic. Paksha is one condition among several, and BPHS Ch. 54 describes its effect as a reduction to some extent.

Why does my panchang app give a different tithi for my birthday?

Most likely because it names the tithi at sunrise for your place, while this page reads the tithi at your birth instant or at noon. For a birth late in the day the two can differ by one. The Daily Panchang on this site uses the sunrise rule when you give a place.

Does the ayanamsa change my tithi?

No. The same ayanamsa is subtracted from the Moon and from the Sun, so it cancels in the difference. In the example the tropical pair 231.5354 and 141.2543 and the sidereal pair 207.7575 and 117.4764 both give 90.28.

Is tithi 15 the exact full Moon?

Tithi 15 is the last twelve degrees before opposition, 168 to 180, and tithi 16 the first twelve after it. Exact opposition is the boundary between them, where the engine's illumination reaches 100.0. Both tithis read 98.9 percent or higher.

Can two people born on the same date have different tithis?

Yes, in two ways. Their birth times can fall either side of a boundary on the same clock, and the same calendar date begins hours apart in different time zones. Read at local noon, 14 August 1994 gave an elongation of 90.28 in India and 95.48 in New York.

Does the calculator tell me whether my tithi is auspicious?

No. It returns the five values and the day range and nothing else. The tithi groups and the classical passages on this page are there so you can see what the sources say and where they stop; the app does not grade a birth tithi and does not prescribe remedies for any tithi.

How close to a boundary is too close to trust?

The engine's Moon is good to about a tenth of a degree, which at the example's speed is about eleven minutes of time. If the elongation is within roughly 0.15 of a multiple of 12, or your birth time is rounded to the nearest quarter hour and falls within that of a boundary, treat the tithi as one of two.

Is the illuminated percentage what I would have seen in the sky?

Closely, but it is an idealised figure from the longitude difference alone. It also cannot tell you whether the Moon was above the horizon at that hour, which this calculator does not compute.