The planetary data export includes visibility based on 3 different criteria; our approximation to the criteria used in the USNO Astronomical Phenomena and Astronomische Grundlagen publications, as well as our own visibility criteria.
Our own planetary visibility criteria require at least 30 uninterrupted minutes of visibility within three hours after sunset or before sunrise, between 04:30 and 23:30 local time. Mercury must be at least 5° above the horizon, and Venus through Saturn 10°, with brightness at least half a magnitude better than the predicted naked-eye detection limit. Uranus and Neptune assume optical assistance, requiring an altitude of 15° and the Sun at least 12° below the horizon. Calendar summaries bridge gaps of up to four non-visible days, while retaining the actual daily results.
Our reconstruction of the visibility used by the USNO mainly uses a planet’s angular separation from the Sun at midnight UTC: approximately 10° for Venus and Jupiter, and 15° for Mars and Saturn, with provisional rules for Mercury, Uranus and Neptune. Morning or evening is assigned according to which side of the Sun the planet occupies, switching at opposition for the outer planets. This produces broad visibility seasons largely independent of the observer’s location, rather than predicting a particular night’s observing conditions.
Our reconstruction of the planetary visibility criteria used by the Grundlagen takes a more local approach, based on the city of Kassel in Germany, estimating whether the planet stands out against the sky, considering its brightness, twilight and atmospheric dimming. It requires a planet-dependent visibility span of roughly 30 minutes to two hours within the relevant half of the sunset-to-sunrise night; morning and evening seasons can therefore overlap. These are inferred rules fitted to the publication’s approximate wording; Mercury remains less certain, and the Uranus/Neptune estimates are our own extensions.
See also: exporting planetary data.
Topic 189215, last updated on 15-Sep-2026
