True Phase

Definition

A computed (rather than visually observed) form of a star's phase with respect to the Sun. A 'true' phase is calculated for the exact moment a star is on the horizon at the same time as the Sun — its rising, setting, culminating, or anti-culminating in mathematical coincidence with the solar horizon — without reference to whether the star would actually be visible at that moment given atmospheric conditions, twilight, or solar glare. Contrasts with the apparent phase, which depends on observational visibility.

In Tradition

In the Hellenistic and modern revival fixed-star tradition phase (Greek ϕάσις) is the planetary-or-stellar visibility-condition with respect to the Sun — Lightfoot records that Pseudo-Manetho links 'eastern morning rising' and 'western evening rising' to a planet's sect status. The modern 'true' modifier formalises the distinction between the mathematical horizon-coincidence event (computable from coordinates) and the visible heliacal rising or setting (which depends on the star's brightness, latitude, and atmospheric arcus visionis).

In Practice

Modern paran work with fixed stars is computed against true phases rather than apparent ones because the mathematical horizon-coincidence is unambiguous: every star has exactly one true rising, one culmination, one setting, and one anti-culmination on any given day, and these can be calculated precisely from the star's right ascension and declination together with the observer's latitude and the date. Apparent phases — the visible heliacal rising the Egyptian and Babylonian traditions actually used for calendar marking — vary by star magnitude and by the local atmospheric arcus visionis (the Sun's depression below the horizon at which a star of a given magnitude becomes visible).

Historical Origin

Pseudo-Manetho (Lightfoot ed.) preserves the Greek phase-vocabulary (ϕάσις, ἑῴα ἀνατολή, ἑσπερία ἀνατολή) that names a planet's eastern or western visibility-condition relative to the Sun. The 'true' versus 'apparent' refinement is a modern computational distinction articulated in the Western fixed-star revival (Brady and successors) once digital ephemerides made the precise mathematical event independently calculable from the observed one.

Etymology

Origin: Greek + English. Meaning: phase = the manner in which something appears; 'true' phase = the mathematically exact, not the visually observed, event.

Further Reading

• Bernadette Brady, Brady's Book of Fixed Stars
• J. L. Lightfoot, The Apotelesmatika of Manetho