Month computations were based on solar transits through the zodiac. The Sun transits this sign from approximately April 21 until May 21 in western astrology. Very rarely, Saturn transits the Crab Nebula. The time when the Sun transits the observer's meridian depends on the geographic longitude. Under the tropical zodiac, the sun transits this sign between about May and June. Under the tropical zodiac, the sun transits this area between February 19 and March 20. The Sun transits this sign on average between September 23 (September equinox) and October 22. Thus after 243 years the transits of Venus return. Transits on Mercury are much more common. There are on average 13 transits of Mercury each century. Several of these transits have been photographed by the Mars Rover "Opportunity". These birds are known for their extraordinary flight skills, flying during their migratory transits. So doing makes the time interval involved with transits through the apparatus much longer. Sequences of transits usually repeat every 243 years. The mechanism by which acid transits the barrier layer remains uncertain.