The Sun transit time, also referred to as Sun-meridian transit time, is the time at which the Sun culminates the observer's meridian. Most often this time is used as local noon time and therefore will differ from longitude to longitude. For example, on April 29 the sun transit time in Boston, Massachusetts is 12:42 p.m., whereas the sun transit time for New York, New York is 12:53 p.m. This is due to longitudinal distance between the two cities.
For Australian Sun Transit Time Calculators, calculators can be found on this website. http://www.ga.gov.au/geodesy/astro/sunrise.jsp - US Naval Observatory Sun/Moon calculator: http://aa.usno.navy.mil/data/docs/RS_OneDay.php
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References
Sun transit time Wikipedia(Text) CC BY-SA