Moon Facts & Science

Moon phases are caused by the changing geometric relationship between the Sun, Earth, and Moon as the Moon orbits Earth. The Moon does not produce its own light — it reflects sunlight. As the Moon orbits, we see different portions of its illuminated half from our vantage point on Earth.

One complete cycle of phases — from New Moon back to New Moon — takes approximately 29.53 days. This is called the synodic month, and it's longer than the Moon's actual orbital period (27.32 days) because Earth is also moving around the Sun, so the Moon must travel a bit extra to realign with the Sun.

The eight recognized phases divide this cycle into segments based on the angle between the Sun and Moon as seen from Earth (the phase angle). At 0° (New Moon) the Moon is between Earth and Sun; at 180° (Full Moon) Earth is between the Moon and Sun.

The Moon is tidally locked to Earth, meaning its rotation period equals its orbital period (both ~27.3 days). As a result, the same hemisphere always faces Earth. This is not a coincidence — it's the natural end state of tidal interactions between any two bodies in close orbit.

Earth's gravity created tidal bulges in the Moon, and friction from these bulges gradually slowed the Moon's rotation until it matched its orbital period. This process, called tidal braking, took hundreds of millions of years.

We can actually see about 59% of the Moon's surface over time due to a phenomenon called libration — small oscillations in the Moon's orientation caused by its slightly elliptical orbit and orbital inclination.

The Moon's gravitational pull is the primary driver of ocean tides on Earth. Although the Sun also contributes, the Moon's tidal force is about 2.2 times stronger than the Sun's despite the Sun being far more massive — because tidal force depends on the cube of distance, and the Moon is much closer.

Spring tides occur during new and full moons, when the Sun and Moon are aligned. The combined gravitational pull creates the highest high tides and lowest low tides. Neap tides occur during quarter phases, when the Sun and Moon are at right angles, producing more moderate tides.

The Moon is also slowly moving away from Earth at a rate of about 3.8 cm per year due to tidal interactions. Billions of years ago, the Moon was much closer and appeared much larger in the sky. In the far future, it will be too far away to produce total solar eclipses.

The leading theory for the Moon's formation is the giant impact hypothesis. Roughly 4.5 billion years ago, a Mars-sized body (sometimes called Theia) collided with the young Earth. The impact ejected a massive cloud of debris into orbit, which gradually coalesced to form the Moon.

This theory explains several key observations: the Moon's relatively low iron content (most of the impactor's iron sank into Earth's core), the similar oxygen isotope ratios between Earth and Moon rocks, and the Moon's initial close proximity to Earth.

Analysis of Apollo lunar samples has largely confirmed this model, though some details are still debated, such as the exact composition and trajectory of the impactor.