What is a moon phase?
The Moon does not produce light. What we see is sunlight reflected off its surface, and how much of the lit half faces us depends on where the Moon sits relative to the Sun.
That relationship is measured as elongation — the Moon's angular distance ahead of the Sun, from 0° to 360°. At 0° the Moon is between us and the Sun and we see nothing: New Moon. At 180° it is opposite and fully lit: Full Moon. Everything else is in between.
The complete cycle takes about 29.5 days and is conventionally divided into eight phases of 45° each, lasting roughly 3.7 days apiece.
The eight phases
- New Moon (337.5–22.5°) — the dark. Beginnings, instinct, unformed potential.
- Waxing Crescent (22.5–67.5°) — first light. Momentum, effort, emerging from what came before.
- First Quarter (67.5–112.5°) — half lit. Crisis of action, decision, the will to force an issue.
- Waxing Gibbous (112.5–157.5°) — nearly full. Refinement, analysis, the last ten per cent.
- Full Moon (157.5–202.5°) — complete. Clarity, relationship, seeing yourself through others.
- Waning Gibbous (202.5–247.5°) — first fading. Communication, teaching, passing on what was learned.
- Last Quarter (247.5–292.5°) — half lit again. Crisis of conscience, revision, letting go on principle.
- Waning Crescent (292.5–337.5°) — last light. Completion, intuition, release.
The same measurement, counted in Indian terms
Indian astrology measures exactly the same angle, but divides it into thirty tithis of 12° each rather than eight phases of 45°.
The fifteen tithis of the waxing half make up Shukla Paksha, the bright fortnight, ending at Purnima — the full moon. The fifteen waning tithis make up Krishna Paksha, the dark fortnight, ending at Amavasya — the new moon.
If you have ever fasted on Ekadashi, celebrated Purnima, or been told a date was chosen for being an auspicious tithi, that is this same measurement. Your report gives both readings side by side, because seeing the Western phase and the Vedic tithi describe one moment makes the relationship obvious in a way no explanation does.
Why the illumination figure is worth having
“First Quarter” covers a 45-degree band and nearly four days. Within it the Moon goes from about 15% lit to about 65% lit — a visibly different sky.
So the actual percentage is computed, using illumination = (1 − cos θ) ÷ 2, where θ is your birth elongation. The disc drawn in your report uses that figure: the terminator is an ellipse whose width follows the cosine, which is why it flattens to a straight line at exactly the quarters. It is your Moon, not a stock image of a moon.
Next new and full moon
Your report also gives the exact dates of the first full moon and first new moon after your birth. These are solved by finding the precise instant the elongation reaches 180° and 0°, not by adding an average 29.53-day month — the real interval between new moons varies by up to about thirteen hours, because both orbits are elliptical.
Does the birth moon phase mean anything?
In Western astrology the birth phase is read as a stage in a cycle of development — the New Moon type initiates, the Full Moon type sees whole, the Last Quarter type revises. It is a well-established minor technique rather than a central one.
In Vedic astrology the tithi carries more practical weight: it is used in choosing auspicious moments, in fasting observances, and as one input to compatibility. Neither system treats the phase as decisive on its own, and it would be overstating it to suggest otherwise.
Frequently asked questions
What moon phase was I born under?
Enter your birth date, time and place above. The calculator computes the Sun–Moon elongation for that exact moment and returns the phase, the illumination percentage, and the corresponding Vedic tithi.
Do I need my exact birth time?
It refines the answer rather than deciding it. The Moon gains about 12 degrees a day on the Sun, so a phase lasts nearly four days — the date alone almost always gives the right phase, and the time sharpens the illumination percentage.
What is the difference between a moon phase and a tithi?
They measure the same thing — the angle between the Sun and the Moon — divided differently. Western astrology uses eight phases of 45 degrees; Indian astrology uses thirty tithis of 12 degrees. Your report gives both.
What are Shukla and Krishna Paksha?
The two fortnights of the lunar month. Shukla Paksha is the waxing half, from new moon to full. Krishna Paksha is the waning half, from full moon back to new.
Why is my illumination not exactly 50% at the quarter?
Because “First Quarter” names a 45-degree band, not a single instant. Exactly 50% occurs at 90 degrees elongation, which is one moment inside a phase lasting nearly four days.
Is the moon image real?
It is drawn from your data rather than photographed. The lit region is computed from your birth elongation, with the terminator as an ellipse whose width follows the cosine of that angle — which is why it becomes a straight line at exactly the quarters.
