- distinguish between the different phases of the Moon
- investigate how the Moon changes phases throughout a month
- predict where to locate the Moon in one of its phases
As the Earth rotates, the Moon also revolves around
the planet. Because of this, not only can the Moon be found in
different locations in the sky depending on the day, but the Moon also
looks different to an observer on the Earth. Over the course of the
Moon’s rotation period (the amount of time it takes the Moon to go
around the Earth once, equal to about 27.3 days) the Earth-Moon-Sun
geometry changes so that there is always a different amount of sunlight
shining on the Moon. This is what gives us our phases
A waning crescent Moon
Perhaps the easiest phases of the Moon to visualize are the New Moon
phase and the Full Moon
phase. A New Moon occurs when the Moon comes between the Earth and the
Sun. As a result, the Sun only lights up the side of the Moon away from
us, and the Moon appears dark, hard to see. Occasionally, when the Sun
and Moon line up perfectly, we see a solar eclipse (see Unit 4, Lesson 2).
Full Moon is the exact opposite. When the Moon is on the opposite side
of the Earth from the Sun, sunlight reflects off the entire surface of
the Moon facing the Earth, and we see a round, lit up Moon. Full Moons
are often so bright that many astronomical ground observations cannot
be made during this phase, as the light from the Moon washes out the
fainter stars. Occasionally the Earth comes directly in between the
Moon and the Sun, creating conditions for a lunar eclipse (see Unit 4, Lesson 2).
A full lunar month
goes from New Moon to New Moon and takes approximately 29.5 days. This
is slightly longer than the Moon’s rotational period, because we have
to keep in mind that the Earth is also moving around the Sun,
constantly changing the Earth-Moon-Sun geometry.
Even though it takes the Moon 27.3 days to go around the Earth once,
by the time it gets back to its original position, the position of the Sun
has changed, slightly altering the visible phase of the Moon. Because
of this, a lunar month takes slightly longer: 29.5 days.
Exactly between the New Moon and the Full Moon, comes two more common phases: the First Quarter Moon
and the Last Quarter Moon
The First Quarter Moon occurs during the first half of the lunar month,
when the Moon-Earth-Sun system forms a 90? angle. At this time, rays
from the Sun hit one half of the Moon’s surface, of which we can only
see a half, leaving us with a quarter - half of the half which is being
lit. Likewise, the Last Quarter Moon occurs during the last half of the
lunar month, when the Moon-Earth-Sun system forms another 90? angle.
During this phase, we again only see a quarter of the Moon.
The New, First Quarter, Full and Last Quarter phases of the Moon,
as seen from far above the Earth’s North pole, and from the surface
of the Earth.
It should be remembered that the Sun’s light always
hits half of the surface of the Moon, just like it always hits half of
the surface of the Earth. The phases occur because of our angle to the
As the Moon travels around
the Earth, we see “in between” phases of the Moon as it goes from one
of the four main phases to another. Whenever the Moon is in between the
New Moon phase and one of the Quarter phases, it is said to be a
crescent Moon (due to it’s croissant-like appearance!). When the Moon
is in between one of the Quarter phases and the Full Moon, it is said
to be a gibbous Moon. This rarely-seen word comes to us from the Latin
gibbus, meaning hump, or humpbacked.
as the Moon goes from New Moon to Full Moon during the first half of
the lunar month, it is known as a waxing Moon, and when it returns from
Full Moon to New Moon during the latter half of the lunar month, it is
known as a waning Moon. The terms waxing and waning are also sometimes
used in reference to the tides on the Earth (see Unit 4, Lesson 1) – the tide coming in and the tide going out, respectively.
In summary, the Moon goes through phases in the following order:
Moon - waxing crescent Moon - First Quarter Moon - waxing gibbous Moon
- Full Moon - waning gibbous Moon - Last Quarter Moon - waning crescent
Moon - New Moon
All phases of the Moon, as seen from far above the Earth’s North
Pole and from the surface of the Earth
Using a telescope, we can see similar phases on
planets closer to the Sun than us – venus and Mercury. When Galileo
first saw the phases of Venus through a telescope, his observations
helped contribute to a Sun-centred solar system, as opposed to an
Earth-centred one which was favoured at the time.
Though the Moon goes through all of these phases many times a year, certain, special phases stand out from the others. A Blue Moon
is the second Full Moon within one calendar month (such as, a Full Moon
on January 2, and then another one on January 31). While the expression
“once in a blue Moon” denotes a rare activity, Blue Moons actually
occur roughly every 3 years. The Full Moon that occurs closest to the
autumnal equinox (see Unit 1, Lesson 2) is known as the Harvest Moon
as the light of that particular Full Moon was useful to farmers working
late into the night to finish the harvest before it got cold. Likewise,
the following Full Moon is known as the Hunter’s Moon
, due to its same contribution to hunting at night.
phases of the moon are very apparent and consistent for observations.
That’s why many cultures have used the moon to keep track of time,
employing the lunar calendar
, based on the lunar month, or lunation
of 29.53 days. Because this month is shorter than the modern solar
month, 12 lunations (or 354.37 days) is not able to match the 365.25
days of a solar year. The seasons therefore become out of synch with
the lunar months. A lunisolar calendar
brings the lunar and solar calendars back into synch with a leap month
that occurs every two or three years to rectify the problem.
Islamic calendar is a modern example of a lunar calendar that repeats
itself every 360 lunar months with a correction of one day. Many other
cultures such as the Chinese, Korean, Hebrew, Tamil and Hindu cultures
have adapted their lunar calendars to lunisolar calendars to some
extent. A lunisolar calendar is still used by Christians to determine
the date of Easter, as it is falls on the Sunday after the first full
moon after March 21, the vernal equinox.