The phases are caused by the Moon's orbit! The Moon orbits Earth once every 27.3 days (sidereal month), but due to Earth's movement around the Sun, the lunar phases cycle every 29.5 days (synodic month). Half of the Moon is always illuminated by the Sun, but we see different fractions depending on where the Moon is in its orbit. The phase names include new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent. The Moon appears to rise about 50 minutes later each day because it moves about 13 degrees eastward in its orbit each day. The word "month" comes from "moon" – many ancient calendars (Hebrew, Islamic, Chinese) are based on lunar months.

New moon is invisible! During new moon, the Moon rises at sunrise and sets at sunset. It is up during the daytime but cannot be seen because it is close to the Sun in the sky. A solar eclipse occurs when the new moon passes directly between the Sun and Earth, casting a shadow on Earth. The term "new moon" also refers to the beginning of the lunar month in many calendars (Islamic calendar, Hebrew calendar). The dark side of the Moon (the side always facing away from Earth) is not the same as the new moon; the "dark side" gets sunlight half the time. The far side of the Moon was first photographed by the Soviet Luna 3 probe in 1959. The new moon is sometimes called the "dark moon." During new moon, the Moon is in conjunction with the Sun (same right ascension). The gravitational pull of the Sun and Moon combine during new moon, causing higher tides (spring tides).

Full moon is the correct answer! The Moon is opposite the Sun, so the entire sunlit side faces Earth. The full moon is the brightest phase. The full moon has been associated with various myths and behaviors (lunacy, werewolves). Some studies suggest a slight correlation with sleep and mood, but no strong evidence for increased crime or hospital visits. The names of full moons: Wolf Moon (January), Snow Moon (February), Worm Moon (March), Pink Moon (April), Flower Moon (May), Strawberry Moon (June), Buck Moon (July), Sturgeon Moon (August), Harvest Moon (September), Hunter's Moon (October), Beaver Moon (November), Cold Moon (December). The Harvest Moon is the full moon closest to the autumnal equinox; it rises soon after sunset for several nights, providing extra light for farmers. The Blue Moon is the second full moon in a calendar month (about once every 2.5 years). Lunar eclipses occur only during full moons. During a total lunar eclipse, the Moon can appear reddish (blood moon) due to sunlight scattered through Earth's atmosphere.

Quarter moons are named for the fraction of the lunar cycle completed! First quarter = 1/4 of the way from new moon to new moon; third quarter = 3/4 of the way. The term "half moon" is also common. During first quarter, the right half is illuminated (in the Northern Hemisphere). During third quarter, the left half is illuminated. The illumination flips in the Southern Hemisphere (left half illuminated at first quarter). First quarter moon is visible in the afternoon and evening; third quarter moon is visible in the early morning and daytime. The Moon appears high in the sky at sunset during first quarter. The gravitational pull of the Sun and Moon are perpendicular during quarter moons, causing lower tides (neap tides). Some cultures call these phases "half moon" (no confusing terminology). The term "quarter" can confuse beginners because it describes the cycle fraction, not the appearance. The Moon is often visible during the day (especially in quarter phases). About 25-50% of the time, the Moon is above the horizon during daylight.

Waxing means increasing! The illuminated portion grows from new moon to full moon. The term "waxing" is related to the same root as "waist" (to grow). A simple mnemonic: "Waxing" sounds like "maxing" (maximizing). Waning is decreasing. In the Northern Hemisphere, the right side of the Moon is illuminated during waxing; the left side is illuminated during waning. In the Southern Hemisphere, it is reversed (left side waxing, right side waning). The "waxing crescent" appears as a thin sliver after new moon. The "waning crescent" appears as a thin sliver before new moon. The Moon rises about 50 minutes later each day, so the waxing phases are visible in the afternoon and evening; the waning phases are visible in the early morning and daytime. The amount of illumination changes slowly (about 1% per hour). The term "gibbous" means more than half but less than full (from Latin "gibbus" meaning hump). Waxing gibbous occurs between first quarter and full moon; waning gibbous occurs between full moon and third quarter.

Gibbous is the correct answer! A gibbous Moon appears as a hump-like shape. The term "crescent" comes from the Latin "crescere" (to grow). The crescent Moon is a familiar symbol in many cultures (Islamic flag, Ottoman Empire). The crescent Moon is often the first visible phase after new moon (about 24-48 hours after new moon). The Earthshine (the "old moon in the new moon's arms") is sunlight reflected from Earth onto the dark part of the crescent Moon. The crescent Moon can be seen in the evening sky (waxing) or morning sky (waning). The gibbous Moon is often visible most of the night. The Moon appears larger near the horizon (Moon illusion) – an optical illusion, not a real change in size. The Moon's actual angular size is about 0.5 degrees (half the width of your pinky finger at arm's length). The Moon's distance varies (perigee = closest, apogee = farthest), causing supermoons (full moon at perigee) and micromoons. A supermoon appears about 14% larger and 30% brighter than a micromoon. The term "blue moon" has nothing to do with color – it is the second full moon in a month (or third full moon in a season with four).

Lunar eclipses occur only during full moon! However, not every full moon produces an eclipse because the Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the Sun. Eclipses occur when the full moon crosses the ecliptic plane (the nodes). There are usually 2-4 lunar eclipses per year. A total lunar eclipse can last up to 1 hour and 47 minutes. A partial lunar eclipse occurs when only part of the Moon enters the umbra (darkest shadow). A penumbral lunar eclipse occurs when the Moon passes through the penumbra (fainter outer shadow). The red color during a total lunar eclipse is caused by Rayleigh scattering (same effect as sunsets). The Moon can appear orange, red, or brown depending on atmospheric conditions (dust, clouds, volcanic ash). The "blood moon" term has become popular but is not scientific. Lunar eclipses have been recorded for thousands of years (ancient Babylonians, Chinese, Greeks). Christopher Columbus used a predicted lunar eclipse in 1504 to intimidate the indigenous people of Jamaica into providing supplies. The next total lunar eclipse visible from a given location can be calculated using astronomical tables. Lunar eclipses are safe to view without eye protection (unlike solar eclipses). You can see a lunar eclipse from anywhere on the night side of Earth.

Solar eclipses occur only during new moon! However, not every new moon produces an eclipse because the Moon's orbit is tilted. Total solar eclipses are rare at any given location (once every 375 years on average). The path of totality is narrow (about 100-200 km wide). The longest possible duration of totality is about 7.5 minutes. The total solar eclipse of July 22, 2009, lasted 6 minutes and 39 seconds. Solar eclipses are dangerous to view without special eye protection (eclipse glasses). Looking directly at the Sun can cause permanent eye damage or blindness. The Sun's corona (outer atmosphere) is visible only during a total solar eclipse. The diamond ring effect occurs just before and after totality. Ancient peoples often interpreted solar eclipses as omens. The 2017 "Great American Eclipse" crossed the United States from Oregon to South Carolina. The 2024 eclipse crossed from Texas to Maine. The next total solar eclipse in the United States will occur in 2044 (Montana, North Dakota) and 2045 (across the country). Annular eclipses are more common than total eclipses. A hybrid eclipse switches between annular and total along the path (rare). Eclipses have helped scientists confirm Einstein's theory of general relativity (bending of starlight). The Saros cycle (about 18 years) predicts eclipse patterns.

Spring tides are the correct answer! The word "spring" refers to "springing forth" (not the season). Spring tides can be 20% higher than normal tides. Neap tides (from Old English "nep" meaning scarce) are lower. The tidal range (difference between high and low tide) varies with the phases. The Bay of Fundy in Canada has the highest tides in the world (over 50 feet / 15 meters). Tides are actually bulges in the oceans caused by gravitational pull and the centrifugal force of Earth-Moon orbit. There are two high tides and two low tides per day (semidiurnal tides). Some locations have diurnal tides (one high and one low per day) or mixed tides. The Moon's pull also causes Earth's rotation to slow down gradually (about 1.8 milliseconds per century). This means the day gets longer over geologic time. The Moon is also moving away from Earth (about 3.8 cm / 1.5 inches per year). Tidal forces affect not just oceans but also Earth's crust (Earth tides) and even the atmosphere. Tidal power (tidal barrages, tidal turbines) generates electricity using tidal currents. The La Rance tidal power plant in France (1966) was the first. Tidal energy is predictable (unlike wind or solar). King tides are especially high tides that occur when the Moon is at perigee (closest to Earth). King tides can cause coastal flooding, especially with sea level rise.

Waning gibbous is the correct answer! After full moon, the Moon becomes a waning gibbous (still more than half illuminated), then third quarter (half), then waning crescent (thin sliver), then new moon (invisible). The entire cycle takes about 29.5 days. A mnemonic: "Documentary: M...W...F... really boring" (Moon, Waxing, Waning, Full, Crescent, Gibbous – not great). Another common mnemonic: "Mom's Very Expensive Purse Just Bought New Shoes" – new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent, new. The Moon's phases are not caused by Earth's shadow (that is a lunar eclipse). The shadow we see on the Moon during a crescent is the Moon's own night side (unlit by the Sun). You have learned about the cause of moon phases, the eight phases, waxing/waning, crescent/gibbous, lunar eclipses (full moon), solar eclipses (new moon), tides (spring and neap), and the order of phases. You are now a Moon phase expert!