The line separating the sunlit and shadowed sides of the Moon.
- In theory, the terminator is close to a great circle at 90° from the solar direction. However it is not a sharply-defined line: first because the Sun has a finite width (of about 0.5° as seen from the Moon); and second because variations in the surface topography cause some high points to catch light well beyond the theoretical limit, and some low spots to be in shadow well before it. As a consquence, the observed terminator is a broken patchwork of isolated areas of light and shadow.
- Unlike the limb, at a given instant the terminator is seen at the same position (relative to the lunar surface features) by all observers.
- The location of the terminator relative to the surface features is totally determined by the location of the subsolar point. The theoretical terminator (for an idealized smooth spherical Moon) is usually taken, as described above, as the great circle at 90° from this point; but it is sometimes shown (as in LTVT) corrected for the finite size of the Sun.
- The location of the terminator is frequently expressed by stating the Sun's colongitude; but because the terminator can be significantly inclined relative to meridians of constant longitude, a complete description requires stating the Sun's latitude north or south of the equator as well. By definition, colongitude is not corrected for the finite size of the Sun or variations in topography.
- The terminator is sometimes divided near the Moon's poles into two halves. Along one of these halves the sun angle over the surface features is increasing, while along the other the sun angle is decreasing. The former is referred to as the Morning Terminator (the line of sunrise) and the latter as the Evening Terminator (the line of sunset). Both of these sweep across the surface features from east to west.
- See also: Saber's Beads, which is a curious optical phenomenon related to the very young (and extremely thin) moon's crescent, or the very old (immediately after and/or before New Moon). The location of the morning's or evening's terminator near the moon's eastern (or western limb) plays a crucial role! - DannyCaes Feb 12, 2008
Twirling World (J-P Metsavainio's rather "unusual" look at the moon's terminator)
- All books about the moon and lunar observing.