Understanding and Observing the Moon's Phases ... - Calgary Centre

Understanding
and Observing
Because of the Moon’s trek around the sky,
from time to time it passes near or even
drifts over other celestial objects. Close
approaches are known as conjunctions, whereas
covering another object is called an occultation
or eclipse, depending on whether a night time
object or the Sun is covered.
The Moon,
its Phases
and
Eclipses
The Moon is Earth’s closest companion in
space. Though well studied, to many people,
Earth’s rocky ‘sibling’ represents an
unknown entity. This brochure will
introduce you to lunar phases and motions
as well as explain the basics of eclipses.
Lunar Motions
The Moon is a cold, rocky body 3,476 km
across and can easily fit between Vancouver
and Ottawa. It has no light of its own but
shines by sunlight reflected from its surface.
Tight groupings of the Moon with bright
planets or stars are striking sights and make
excellent photographs, particularly if a scenic
earthly foreground is included. Use a camera
tripod to make sure your camera stays steady
during the time exposure. Depending on
your choice of camera, exposures of about ¼
to a few seconds should give useable results.
Don’t forget to experiment with subtle
foreground lighting for dramatic effects.
Lunar Phases
A common misconception is that lunar
phases are somehow caused by the shadow
of the Earth falling onto the Moon. In
reality, the Moon’s phases are the result of
our changing viewing angle of the Moon as it
orbits the Earth.
Understanding the Moon’s phases cannot
really be done without also understanding
how the Moon’s position in the sky varies
throughout the lunar month.
A New Moon occurs when the Moon lies
hidden in the Sun’s glare and not visible
because only the shaded side is facing us.
Over a few days, a thin waxing crescent will
become apparent in the evening sky shortly
after sunset. Because the Moon is now
surrounded by darker sky, the faintly
luminous shaded part of
the Moon is now visible
due to earth shine, the
reflected light from a
nearly fully-illuminated
Earth (as seen from the
Moon). This stage is commonly described as
the ‘old Moon in the new Moon’s arms.’
About 7 days after New Moon, the shadow
line separating the lit
and unlit portions of
the Moon (known as
the terminator) is
straight. This represents
the First Quarter Moon
because we are now
one quarter through the Moon’s orbit
around the Earth. This Lunar phase is
visible throughout the afternoon in a bright

daylight sky. Even though the Moon is high
in the south at sunset, and in dark skies
during the evening, Earth shine on the
shaded part of the Moon is difficult to see,
since the Earth, as seen from the Moon, is
also only half illuminated and not as bright as
earlier in the cycle. In binoculars and small
telescopes, craters are easily visible along the
terminator.
Two weeks after New Moon, the Moon is
Full and rises as the Sun sets and sets when
the Sun rises. In other words, it is visible
only in a darkened sky.
An astronaut on the
Moon would now see a
‘New Earth’, completely
in shadow. Now the
craters are only seen
with difficulty, but
surface brightness
variationss are best seen. Darker maria
(pronounced mar’ ee uh, Latin for seas)
stand out from lighter highlands and both
are dusted by bright white rays of material
spewed from younger craters.
A Third or Last Quarter Moon is seen in
dark skies only in the
predawn and sets in the
west at about noon.
The Moon’s phase
continues to wane and
Earth shine is again
obvious just before
sunrise where we see the ‘new Moon in old
Moon’s arms.’ It takes the Moon 29.5 days
to complete this cycle; one lunar month has
passed. In fact, this is the origin of the word
‘month.’
Named Moons
A number of full
Moons in a given year
have widely recognised
names Honey, Hunter,
Harvest, but all Full
Moons are named,
albeit with sometimes
lesser-known monikers.
January - Wolf, Hunger, Old
February - Snow, Ice
March - Worm, Sap, Sugaring, Crow, Storm
April - Pink, Egg, Gras, Rai, Growing
May - Flower, Planting, Milk, Hare
June - Strawberry, Rose, Honey, Mead
July - Buck, Thunder, Deer, Hay
August - Sturgeon, Corn, Fruit, Barley
September - Harvest (nearest autumn equinox)
October - Hunter's (after Harvest)
November - Beaver, Frost, Snow
December - Cold, Long Night, Winter
‘Once in a blue moon’ is used to refer to a
rare event, four Full Moons in a season.
However, due to a mistake made in an
astronomy magazine in the late 1940’s,
popular culture has embraced a revised
definition: the blue moon is the second Full
Moon in the same month. Both occur with
roughly equal frequency; about one every
two and a half years.
Eclipses
There is a time that the Earth’s shadow can
change the Moon’s appearance, and that is
during a lunar eclipse, when the Moon passes
through the Earth’s shadow. A Lunar
Eclipse only occurs during a Full Moon,
though not all Full Moons produce an
eclipse because the Moon’s orbit is oblique
to the Earth’s orbit around the Sun. An
eclipse can only occur when the Moon is
crossing the Earth’s orbital plane and the
Moon, Earth and Sun are lined up. For
more information, see our companion
brochure, Observing and Understanding Lunar
Eclipses.
A related phenomenon is the solar eclipse
when the Sun is covered because the Moon
is passing between the Sun and the Earth.
This event takes place only on select New
Moons.
Tides
The tides on the Earth are primarily due to
the change in intensity of the Moon's
gravitational pull from one side of the Earth
to the other. This forms two tidal bulges,
one on either side of the Earth.
Full Moon Effect
Some people blame the Moon for affecting
people’s moods, but careful scientific studies
over the years have found no measurable
effect.
Summary
The Moon presents an ever-changing face to
us because of its movement through our day
and night skies and is an ideal way for a
beginner to enjoy astronomy as well as for
experts to ponder its movements and origin.
More information on the topics covered in
this brochure is available in the Observer’s
Handbook, which is included with
membership in the Royal Astronomical
Society of Canada.
Find out more
To learn more about the Royal Astronomical
Society of Canada, or membership in the
Society, contact your local Centre or the
Society’s National Office:
Website:
www.rasc.ca
E-mail:
Nationaloffice@rasc.ca
Telephone:
Phone: (416) 924-7973
Fax: (416) 924-2911
Mailing Address:
The Royal Astronomical Society of Canada
203 - 4920 Dundas St W
Toronto ON M9A 1B7