Lunar month facts for kids

A lunar month is how we measure time based on the Moon's journey around Earth. It's the time it takes for the Moon to go through all its different phases, like from one new moon to the next, or from one full moon to the next. This period is super important for lunar calendars used by many cultures around the world.

Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere. The apparent wobbling of the Moon is known as libration.

How People Measure a Lunar Month

Different cultures have unique ways of deciding when a lunar month begins.

• In some traditions, like those in the Middle East and Europe, a new month starts when the thin crescent moon is first seen in the evening sky. This happens a day or two after the Moon has lined up with the Sun and Earth. The Islamic calendar uses this method.
• In ancient Egypt, the lunar month began when the old, fading moon could no longer be seen just before sunrise.
• Other calendars might count from one full moon to the next.
• Some calendars, like the Hebrew calendar and the Chinese calendar, use special calculations to figure out the start and length of their months. These months can be 29 or 30 days long.
• Ancient Hindu calendars, called Panchangam, also track lunar cycles with great detail.

Different Kinds of Lunar Months

The term lunar month usually refers to the time it takes for the Moon to complete its visible phases. But astronomers have identified several other types of lunar months, each measuring a slightly different aspect of the Moon's journey. Many of these were first studied by ancient Babylonian astronomers!

The Synodic Month: The Phase Cycle

The synodic month (which means "meeting" in Greek, referring to the Sun and Moon meeting) is the most common type of lunar month. It's the average time it takes for the Moon to complete one full cycle of its phases, like from one new moon to the next. This period is about 29 days, 12 hours, 44 minutes, and 3 seconds.

Why is it this long? While the Moon orbits Earth, Earth is also moving around the Sun. So, after the Moon finishes one full trip around Earth compared to the stars (which is a shorter time), it still needs to travel a little further to catch up and appear in the same phase relative to the Sun as seen from Earth. This is why the synodic month is longer than the sidereal month.

The Sidereal Month: Tracking the Stars

The sidereal month is the time it takes for the Moon to complete one orbit around Earth when measured against the distant, seemingly fixed stars. This period is about 27 days, 7 hours, 43 minutes, and 11.6 seconds.

Many ancient cultures, including those in the Middle East, India, and China, used this type of month. They divided the sky into "lunar mansions" or sections, each marked by specific stars. The Moon would pass through one mansion each day.

The Tropical Month: Crossing the Equator

The tropical month is the average time between successive moments when the Moon crosses from the southern part of the sky to the northern part (or vice versa). This is similar to how we measure a tropical year for Earth. This month is slightly shorter than the sidereal month, lasting about 27 days, 7 hours, 43 minutes, and 4.7 seconds.

The Anomalistic Month: Moon's Distance from Earth

The Moon's path around Earth is not a perfect circle; it's more like an oval, or an ellipse. This means the Moon's distance from Earth changes.

• When the Moon is closest to Earth, it's called perigee.
• When it's farthest, it's called apogee.

The anomalistic month is the time it takes for the Moon to travel from its closest point to Earth (perigee) and return to that same closest point. This period is about 27 days, 13 hours, 18 minutes, and 33.2 seconds. Because the Moon's distance changes, its apparent size in the sky also changes. This is important for predicting eclipses, as the Moon's size affects whether an eclipse will be total or annular.

The Draconic Month: The Eclipse Month

A draconic month is also known as a nodal month. The Moon's orbit is slightly tilted compared to Earth's orbit around the Sun. The points where the Moon's orbit crosses Earth's orbital plane are called lunar nodes.

The draconic month is the time it takes for the Moon to pass through one of these crossing points and return to it again. This period is about 27 days, 5 hours, 5 minutes, and 35.8 seconds. This month is very important for predicting eclipses! Eclipses can only happen when the Moon is very close to one of these nodes. The name "draconic" comes from an old myth about a mythical dragon that was thought to "eat" the Sun or Moon during an eclipse.

How Long Are Lunar Cycles?

All lunar calendar months are designed to match the average length of the synodic month, which is the cycle of the Moon's phases (about 29 to 30 days). The Moon orbits Earth in about 27.3 days (a sidereal month). However, because Earth is also moving around the Sun, the Moon needs a bit more time to get back to the same position relative to the Sun and show the same phase.

Here are the average lengths of the main types of astronomical lunar months:

These lengths are averages. The actual time for each cycle can change slightly due to the complex gravitational pulls from the Sun and other planets.

See also

• Lunar calendars
  • Islamic calendar (Lunar Hijri calendar)
  • Javanese calendar
• Lunisolar calendars
  • Chinese calendar
  • Hebrew calendar
  • Babylonian calendar
  • Hindu calendar
  • Tibetan calendar