New moon calendar

A new moon is the instant the Moon and the Sun share the same ecliptic longitude — the Moon passing between us and the Sun, with its lit hemisphere turned entirely away. Like the full moon it is a moment rather than a night, and this new moon calendar gives the moment.

It is invisible for two reasons at once, and the second one surprises people. Its lit face points away from us, so there is nothing to see; and it is in the sky at the same time as the Sun, rising and setting with it, so even the sliver of light along its limb is lost in daylight. A new moon is a daytime object.

The earliest you can catch the returning crescent is usually 15 to 20 hours after the exact instant, low in the west just after sunset. Under ideal conditions observers have recorded it under 16 hours; the record attempts are a small and competitive field.

This is also the darkest sky of the month, which is why observatories and anyone chasing faint objects plan around the new moon rather than the full one. If you want to see the Milky Way, this calendar is the one to check.

Every solar eclipse happens at a new moon, and for the same reason every lunar eclipse happens at a full one: the three bodies have to be in a line. What decides whether a given new moon produces an eclipse is how close it falls to the lunar nodes, the two points where the Moon's tilted orbit crosses the Earth's.

If the Moon's orbit were not tilted about 5°, there would be a solar eclipse at every single new moon. The tilt is why we get two to five a year instead, clustered into eclipse seasons about six months apart.

A solar eclipse is also the one case where the difference between a new moon and a visible event is stark. The Moon's shadow is small: even a total eclipse is only visible from a band a hundred kilometres or so wide, while the new moon itself is a fact about the whole planet at one instant.

This calendar does not flag solar eclipses on new moons, because unlike a lunar eclipse — which everyone on the night side sees identically — a solar eclipse is a different event depending on where you stand. The eclipse pages handle that properly, with local circumstances.

There is a nice symmetry between the two ends of the cycle. A lunar eclipse needs a full moon near a node and is seen identically by everyone on the night side of the planet; a solar eclipse needs a new moon near a node and is seen completely differently a hundred kilometres apart. Same alignment, same nodes, opposite viewing experience — because in one case the small body casts the shadow and in the other it receives it.