Landscape by Month. A month is a unit of time, used with calendars, which is approximately as long as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of Moon phases; such months are synodic months and last approximately 29.53 days. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth-Sun line, are still the basis of many calendars today, and are used to divide the year. Main article: Lunar month The following types of months are mainly of significance in astronomy, most of them first recognized in Babylonian lunar astronomy. The sidereal month is defined as the Moon's orbital period in a non-rotating frame of reference. It is about 27.32166 days. It is closely equal to the time it takes the Moon to pass twice a fixed star. A synodic month is the most familiar lunar cycle, defined as the time interval between two consecutive occurrences of a particular phase as seen by an observer on Earth. The mean length of the synodic month is 29.53059 days. Due to the eccentricity of the lunar orbit around Earth, the length of a synodic month can vary by up to seven hours. The tropical month is the average time for the Moon to pass twice through the same equinox point of the sky. It is 27.32158 days, very slightly shorter than the sidereal month days, because of precession of the equinoxes. An anomalistic month is the average time the Moon takes to go from perigee to perigee-the point in the Moon's orbit when it is closest to Earth. An anomalistic month is about 27.55455 days on average. The draconic month, draconitic month, or nodal month is the period in which the Moon returns to the same node of its orbit; the nodes are the two points where the Moon's orbit crosses the plane of the Earth's orbit. Its duration is about 27.21222 days on average. A synodic month is longer than a sidereal month because the Earth-Moon system is orbiting the Sun in the same direction as the Moon is orbiting the Earth. The Sun moves eastward with respect to the stars and it takes about 2.2 days longer for the Moon to return to the same apparent position with respect to the Sun. An anomalistic month is longer than a sidereal month because the perigee moves in the same direction as the Moon is orbiting the Earth, one revolution in nine years. Therefore, the Moon takes a little longer to return to perigee than to return to the same star. A draconic month is shorter than a sidereal month because the nodes move in the opposite direction as the Moon is orbiting the Earth, one revolution in 18.6 years. Therefore, the Moon returns to the same node slightly earlier than it returns to the same star. Further information: lunar calendar and lunisolar calendar At the simplest level, most well-known lunar calendars are based on the initial approximation that 2 lunations last 59 days: a 30-day full month followed by a 29-day hollow month, but this is only roughly accurate, and eventually needs correction by using larger cycles, or the equivalent of leap days. Additionally, the synodic month does not fit easily into the year, which makes accurate, rule-based lunisolar calendars complicated. The most common solution to this problem is the Metonic cycle, which takes advantage of the fact that 235 lunations are approximately 19 tropical years. However, a Metonic calendar will drift against the seasons by about 1 day every 200 years. Metonic calendars include the calendar used in the Antikythera Mechanism about 2000 years ago, and the Hebrew calendar. The complexity required in an accurate lunisolar calendar may explain why solar calendars have generally replaced lunar calendars for civil use in most societies. The Hellenic calendars, the Hebrew Lunisolar calendar and the Islamic Lunar calendar started the month with the first appearance of the thin crescent of the new moon. However, the motion of the Moon in its orbit is very complicated and its period is not constant. The date and time of this actual observation depends on the exact geographical longitude as well as latitude, atmospheric conditions, the visual acuity of the observers, etc. Therefore, the beginning and lengths of months defined by observation cannot be accurately predicted. While some like the Jewish Karaites still rely on actual moon observations, most people use the Gregorian solar calendar. Pingelapese, a language from Micronesia, also uses a lunar calendar.