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True solar time: relate clock time to the Sun at a location
Civil time lets a whole time zone share one clock. Apparent solar time depends on local longitude and the Sun's observed position. They are not the same time scale, so a conversion must preserve both the original time and its method.
The short answer
A true-solar-time conversion generally needs the longitude offset from the time-zone meridian and the date's equation of time, after historical time-zone and daylight-saving rules are resolved.
Four required inputs
- Original civil date and time
- Precise latitude and longitude of the birthplace
- Time zone and historical daylight-saving rule in force
- Equation-of-time algorithm or astronomical source
Why longitude changes local time
Earth rotates about 15 degrees per hour. Locations at different longitudes within one civil time zone therefore have a fixed difference between local mean solar time and the shared clock. This is a time conversion, not a metaphysical conclusion about an address.
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What is the equation of time
The U.S. Naval Observatory defines it as apparent solar time minus mean solar time. Because of Earth's orbit and axial tilt, the difference varies by date and can accumulate to more than ten minutes during the year.
Using it in a chart
Traditional methods do not all apply solar time in exactly the same way. A calculator should state whether it is enabled, show before/after times and name the method rather than silently changing a birth time.
Continue reading
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Frequently asked questions
Is true solar time the same as time-zone time?
No. Time-zone time is a civil standard; true solar time depends on the Sun's position at a specific longitude.
Can conversion change the hour pillar?
It may if the converted time crosses a traditional two-hour boundary, which is why before/after times and the method must remain visible.