Imagine a warm summer night, looking up at the twilight sky and you see a Full Moon. Now imagine looking up at the sky sometime later that month, but this time the Moon is only partially full. Ever wonder why this happens?
As the Moon orbits the Earth, the parts of the Moon being illuminated by the Sun change.
- New Moon:
When the Moon is between the Earth and the sun, it appears dark, a "new" Moon. The three objects (Moon, Sun, and Earth) are in approximate alignment. The entire illuminated portion is hidden on the back side of the Moon; it is the side that we can’t see.
- Waxing Crescent: After New Moon, the sunlit portion is increasing, but is still less than half so it’s a waxing crescent.
- First Quarter: The Moon is at a 90-degree angle with respect to the Earth and the Sun, so the right half is illuminated.
- Waxing Gibbous: After the first quarter, the sunlight portion is still increasing, but it is more than half, so its phase is waxing gibbous.
- Full Moon: When the sun and Moon are on opposite sides of the Earth, the Moon appears "full" to us, a bright, round disk.
- Waning Gibbous: After the full Moon (maximum illumination), the light continually decreases. So, the waning gibbous phase occurs next.
- Third Quarter: The Moon is again at a 90-degree angle with respect to the Earth and the Sun and the left half is illuminated.
- Waning Crescent: The sunlit portion of the Moon is still decreasing; the Moon shrinks to a crescent, with the left portion illuminated.
As it appears to a viewer from Earth, the Moon goes through a complete cycle of Moon phases in almost a month, 29.5 days, also known as the synodic period. As it appears from the stars, the Moon goes through a complete cycle of Moon phases in 27.3 days, also known as the sidereal period. The synodic is different from the sidereal because on Earth, we are viewing the Moon, while the Earth is still rotating around the sun. During the lunar cycle, the Earth has moved almost a month along its year orbit around the Sun, which alters our angle of view of the Moon, which then alters the phase. The Earth’s orbital direction lengthens the period for Earth bound observers, which is us.
You may also be curious at why at new Moon, the Moon doesn’t block the Sun; and at a full Moon, why the Earth doesn’t block all the sunlight that reflects on the Moon. This is because the Moon’s orbit around the Earth is about five degrees off from the normal Earth-sun orbit.
But, there are special times during the year when our Earth, Sun, and the Moon line up. When the Moon blocks the Sun or a part of the Sun, a solar eclipse occurs, this can only occur during the new Moon phase. The Sun’s outermost layer, the corona, becomes apparent when the disk of the Sun is covered.
When the Earth is in between the Moon and the sun a lunar eclipse occurs, which only occurs during a full Moon phase. About four to seven eclipses happen a year, but some are “partial” eclipses.