What is a new Moon?

What is a new Moon, astronomically?

It is the instant when the Moon and the Sun have the same ecliptic longitude as seen from the Earth — a conjunction, an elongation of 0°. Geometrically the Moon lies between the Earth and the Sun. Because the Sun lights the far side of the Moon at that moment, the hemisphere facing us is unlit, and the new Moon is invisible from the ground.

The ecliptic longitude is the position measured around the zodiac, the same coordinate a chart uses. So “new Moon” is exactly “Sun and Moon at the same degree” — which is why, in a chart, a new Moon always falls in one sign, at one shared degree of the Sun and Moon.

The diagram shows the arrangement from above: sunlight from the left, the Moon between Earth and Sun, its lit side toward the Sun and its dark side toward us.

The exact instant versus the visible crescent

The astronomical new Moon is a single calculated instant, and at that instant the Moon is invisible — lost in the Sun’s glare, its dark side toward us. The slender crescent people call “the new Moon” in everyday speech is not that instant: it is the first thin sliver seen low in the west after sunset about a day or so later, once the Moon has moved far enough from the Sun to catch a visible edge of sunlight. The photograph above is that young crescent.

This gap matters for calendars. The astronomical new Moon can be computed centuries ahead, but the first crescent sighting depends on the sky, the season and the observer’s latitude, so the two can fall on different dates. Several lunar calendars are still reckoned from the sighting, not the instant.

A worked example, from the engine

Take the new Moon after 1 January 2025. OrbHeaven’s engine finds the conjunction at 29 January 2025, 12:36 UTC, with the Sun and the Moon both at 9°51′ Aquarius — their ecliptic longitudes agree to within about six thousandths of a degree, which is what “conjunction” means here. The Moon, though, was about 3.8° south of the ecliptic at that instant, and the nearest point where its orbit crosses the ecliptic was more than three days away.

The assumptions are the engine’s defaults: geocentric apparent ecliptic longitude, true equinox of date; the new-Moon instant is found by searching for the moment the Sun–Moon elongation is zero. No value here is typed in — the date, the shared degree, the Moon’s latitude and the gap to the node are all read from the same ephemeris the charts use, and the test suite re-derives them.

That 3.8° of latitude is the whole reason this new Moon was an ordinary one and not an eclipse, as the next section explains.

Why isn’t every new Moon a solar eclipse?

Because the Moon’s orbit is tilted about 5.1° to the ecliptic — the plane of the Sun’s apparent path — so at most new Moons the Moon passes a little above or below the Sun rather than exactly across it. A solar eclipse needs the new Moon to fall close to one of the two nodes, the points where the tilted orbit crosses the ecliptic; only there are the Sun, Moon and Earth aligned closely enough in latitude for the Moon’s shadow to reach the Earth.

In the worked example the Moon was about 3.8° south of the ecliptic at the conjunction, with the nearest node more than three days away — far outside the narrow window (roughly a degree and a half of latitude) in which the Moon can cover the Sun. So its shadow passed well below the Earth, and the new Moon was ordinary.

This is exactly the node geometry behind every eclipse season. The full treatment — the nodes, the tilt and why eclipses come in pairs — is in The Lunar Nodes; the figure below is the same tilted-orbit picture.

A short history

The new Moon has marked the start of the month for most of recorded history. Babylonian, Hebrew and Islamic calendars all begin the month at the new Moon — traditionally at the first sighting of the young crescent, not the invisible conjunction — and the English word “month” shares its root with “Moon”. Distinguishing the computed conjunction from the observed crescent is an old problem, worked on by Babylonian and later Islamic astronomers who built arithmetic and geometric methods to predict first visibility.

The ability to compute the conjunction itself, rather than wait for the sky, is what let lunisolar calendars be fixed in advance — while religious calendars that still depend on the sighting keep the older, observational new Moon.

What a new Moon means in the tradition

Interpretation, labelled as such: astrology reads the new Moon as a beginning — the Sun and Moon together, the monthly cycle restarting — and treats it as a natural point to set an intention in the area of life marked by the sign and house it falls in. The following Full Moon, the opposition, is then read as the culmination of what the new Moon began.