The Phases of the Moon
- The Moon orbits the Earth roughly once a month.Looking down on the Earth and Moon from above the Earth’s north pole, we see that its revolution is in the same direction as the Earth’s rotation (and also the Earth’s revolution around the Sun).
- The Moon shines by reflected sunlight.Therefore, at any time only one half of the Moon, the side facing the Sun, is illuminated.
The dividing circle between the light side and the dark side is called the terminator.
- The illuminated side of the Moon is not necessarily the half which faces the Earth.Depending on the relative positions of the Sun, Moon, and Earth, we see different fractions of the Moon illuminated.These are called the phases of the Moon.
- At new moon, we can’t see any of the illuminated half of the Moon; at full moon we can see all of it.
- Halfway in between new and full moon, we see half of the illuminated half of the Moon, or a quarter of the Moon.First quarter occurs as the Moon moves from new to full; third or last quarter occurs as the Moon moves from full to new.
- Between the new and quarter moons, only a small fraction of the Moon is illuminated; we call this a crescent moon.
Between the quarter and full moons a larger fraction of the Moon is illuminated; we call this a gibbous moon.
When the Moon moves from new to full, it becomes more illuminated, and we say that it is waxing. When it moves from full to new, it becomes less illuminated, and we say that it is waning.
- On any particular night, the Moon will essentially be motionless.As can be seen from the diagram above, a full moon must therefore rise around 6 P.M., be overhead at midnight, and set around 6 A.M.A first quarter moon must rise around noon, be overhead around 6 P.M., and set around midnight.
Crescent moons are overhead during the day, but they are generally only visible near sunrise/sunset (both because of their small illumination and the brighter light from the Sun).
Question: if it’s 3 A.M. and the Moon is rising, what phase is it?
- The synodic month is defined as the time it takes for the Moon to return to the same position relative to the Sun, e.g. from full moon to full moon.The synodic month is equal to 29.5 days.
- The sidereal month is defined as the time it takes for the Moon to return to the same position relative to the stars; it is equal to 27.3 days.The sidereal month is shorter than the synodic month because of the revolution of the Earth around the Sun, as can be seen at the right.The Moon doesn’t have to travel as far around its orbit to line up with the same distant star.
4.3 Lunar Eclipses
(Discovering the Universe, 5th ed., §1-10)
4.4 Solar Eclipses
(Discovering the Universe, 5th ed., §1-11)
4.5 The Frequency of Eclipses
(Discovering the Universe, 5th ed., §1-9)