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An equinox occurs when the angle formed at the Earth's center between its axis of rotation and the line joining the Earth to the Sun is a right angle. At such a point in the Earth's orbit the length of day and night is equal. The northern vernal equinox (that is, the vernal equinox in the northern hemisphere) occurs around March 21st of each year, and the northern autumnal equinox occurs around September 21st. In the southern hemisphere these dates are reversed. These dates are independent of the motion of the Moon.
A solstice occurs when this angle reaches a maximum or a minimum. At such a point the duration of the day and the night is either longest or shortest. The northern winter solstice occurs around December 21st of each year, and the northern summer solstice occurs around June 21st. In the southern hemisphere these dates are reversed. (For more information see Equinoxes, Solstices and Cross-Quarter Days.)
A dark moon occurs when the Sun and the Moon are astronomically conjunct (or more exactly, when either the Moon's center lies on the line joining the centers of the Earth and the Sun or the plane defined by the Sun, Earth and Moon is perpendicular to the Earth's orbital plane).
The term "new moon" is not used here, since it is ambiguous. It can mean either a dark moon or the phase of the Moon when a crescent is first visible (in which sense a month in the Muslim calendar begins at new moon).
A lunation is a passage of the Moon from one dark moon to the next. A lunation begins at the dark moon (astronomical conjunction of Sun and Moon), and the next dark moon marks the beginning of the next lunation.
A date and time displayed by the Chinese Calendrics program identifies a moment within a particular lunation, and the number of that lunation is shown in square brackets [] thus:
Lunations are numbered -1, 0, 1, 2, ... in temporal order. Lunation number 0 began with the dark moon at 18:13 GMT on 2000-01-06, following the numbering used by Jean Meeus (the lunation numbers LN are such that LN = Brown Lunation Number - 953 and LN = Goldstine Lunation Number - 37105).
The age of the Moon is the time since the previous dark moon. The percentage in parentheses gives the position of the Moon in the current lunation. Full moons usually do not occur at exactly halfway through the lunation, but usually somewhere between 48% and 52%. Dark moons, by definition, occur at exactly 0%.
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