Shukla and Krishna paksha
A paksha is a fortnight of the lunar month. Shukla paksha is the bright fortnight, growing toward full Moon; Krishna paksha is the dark fortnight, waning toward new Moon.
What it is
Shukla paksha runs from the day after new Moon (Amavasya) to full Moon (Purnima), tithis 1 to 15. Krishna paksha runs from the day after full Moon to new Moon, tithis 16 to 30, where the 30th is Amavasya.
How it is worked out or read
The paksha follows directly from the elongation: below 180 degrees the Moon is waxing (Shukla), and from 180 to 360 it is waning (Krishna).
Where it appears on this site
The paksha is shown with each tithi in the panchang. The glossary lists the terms.
Why the bright half is not always the same length
Each paksha has fifteen tithis, but the calendar days of each half vary because the tithis vary. The bright half may run a day longer or shorter than the dark half in a given month. This is why a festival tied to the dark fortnight can fall on different calendar dates in different years. The paksha is a count of tithis, not a count of days, and the reader should remember that difference when planning anything that depends on the phase of the Moon.
The phase and the sky
Traditionally the bright half is the time when the Moon grows from new to full, and the dark half is the time it wanes. Looking at the Moon each evening, a reader can check the phase against the panchang. In the first days of the bright half, the Moon is a thin crescent low in the west after sunset. Near full, it rises at sunset in the east. These observations are a simple way to confirm that the tithi count matches the sky.
Paksha in the regional calendars
Different regions in India count the lunar month differently, as explained in the lunar month article. In the amanta system the dark fortnight ends the month, and in the purnimanta system it begins the next month. The paksha itself is the same in both, but its position in the month differs. A reader who moves between regions should note which system a date belongs to before comparing it with another calendar.
The paksha in ritual calendars
Many ritual calendars give the paksha as a condition for festivals and observances. A reader planning around a paksha should check whether the observance uses the bright or the dark fortnight, and in which calendar system, since the two can differ at the month's start.
Bright and dark halves in the year
Over a year the bright and dark halves alternate twelve or thirteen times, and their rhythm shapes the calendar's sense of waxing and waning. A reader who notices this rhythm can see the seasons of the Moon as a regular pattern rather than as isolated dates.
The word paksha
Paksha means side or wing, and the two fortnights are the two sides of the lunar month.