Astronomy Answers: From the Astronomical Dictionary
The description of the word you requested from the astronomical dictionary is given
the First Quarter
The First Quarter is a phase of the Moon, equal to
- [daily life] the period when about half of the lunar disk is
illuminated, about a week after New Moon.
- [astronomy] the instant at which the ecliptic longitude of the
center of the Moon is exactly 90 degrees greater than that of the
center of the Sun.