Lunation Calendar · — Deep Dives · — Sun-days and Moon-days

Across one horizon

A daily passage begins when a body rises above the local horizon, climbs to its highest point, and sets below the horizon again. That highest point is its culmination; it need not be overhead. Here, a Sun-day or Moon-day means one such arc above the horizon, not a civil date or a fixed twenty-four-hour interval.

The Lunation Chart’s Moon and Sun Arcs Band traces these passages through local time. The horizon is the band’s baseline. A curve rises away from it while its body is above the horizon, and its height records elevation in the sky: blue for the Moon, gold for the Sun. A radial line marks the culmination of each Moon arc, aligning it with the corresponding image in the Moon Faces Band.

The width of an arc shows how long the body remains above the horizon; its height shows how high it climbs. Read together, the blue and gold curves reveal periods when Sun and Moon share the sky, when only one is above the horizon, and when both are below it.

The Moon slips through the day

The Sun returns to nearly the same place in the local sky once each solar day. The Moon is meanwhile moving eastward in its orbit, so Earth must turn a little farther before the Moon reaches its next culmination. Successive moonrises and culminations occur about fifty minutes later on average, though the difference varies substantially. Moonrise, culmination, and moonset therefore tend to move later through local time from one passage to the next.

Near New Moon, Sun and Moon rise and set at roughly the same times. Near First Quarter, the Moon is prominent in the afternoon and evening. Near Full Moon, it rises around sunset and crosses the sky at night. Near Last Quarter, it belongs chiefly to the hours after midnight and the morning. The next New Moon restores the alignment.

This connection between phase and time of day follows directly from Sun-Moon-Earth geometry. The Moon Phases Deep Dive explains why each phase places the Moon at a different angular distance from the Sun.

Presence is not brightness

An arc says that a body is geometrically above the horizon; it does not promise a bright or visible light. A New Moon can spend much of the day above the horizon while its illuminated face is turned away from us. A thin crescent may be lost in twilight, and weather or obstructions can hide either body. The band describes position, not atmospheric conditions or naked-eye visibility.

Even so, the overlap of the arcs gives useful context for light. A waxing Moon extends the period of potential moonlight beyond sunset; a Full Moon can carry it through most of the night; a waning Moon brings it before sunrise. The chart lets this changing relationship emerge from the paths themselves rather than reducing it to a single rise or set time.

Different paths through the same sky

Earth’s axial tilt carries the Sun high through the sky in summer and low in winter. The Moon’s path varies for the same reason, but the Moon’s orbit is also tilted relative to Earth’s orbit around the Sun. That additional tilt carries the Moon above and below the Sun’s path.

These changing paths have turning points. The Sun’s path reaches its highest and lowest arcs at the summer and winter solstices. The Moon has corresponding northern and southern turns called lunistices. Its arcs reach their highest peaks around the northern lunistice and their lowest around the southern lunistice.

Lunistice Rare Event marks each northern or southern turn, while the surrounding blue arcs show the corresponding high or low passages through the sky.