Lunar phase

The Moon looks like it changes shape.

This happens as it moves around Earth. The sun shines on the Moon. We only see the bright parts.

Sometimes the Moon looks like a thin sliver. Other times it looks like a big circle. This is called a full moon.

One full cycle takes about 29 days. It is fun to watch the Moon change.

The Moon seems to change its shape every night.

These shapes are called lunar phases. This happens because of the Moon's orbit around Earth. The Sun always shines on one side of the Moon. As the Moon moves, we see different amounts of that light.

There are four main phases. First is the new moon. During this phase, the bright side faces away from us. Next is the first quarter. Then comes the full moon, when we see the whole lit side.

Finally, there is the last quarter.

Between these big phases are smaller ones. We call these intermediate phases. Some look like a crescent, which is a thin sliver. Others look gibbous, which means they are bulging out. One full cycle is called a lunation. It takes about 29.5 days to finish.

People have used these cycles to make calendars for a long time. Some cultures even have 30 different phases.

The Moon seems to change its shape every night. These changing shapes are known as lunar phases.

This happens because the Moon orbits our Earth. The Sun always shines on one side of the Moon. As the Moon moves, we see different amounts of that light. This cycle is very important for understanding our sky. It can take about 29.5 days to finish one full cycle.

To understand how it works, we must look at the Moon's position. The Moon is tidally locked to Earth. This means the same side always faces us. During a new moon, the Moon is between the Earth and the Sun. The side facing us is dark. As the Moon moves, it begins to wax. Waxing means the lit part is growing. It moves from a crescent to a first quarter. Then it becomes a waxing gibbous before reaching a full moon.

Humans have studied these patterns for a very long time. Archaeologists found that people used the Moon to keep time in prehistoric times.

Many cultures created calendars based on these cycles. The Islamic Hijri calendar uses the new moon as a marker. Some calendars are lunisolar, meaning they use both the Moon and the Sun. The Sumerian calendar used a special way to stay in step with the seasons. The Chinese calendar also uses the Sun and Moon together.

There are several specific phases to watch for. The four main phases are the new moon, first quarter, full moon, and last quarter. Between these, there are intermediate phases. These include the waxing crescent and the waning crescent. You might also see a waxing gibbous or a waning gibbous. A gibbous shape looks like it is bulging outwards. A crescent shape looks like a thin sliver.

Where you live on Earth changes how you see the Moon. The phases look different to people in the Northern Hemisphere than in the Southern Hemisphere.

In the North, if the left side is dark, the Moon is waxing. If the right side is dark, the Moon is waning. The Moon also moves higher or lower in the sky depending on the season. For example, a full moon is highest during the winter solstice. These patterns help us connect the sky to the time of year.

The Moon appears to change its shape throughout the month. These changing shapes are known as lunar phases.

A lunar phase is the apparent shape of the Moon's day and night phases as viewed from a distance. This cycle is vital because it helps us track time and understand our place in the solar system. The phases change as the relative positions of the Moon, Earth, and Sun shift in space. This movement creates a predictable cycle that has guided humans for thousands of years.

To understand this mechanism, we must look at how the Moon orbits our planet. The Moon is tidally locked to Earth. This means the same side of the Moon always faces our planet. As the Moon orbits, the Sun illuminates different portions of its surface. The amount of sunlight we see from Earth varies from 0% during a new moon to nearly 100% during a full moon. The Moon rotates as it orbits, creating a lunar day. Because it is tidally locked, one lunar day is equal to one full lunar month, also called a synodic orbit.

Astronomers categorize the Moon's appearances into four principal phases. These are the new moon, the first quarter, the full moon, and the last quarter.

During a new moon, the Moon is at an ecliptic longitude of 0 degrees relative to the Sun. At the first quarter, the angle is 90 degrees. The full moon occurs at 180 degrees, and the last quarter occurs at 270 degrees. Between these major stages are intermediate phases. These include the waxing crescent, waxing gibbous, waning gibbous, and waning crescent.

We use specific terms to describe whether the Moon is growing or shrinking. When the illuminated portion is increasing, we say the Moon is waxing. This happens as it moves from a new moon toward a full moon. When the illuminated portion is decreasing, the Moon is waning. This occurs as it moves from a full moon back toward a new moon. The term "gibbous" describes a shape that bulges outward. A "crescent" describes a thin, curved shape. On average, these intermediate phases last about 7.38 days.

One full cycle of these phases is called a lunation. A lunar month is the time between the same phases occurring again. Due to the eccentricity of the Moon's orbit, this duration is not perfectly constant. It averages about 29.53 days.

You can calculate the "age" of the Moon by counting the days since the last new moon. This calculation is useful for novelty clocks, though specialists use more complex math. They must account for the Moon's apogee and perigee, which are the points where its orbit is farthest from or closest to Earth.

Your location on Earth changes how you observe the Moon. The orientation of the phases is opposite for observers in the Northern and Southern Hemispheres.

In the Northern Hemisphere, if the left side of the Moon is dark, it is waxing. If the right side is dark, it is waning. The Moon's height in the sky also changes with the seasons. For example, a full moon appears highest at the winter solstice in the North. During a new moon, the Moon appears highest at the summer solstice.

Humans have used these cycles for timekeeping since the Neolithic period. Many cultures developed calendars based on the Moon. The Islamic Hijri calendar is a pure lunar calendar that relies on the new moon. Other systems are lunisolar, meaning they combine lunar months with the solar year. The Sumerian calendar used thirteen lunar months every few years to stay in step with the seasons. The Chinese calendar restarts its count based on the new or full moon after the winter solstice.

This connection between the sky and time shows how the Moon's phases shape human history.