Astrology wiki

A backward clock change can map one local reading to two instants. The clock record alone cannot choose between them; an offset, occurrence choice or additional historical evidence is needed.

Calendar rules determine the date label. A date-only tool can operate on that label; an astronomical calculation additionally needs enough information to resolve an instant. Atlas states which kind of input each method uses.

Seasonal clock changes are institutional decisions documented by place and year. They are not corrections for the Sun’s actual position, and one country’s schedule cannot stand in for another’s.

The difference between TT and UT1 depends on knowledge of Earth’s rotation. Recent determinations and distant historical reconstructions have different uncertainties; many decimal places do not remove that distinction.

An epoch anchors positions or model parameters to an instant. It must be read with the associated frame and motion model; it does not necessarily identify when an observation was made.

Earth's orbital geometry produces a calculable difference between apparent and mean solar time. Sources may reverse the subtraction order; comparing numerical values requires checking the sign convention before applying the correction.

Gregorian arithmetic follows a defined leap-year rule. Historical documents did not adopt it everywhere at once, so a correctly calculated proleptic date may differ from the date originally recorded.

The epoch is defined at a specified TT instant. A catalogue labeled J2000 still needs its frame and proper-motion information; an epoch label alone cannot place a star at another date.

A continuous day count makes time arithmetic reproducible. The number needs an accompanying time scale and day-boundary convention; it is not itself evidence for a historical calendar identification.

Leap seconds are administrative adjustments linked to Earth-rotation monitoring. Their presence must come from the applicable time standard and date, rather than a repeating rule inferred from calendar leap years.

The Sun's actual apparent position determines this time scale. It differs seasonally from a uniform clock, so a conversion needs the equation of time as well as longitude rather than only a timezone offset.

Longitude sets the offset of a uniform solar clock from Greenwich. This can reproduce a stated historical time basis, but it does not include legal timezone changes or daylight saving and must not be substituted silently for civil time.

An observed lunar cycle and a traditional computational month are related concepts, not automatically identical numerical objects. Phugpa month arithmetic uses the constants and correction rules of its specified model.

A forward clock change can leave a local reading with no corresponding instant. Software should surface that conflict; silently shifting the time adds information the record did not provide.

Extending a calendar’s rules backward creates consistent arithmetic. It does not establish that a historical community actually used those rules or wrote the resulting date.

Sidereal time expresses rotational orientation against a celestial reference direction. Mean and apparent versions differ through nutation conventions; neither is simply another local civil time zone.

TT supplies the time argument for uniform astronomical models. It is not the time shown on an ordinary birth record, and the correct conversion matters when comparing ephemerides.

A zone database records political clock rules, including changes over time. These records can be revised, especially for older dates; a present-day offset is insufficient for historical conversion.

UT1 is determined from Earth-orientation measurements and models. Earth does not rotate perfectly uniformly, so using civil UTC as an exact substitute can affect precise horizon calculations.

UTC is an agreed time standard realized through timekeeping institutions. A UTC timestamp identifies an instant, but converting an old local birth record still requires the relevant historical clock rules.

The offset follows the clock rules at a particular instant. Two dates in the same city may have different offsets, so storing only today’s value can misidentify a birth instant.