Lunar month

The lunation (from the Latin luna, moon ') is the variable length of time for one revolution of the moon around the earth, based on its position relative to the sun, and thus the synodic period of the moon. Is measured each time for one complete cycle of the moon phases by a particular new moon to new moon following, when the moon so again is conjunct the Sun. Lunations take different lengths of time, the average value is called mean lunation length and is also called the synodic month.

The term lunar month refers generally to an astronomical lunar orbit with respect to a certain reference point. For the lunar month ( lunar month ), the calendar account can form the basis of various lunar periods.

In obstetrics, the period of 28 days is sometimes called a lunar month, or lunar month and divided a pregnancy in ten such sections.

The astronomical lunar months

Synodic month (average lunation )

Lunations are a characteristic feature of the system Sun-Earth - Moon. In good approximation, the orbit of the moon around the earth and the earth's orbit around the Sun can be represented respectively by Kepler ellipses, and for this idealization allows the synodic period of the moon calculated. For various reasons, the actual duration will vary for a single lunation. The decades -averaged average lunar month ( synodic month ) averaged is currently:

Fluctuations of the lunation period

Since change their orbital speeds of Earth like moon during the rotation (see the second Kepler 's law ), thus also changes the duration of the period of each one new moon to the next or one full moon to the next. The calculation of these appointments and the current lunation period is one of the most complex tasks of the lunar theory and the ephemerides of the moon.

• Annuale (annual ) fluctuations: The plane of the moon's orbit and the Earth's orbit ( the ecliptic ) are relative to the fixed stars ( sidereal ) relatively fixed in space, their inclination to each other of about 5 ° varies only slightly and the counter-rotation of the lunar orbit plane takes about 19 ​​years. In general, the moon is moving faster on its approximately elliptical path when he joined the perigee, perigee, approaches, and slower at apogee, the most distant point on the path. The dates of the new moon and full moon but depend on the position of the sun: At new moon the moon is sonnennah between Earth and the Sun conjunct, the full moon dates reversed sun away in opposition. On the new moon date to move it in the direction of the sun. If even then the perigee direction of the sun, the moon moves faster, and as a result of the stronger gravitational effect of the sun even more quickly than would be expected solely on the Kepler 's laws for the undisturbed two-body Earth-Moon system. In addition, these two form a double system and overwhelmed by the the Earth-Moon gravity ( EMS); however, since ( ecliptic ) Conjunction is using the midpoints of the Earth, Moon and Sun defined term, add or subtract next to those for the web path of the moon even those periods, it needs to not EMS - moon - sun, but center of the Earth - moon - sun are in a line. If the new moon close to the earth, the earth puts this distance back in less time than when the moon is erdfern. Therefore, the lunations reach a minimum when the new moon date and Perigäumsdurchgang coincide, or take shorter and shorter until the date on which the Apsidenline ( the line joining the perigee - apogee ) with the line coincides Earth-Sun. Then the lunation periods during the year to take over and reach a maximum when the full moon falls on the date Perigäumsdurchgang. The moon goes around the earth prograde, in corotating sense as the Earth around the Sun; to new moon it moves, therefore, seemingly against the direction of the earth.
• Fluctuations in the cycle of apses: the perigee of the Moon's orbit ellipse moves in an interval of about 8.85 years around the Earth ( Apsidendrehung of the moon; difference of slightly shorter anomalistic month to the sidereal lunar month ). Therefore, there are over a longer period in some years relatively small, in other years relatively strong differences within the annual fluctuations in the lunation periods. Because when the perigee is run on a date that is in the vicinity of the perihelion of the earth ( the sun near point of the Earth's orbit, to January 3, to which the earth 's fastest moving), the difference between the short lunations is perigäumsnahen the new moons, and the longer the apogäumsnahen new moons damped. Conversely, when the moon be perigee near the aphelion of the earth passes ( about July 5 ), the differences between the short and the long lunations by about twice stronger.
• Other variations: Due to different degrees of perturbations by the other bodies of the solar system deviates the shape of the lunar orbit considerably from the exact ( Kepler ) from ellipse; this is also the lunation period other short-and long -term periodic fluctuations that overlay each other with clearly lying under an hour fluctuation range.

The value of the current lunations varies between about:

The full moon is not in the middle of the lunation margin but always later than this. Depending on the constellation of earth to the sun ( perihelion / aphelion ), but also the position of the lunar perigee to the sun, then pass on further variations in the time interval full moon to new moon dates of appointment. These fluctuations are even larger than that of the new moon date, so you calculate in modern astronomy - no longer based primarily on this issue on observation, but computational models - always the lunations of conjunction to conjunction, although their exact time because he on day sky or at night takes place below the horizon, is not easily measured.

Astrometric new and full moon dates are expected ekliptikal, true new and full moon ( phase angle maximum / minimum, ie minimum / maximum illumination of the moon ) then fluctuate again to the tabulated date and are also available from observation ( topocentric coordinates) addictive, then enter one when observers, moon and sun are in a line (or the distance of the moon from the line of sight observer sun is minimal, exactly in a line they never are - and he is very close, there is a lunar eclipse, therefore, the moon never 100% "full" ). These fluctuations remain under an hour.

The calendrical lunar month

The lunar month is probably - in addition to day and night - the most obvious astronomical amount of time - and should therefore also the most primitive calendar models underlying. Today astronomical lunar calendar, ie those that determine the calendar date after the actual lunations, nurmehr in Saudi Arabia ( the moon sighting Neulichts ) and some indigenous cultures are common. All other cultures that use Lunar calendar to work with an arithmetic calendar system based on the notional size of the synodic month.

The calendar month, BC only by name to do since the introduction of the Julian calendar in the year 46 with the lunations, the phases of the moon no longer correlate with it, and move during one year decline against the monthly data, because a calendar month average lasts, so is almost exactly one day longer than the synodic period of the moon.

The lunation number

The lunations are numbered consecutively in astronomy. This number is called a lunation number and goes back to EW Brown:

Alternatively, today an updated count is used:

The conversion is done by:

The current new moon date can thus be estimated by: