Moon phases: from new moon through waxing to full moon

Why the shape changes

The Moon reflects sunlight rather than producing the light we see. The hemisphere facing the Sun is illuminated; our view of it changes as the Moon orbits Earth. Near the Sun’s direction we see mostly the night side: new moon. Opposite the Sun we see almost all the day side: full moon.

The main phases are defined through the difference in ecliptic longitude between Moon and Sun:

The exact illuminated fraction need not be an ideal round percentage: distances and lunar latitude add small differences. The table describes the basic geometry, not a live phase result.

A “quarter” refers to the Moon’s progress through the phase cycle, not to a quarter of the visible disc. Between the main phases are crescents and gibbous phases, when more than half the disc is illuminated but it is not yet full, or is no longer full.

The synodic month and lunar age

One orbit relative to the stars takes approximately 27.32 days: the sidereal month. Returning to the same alignment with the Sun takes longer because Earth has moved along its own orbit. The **synodic month** averages about 29.53 days.

Neither Earth nor Moon moves at a perfectly constant orbital speed. Individual lunations therefore vary by several hours, and the main phases do not divide every cycle into four identical time intervals.

**Lunar age** is the time since the previous new moon. A few days old, the Moon is a thin evening crescent; first quarter is around 7.4 days and full moon around 14.8 days. Those are average guides, not exact predictions.

Illumination is not proportional to age. The apparent geometry changes non-linearly, and orbital speed varies. Two Moons of the same age can have slightly different illuminated fractions. A lunar theory such as the one used by Apphemeride calculates the position and phase for the chosen instant.

Waxing or waning

From a typical northern-hemisphere viewpoint, the right side is commonly illuminated while waxing and the left while waning. The orientation depends on the Moon’s place in the sky and how you view it. A telescope can invert or reflect the image, so a left–right rule is not universal.

In the southern hemisphere the familiar orientation is reversed. Near the equator, a crescent may look almost like a horizontal bowl. The physical phase is shared across Earth, while its orientation relative to the local horizon changes.

Time of appearance offers another clue:

• A waxing crescent is an evening object towards the west after sunset.
• First quarter is well placed around dusk and sets roughly near solar midnight.
• Full moon occupies the sky opposite the Sun and can be visible through much of the night.
• Last quarter favours the second half of the night and the morning.
• A waning crescent is found before sunrise towards the east.

These are broad patterns rather than local clock times. Latitude, season, declination and horizon can change the details considerably.

Which phase is worth observing

Full moon is striking, but it is not always best for exploring craters. Sunlight falls steeply on much of the visible hemisphere, shortening shadows and flattening the apparent landscape.

Near the **terminator**, the boundary between lunar day and night, low sunlight casts long shadows. Mountains, crater walls and small elevations become much easier to interpret. From a crescent through the days around first quarter, many features are also available at convenient evening hours.

For meteors, faint stars or the Milky Way, reverse the strategy. New-moon periods and intervals when the Moon is below the horizon offer darker backgrounds. A full moon can wash out faint stars even when the Sun is below −18°: it is still astronomical night, but not a moonless sky.