The visible portion of the Moon to observers on Earth seems to change shape and appears closer or further away at differing times throughout a calendar month. These changes are part of lunation, more commonly known as the lunar cycle. This cycle produces different images of the Moon, in an orderly fashion with no variances in the order of the phases. The only thing that seems to change during each lunation is the Moon’s proximity to Earth. This accounts for the Moon appearing larger (closer) and smaller (further) to viewers on Earth.
The Moon’s orbit
The Moon travels around the Earth in its nearly circular orbit with an eccentricity of 5.49%. This means that the Moon’s orbit is elliptical. As viewed from Earth, the Moon appears to take about 27.3 days to complete its orbit around the celestial sphere, called a sidereal month. However, on average, the lunar cycle lasts about 29.5 solar days from beginning to end, known as a synodic month. According to Stardate.org, the moon spends the difference of 2.2 days “catching up” to Earth. This is because during the time the Moon completes one orbit around Earth, the Earth travels around 45 million miles around the Sun. To be able to complete the orbit in 27.3 days the Moon must move Eastward through the constellations by 13.2° each day, meaning that every day the Moon crosses the celestial meridian about 52.8 minutes later each day.
Perigee and apogee
Travelling on elliptical orbits, the Moon and the Earth will at times during their orbits reach their maximum and minimum distances to each other. The Moon’s mean distance from Earth (from center to center) is about 238,855 miles (384,401 km), with distance variations of over 30,000 miles.
Perigee is when the Moon is at its closest proximity to earth in a given orbit (month) and its antonym is apogee, apogee being the furthest distance from earth in an orbit. According to the article “Inconstant Moon” by John Walker, the closest perigee occurring from 1750 through 2125 was on January 4, 1912, at a distance of only 221,441 miles (356,375 km); the furthest apogee occurring on February 3, 2125, at a distance of 252,724 miles (406,720 km). Richard Nolle’s article “Lunar Extremes” includes a table with the dates, times and distances of the 2012 lunar apogees and perigees.
These extremes sometimes coincide with other celestial phenomena. A SuperMoon occurs when the Moon at perigee is in either the full or the new Moon phase of the lunar cycle. In the case of a full Moon SuperMoon, the Moon, because of its closeness to Earth, appears up to 14% bigger and 30% brighter than other full Moons. Astronomer Richard Nolle has also published a table titled “21st Century SuperMoon Alignments” that includes dates for other phenomenon such as solar and lunar eclipses.
The Moon’s cycle through its orbit has eight phases. These phases stem from the Moon’s rotation around the Earth. This rotation causes the visible portion of the Moon to change as observed from Earth. According to NASA, the phases are as follow:
Waxing crescent: The Moon appears less than half-illuminated, with illumination increasing.
First Quarter: Half of the Moon appears illuminated with illumination increasing.
Waxing Gibbous: The Moon is more than half-illuminated and increasing.
Full Moon: To observers on the ground the Moon appears fully illuminated.
Waning Gibbous: Greater than half-illuminated, with illumination decreasing.
Last Quarter: Half of the moon appears illuminated, with illumination decreasing.
Waning Crescent: Less than half of the Moon appears illuminated, with illumination decreasing.
New Moon: To observers on the ground the Moon appears cloaked in darkness.
The lunar cycle is also responsible for the tides in Earth’s oceans. The Moon’s proximity to Earth affects its gravitational pull on the planet with the Sun entering the equation at certain times. The closer the Moon is, the greater the tides are and vice-versa. When the Moon is at perigee, perigean tides occur. When it reaches apogee, apogean tides occur. When the Moon is either in its full or new phase (syzygy), greater range spring tides occur. When the Moon is in its first and last quarter, neap tides occur. At times, perigee will coincide with syzygy (SuperMoon) creating even greater tides than perigean tides, known as perigean spring tides.
The Moon, according to scientists, is about a four-day trip away, therefore Earth bound humans must content themselves with the beautiful picture show that plays out monthly in the sky above from apogee to perigee.