Tropical month

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A tropical month is the length of time between two consecutive passes of the moon through the hour circle of the vernal equinox . In general, in astronomy, “tropical” is synonymous with “related to the spring equinox”.

Due to the precession of the earth's axis , the vernal equinox moves retrograde (i.e. from east to west) along the ecliptic . Therefore, the moon hangs up his alley during a tropical month - relative to the fixed star background - something less than a full orbit of 360 degrees back. A tropical month is therefore a little shorter than a sidereal month (on average by 6.8 seconds); it lasts on average 27 days 7 hours 43 minutes 4.7 seconds or 27.32158 days (year  2000 ).

This means that a tropical month is also significantly shorter than a synodic month , which is based on the position of the moon relative to the sun.

During one half of a tropical month the moon is nosing , that is, falling, the other half is obsessive , that is, rising; these terms are not to be confused with the phases of the moon increasing and decreasing.

See also


  • Joachim Hermann: dtv atlas for astronomy . 11th edition. dtv, Munich 1993, ISBN 3-423-03006-2 .