Moon's Phases

Mean distance between Moon's and Earth's centers is 239,000 miles. Moon's orbit has a feeble eccentricity and Moon's orbit plane is tilted by 5.1° relative to Earth's equatorial plane (for a more complete table of data about Moon, see the tutorial "Figures About the Moon" in section Theory. Advanced)

Moon Phases

Moon's phases are mainly due to Moon's revolution period equaling its rotation period. Moon orbits Earth in 27.3 days and it rotates in 27.3 days too. This resonance was built along time since Earth and Moon were created. It was created by gravitational interactions between both. A point at Moon's surface has rotated once when Moon has completed one orbit around Earth (see point 1 on following sketch). On another hand, a point located at Moon far side will always remain invisible from Earth. Hence Moon has a near and a far side. Far side remained unknown until space age

27.3 days is the duration of Moon's orbital revolution relative to stars background. Moon's phases have a real duration of 29.5 days, that is the time Moon takes to complete a full cycle starting at the Earth-Sun line and back. Or -what is equal- to accomplish a complete lunar cycle: to go from one phase to the same phase. Moon 27.3 days revolution is called a sidereal month; 29.5 days cycle, from phase to phase, is called a synodic month

Moon's phases are due to Moon differently illuminated by Sun's rays along its orbit around Earth:

- when Moon is between Sun and Earth, it is new moon; Sun reaches Moon's far side only
- when Moon, moving on its orbit counter-clockwise begins to be at an angle to Sun (elongation), first crescent appears
- first crescent increases during a week towards first quarter (waxing crescent)
- Moon comes at its eastern elongation. It's first quarter. Sun lights Moon laterally. We see only half of visible face

click to Moon's phases

- beyond first quarter, Moon is receiving more and more light. Moon is "waxing gibbous"
- Moon then reaches Sun-Earth axis, on the other side of Earth relative to Sun. It's full moon. Seen from Earth whole Moon near side is illuminated
- full moon is then decreasing during a week towards last quarter. Moon is said "waning gibbous"
- Moon reaches its western elongation. Sun is illuminating Moon laterally again. It's last quarter
- during one week Moon heads back to new moon passing through a crescent form (this decreasing crescent is termed "waning crescent")
- eventually, Moon is back to new. Phases sequence is over

Moon Phases For an Observer

In the northern hemisphere, just after the new moon, the first crescent Moon appears West and day after day, it gets distant from the Sun at the same time is increases in size. At first quarter Moon appears full South when Sun sets. The quarter Moon become waxing gibbous and it appears more East each day. When Moon reaches full, it's rising East just when Sun is setting West. Moon then sets in turn about 12 hours later that is by next morning. Waning gibbous Moon rises later each day until it reaches last quarter. Moon then rises at midnight and sets about midday. The last crescent completes the cycle as it rises less and less early before Sun
At the equator the sequence unfolds approximately the same except that Moon is usually seen lying on its side as it is usually high in the sky. The ecliptic there is often at a sharp angle to the horizon
In the southern hemisphere, the sequence is the same than in the North at these exceptions that Moon is seen upside-down and that the ecliptic is seen on the northern horizon hence the sequence of Moon's phases begins West and then turn northwest, North, etc

Each day, in mid-northern latitudes, Moon is rising later by an average of 50 mn each day. This interval of time is varying between 70 mn and 30 mn along the year due to the different inclination of the ecliptic on the horizon. The maximum delay is reached in January as the minimum occurs in September. This Moon is called the "Harvest Moon" as farmers used to make their last harvests before autumn benefiting from this extra light at full Moon time. The Moon behaves exactly in the opposite in the southern hemisphere. Generally, the Moon is moving each night by about 12.5° relative to the background stars

The Ashen Glow or Earthshine

When Moon is in first crescent on western horizon, one may sometimes see the dark part of the lunar disk slightly shining. This is called the "ashen light", the "ashen glow", or the "Earthshine", and it is a fine show. Moon's part which is still in night is lit by "earthshine". For an observer located on the Moon it would be night and Earth would be shortly after "Full Earth". The ashen glow is seen too at last quarter Moon when last crescent is rising on eastern horizon. A popular expression is terming the ashen glow "the old Moon in the New Moon's arms". Seen from Moon, Earth is 2° wide in the sky, that is four times larger than Moon seen from Earth. Earth's albedo -part of sunlight Earth is reflecting back space- is varying. Hence Earthshine was found to vary. A long time variation has been found working on an yearly basis: Earthshine is brighter (about 10 percent) in April and May. Main explanation for such a variation is Earth's cloud cover of course. Cloud cover is Earth's main reflectivity factor. Cloud cover is varying along the seasons due to climate's large scale patterns. The ashen glow may even be seen waxing and waning (by about 5 percent) on an hourly basis to an attentive observer. This is better due to what part of Earth is seen at Moon: Pacific Ocean, with its dark blue, is surely reflecting less light than a continent. Our planet's average albedo in recent years is 0.297 plus or minus 0.005