Moon Phase Calculator

The Moon's phases are caused by the changing relative positions of the Earth, Moon, and Sun as the Moon orbits around Earth. The Moon itself doesn't emit light but reflects sunlight. We always see approximately the same side of the Moon (the near side), but the amount of that side illuminated by the Sun changes depending on where the Moon is in its orbit. When the Moon is between Earth and the Sun (New Moon), the side facing us receives no direct sunlight. When Earth is between the Sun and Moon (Full Moon), we see the entire sunlit side. The phases we observe are simply different portions of the illuminated half visible from our perspective on Earth.

The Moon takes approximately 29.5 days to complete one cycle of phases, from New Moon to New Moon. This is called a synodic month or lunation. It's slightly longer than the Moon's actual orbital period around Earth (27.3 days, known as the sidereal month) because Earth is also moving around the Sun. As the Moon completes one orbit around Earth, our planet has moved further in its orbit around the Sun, requiring the Moon to travel a bit farther to reach the same position relative to the Sun and Earth. This difference accounts for the roughly 2.2 additional days in the synodic month.

We always see approximately the same side of the Moon because it is tidally locked to Earth. This means the Moon's rotation period about its axis exactly matches its orbital period around Earth, causing it to keep the same face toward us. This synchronization occurred over billions of years due to Earth's gravitational pull, which slowed the Moon's rotation until it matched its orbital period. Despite this tidal locking, we can actually see about 59% of the lunar surface over time (not just 50%) due to small wobbles in the Moon's motion called librations. These librations allow us to see slightly around the edges of the Moon's visible disk at different times.

Moon phases have a significant impact on Earth's tides. Tides are primarily caused by the Moon's gravitational pull on Earth's oceans, with a smaller contribution from the Sun. During New and Full Moons, when the Sun, Earth, and Moon align (called syzygy), their gravitational forces combine to produce higher high tides and lower low tides, known as spring tides (unrelated to the season). During First and Last Quarter phases, when the Sun and Moon are at right angles to each other relative to Earth, their gravitational forces partially cancel out, producing weaker neap tides with smaller differences between high and low water. These tidal patterns are crucial for coastal ecosystems and have influenced human maritime activities throughout history.

A Blue Moon has two common definitions. The modern definition refers to the second full moon occurring within a single calendar month, which happens approximately every 2.7 years. The older, traditional definition refers to the third full moon in a season that contains four full moons (normally a season has three), occurring about every 2.7 years as well. Despite the name, the Moon doesn't actually appear blue during these events—the term has nothing to do with color. The phrase "once in a blue moon" comes from this relatively rare phenomenon and indicates something that doesn't happen very often. The next Blue Moon (by the monthly definition) will occur in August 2023, followed by another in August 2024 (a rare double occurrence).

While the Moon's phase appears the same globally, its orientation differs depending on your location. In the Northern Hemisphere, the illuminated portion of the waxing Moon appears on the right side, while in the Southern Hemisphere, it appears on the left. At the equator, the Moon can appear rotated at various angles depending on the time of night. Additionally, the Moon appears "upside down" when comparing views from the northern and southern hemispheres. This is because observers stand on different parts of our spherical planet, facing different directions when they look up. The Moon also appears larger near the horizon than when high in the sky, but this is an optical illusion called the "Moon illusion," as its angular size remains constant.

Modern moon phase predictions are extremely accurate. Astronomers can calculate the exact position of the Moon relative to Earth and the Sun using precise mathematical models that account for various factors affecting lunar motion, including Earth's and the Moon's orbital eccentricities, perturbations from other planets, and relativistic effects. Online calculators and apps (including this one) typically use simplified algorithms that are still accurate to within minutes for current and near-future phases, and within hours for dates centuries in the past or future. For everyday purposes, these simplified models are more than adequate. However, for specialized purposes like predicting exact eclipse times or historical astronomical events, more complex calculations incorporating additional astronomical parameters are used by professional astronomers.