Phases of the Moon

1
Phases of the Moon

• The Phases of the Moon are the variations in the
  Moons appearance as the Moon orbits the Earth.
• They are due to the changing Sun-Earth-Moon angle
  through each month.
• The following is the cycle of lunar phases new,
  crescent, first quarter, gibbous, full, gibbous,
  third quarter, crescent, new

2
Moon Phases

• The Moon orbits the Earth in about one month
  (29.5 days)
• Over one orbit the appearance and rise and set
  times change with the cycle of Lunar Phases.
• The figure is not to scale

3
Light Dark

• We only see the Moon because sunlight reflects
  off its surface.
• At any one time, half the Moon is always lit and
  half is in darkness.
• The amount of the illuminated half we see from
  Earth depends on the position of the Moon in its
  orbit
• The time of day the Moon is visible also depends
  on its phase.

How the Earth and Moon would look high above
Earths North Pole. The figure is not to scale.
4
Example Observing the Moon

• Between January 23-25, 2004, at dusk, the Moon
  was visible in its waxing crescent phase.
• The first night it appeared close to the Sun.
  Subsequent nights it appeared higher in the sky
  (farther from the Sun) and the lit portion
  appeared to be growing larger.
• What do you think you would see the next night?
  How about a week later?

5
Lunar Phase Terms

• The cycle of Lunar Phases starts at the New Moon.
• At New Moon the Moon is not visible to us on
  Earth.
• From New Moon to Full Moon the illuminated
  fraction of the Moon we see from Earth grows. It
  is said to be waxing.
• After Full Moon the fraction of the illuminated
  Moon visible from Earth shrinks. It is said to be
  waning.

We can only see the half of the Moon above and to
the right of the diagonal line drawn on the
Moon in the figure above.
6
The Cycle of Lunar Phases

• The Lunar cycle is split into quarters
• New Moon
• 1st Quarter
• Full Moon
• 3rd (or Last) Quarter
• Since the whole cycle lasts about a month each
  quarter lasts about a week.

7

• As you view the moon from different directions
  (relative to the Sun) its appearance changes.

8
crosses meridian at sundown
crosses meridian at about 900 pm
crosses meridian at about 300 pm
sunlight
crosses meridian at midnight
crosses meridian at noon
sunlight
crosses meridian at about 900 am
crosses meridian at about 300 am
crosses meridian at dawn
9
The Lunar Phases Web Tool

• This is an interactive tool that helps illustrate
  how
• the time of day,
• the phase of the Moon and,
• the position of the Moon in its orbit are all
  related to one another.
• The time that is used in this tool is only an
  average 6 PM represents sunset, 6 AM represents
  sunrise, etc.

Lunar Phases Web Tool located
at http//www.calvin.edu/lmolnar/moon/index.html