Understanding and Observing the Moon's Phases ... - Calgary Centre
Understanding and Observing the Moon's Phases ... - Calgary Centre
Understanding and Observing the Moon's Phases ... - Calgary Centre
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<strong>Underst<strong>and</strong>ing</strong><br />
<strong>and</strong> <strong>Observing</strong><br />
Because of <strong>the</strong> Moon’s trek around <strong>the</strong> sky,<br />
from time to time it passes near or even<br />
drifts over o<strong>the</strong>r celestial objects. Close<br />
approaches are known as conjunctions, whereas<br />
covering ano<strong>the</strong>r object is called an occultation<br />
or eclipse, depending on whe<strong>the</strong>r a night time<br />
object or <strong>the</strong> Sun is covered.<br />
The Moon,<br />
its <strong>Phases</strong><br />
<strong>and</strong><br />
Eclipses<br />
The Moon is Earth’s closest companion in<br />
space. Though well studied, to many people,<br />
Earth’s rocky ‘sibling’ represents an<br />
unknown entity. This brochure will<br />
introduce you to lunar phases <strong>and</strong> motions<br />
as well as explain <strong>the</strong> basics of eclipses.<br />
Lunar Motions<br />
The Moon is a cold, rocky body 3,476 km<br />
across <strong>and</strong> can easily fit between Vancouver<br />
<strong>and</strong> Ottawa. It has no light of its own but<br />
shines by sunlight reflected from its surface.<br />
Tight groupings of <strong>the</strong> Moon with bright<br />
planets or stars are striking sights <strong>and</strong> make<br />
excellent photographs, particularly if a scenic<br />
earthly foreground is included. Use a camera<br />
tripod to make sure your camera stays steady<br />
during <strong>the</strong> time exposure. Depending on<br />
your choice of camera, exposures of about ¼<br />
to a few seconds should give useable results.<br />
Don’t forget to experiment with subtle<br />
foreground lighting for dramatic effects.<br />
Lunar <strong>Phases</strong><br />
A common misconception is that lunar<br />
phases are somehow caused by <strong>the</strong> shadow<br />
of <strong>the</strong> Earth falling onto <strong>the</strong> Moon. In<br />
reality, <strong>the</strong> Moon’s phases are <strong>the</strong> result of<br />
our changing viewing angle of <strong>the</strong> Moon as it<br />
orbits <strong>the</strong> Earth.<br />
<strong>Underst<strong>and</strong>ing</strong> <strong>the</strong> Moon’s phases cannot<br />
really be done without also underst<strong>and</strong>ing<br />
how <strong>the</strong> Moon’s position in <strong>the</strong> sky varies<br />
throughout <strong>the</strong> lunar month.<br />
A New Moon occurs when <strong>the</strong> Moon lies<br />
hidden in <strong>the</strong> Sun’s glare <strong>and</strong> not visible<br />
because only <strong>the</strong> shaded side is facing us.<br />
Over a few days, a thin waxing crescent will<br />
become apparent in <strong>the</strong> evening sky shortly<br />
after sunset. Because <strong>the</strong> Moon is now<br />
surrounded by darker sky, <strong>the</strong> faintly<br />
luminous shaded part of<br />
<strong>the</strong> Moon is now visible<br />
due to earth shine, <strong>the</strong><br />
reflected light from a<br />
nearly fully-illuminated<br />
Earth (as seen from <strong>the</strong><br />
Moon). This stage is commonly described as<br />
<strong>the</strong> ‘old Moon in <strong>the</strong> new Moon’s arms.’<br />
About 7 days after New Moon, <strong>the</strong> shadow<br />
line separating <strong>the</strong> lit<br />
<strong>and</strong> unlit portions of<br />
<strong>the</strong> Moon (known as<br />
<strong>the</strong> terminator) is<br />
straight. This represents<br />
<strong>the</strong> First Quarter Moon<br />
because we are now<br />
one quarter through <strong>the</strong> Moon’s orbit<br />
around <strong>the</strong> Earth. This Lunar phase is<br />
visible throughout <strong>the</strong> afternoon in a bright
daylight sky. Even though <strong>the</strong> Moon is high<br />
in <strong>the</strong> south at sunset, <strong>and</strong> in dark skies<br />
during <strong>the</strong> evening, Earth shine on <strong>the</strong><br />
shaded part of <strong>the</strong> Moon is difficult to see,<br />
since <strong>the</strong> Earth, as seen from <strong>the</strong> Moon, is<br />
also only half illuminated <strong>and</strong> not as bright as<br />
earlier in <strong>the</strong> cycle. In binoculars <strong>and</strong> small<br />
telescopes, craters are easily visible along <strong>the</strong><br />
terminator.<br />
Two weeks after New Moon, <strong>the</strong> Moon is<br />
Full <strong>and</strong> rises as <strong>the</strong> Sun sets <strong>and</strong> sets when<br />
<strong>the</strong> Sun rises. In o<strong>the</strong>r words, it is visible<br />
only in a darkened sky.<br />
An astronaut on <strong>the</strong><br />
Moon would now see a<br />
‘New Earth’, completely<br />
in shadow. Now <strong>the</strong><br />
craters are only seen<br />
with difficulty, but<br />
surface brightness<br />
variationss are best seen. Darker maria<br />
(pronounced mar’ ee uh, Latin for seas)<br />
st<strong>and</strong> out from lighter highl<strong>and</strong>s <strong>and</strong> both<br />
are dusted by bright white rays of material<br />
spewed from younger craters.<br />
A Third or Last Quarter Moon is seen in<br />
dark skies only in <strong>the</strong><br />
predawn <strong>and</strong> sets in <strong>the</strong><br />
west at about noon.<br />
The Moon’s phase<br />
continues to wane <strong>and</strong><br />
Earth shine is again<br />
obvious just before<br />
sunrise where we see <strong>the</strong> ‘new Moon in old<br />
Moon’s arms.’ It takes <strong>the</strong> Moon 29.5 days<br />
to complete this cycle; one lunar month has<br />
passed. In fact, this is <strong>the</strong> origin of <strong>the</strong> word<br />
‘month.’<br />
Named Moons<br />
A number of full<br />
Moons in a given year<br />
have widely recognised<br />
names Honey, Hunter,<br />
Harvest, but all Full<br />
Moons are named,<br />
albeit with sometimes<br />
lesser-known monikers.<br />
January - Wolf, Hunger, Old<br />
February - Snow, Ice<br />
March - Worm, Sap, Sugaring, Crow, Storm<br />
April - Pink, Egg, Gras, Rai, Growing<br />
May - Flower, Planting, Milk, Hare<br />
June - Strawberry, Rose, Honey, Mead<br />
July - Buck, Thunder, Deer, Hay<br />
August - Sturgeon, Corn, Fruit, Barley<br />
September - Harvest (nearest autumn equinox)<br />
October - Hunter's (after Harvest)<br />
November - Beaver, Frost, Snow<br />
December - Cold, Long Night, Winter<br />
‘Once in a blue moon’ is used to refer to a<br />
rare event, four Full Moons in a season.<br />
However, due to a mistake made in an<br />
astronomy magazine in <strong>the</strong> late 1940’s,<br />
popular culture has embraced a revised<br />
definition: <strong>the</strong> blue moon is <strong>the</strong> second Full<br />
Moon in <strong>the</strong> same month. Both occur with<br />
roughly equal frequency; about one every<br />
two <strong>and</strong> a half years.<br />
Eclipses<br />
There is a time that <strong>the</strong> Earth’s shadow can<br />
change <strong>the</strong> Moon’s appearance, <strong>and</strong> that is<br />
during a lunar eclipse, when <strong>the</strong> Moon passes<br />
through <strong>the</strong> Earth’s shadow. A Lunar<br />
Eclipse only occurs during a Full Moon,<br />
though not all Full Moons produce an<br />
eclipse because <strong>the</strong> Moon’s orbit is oblique<br />
to <strong>the</strong> Earth’s orbit around <strong>the</strong> Sun. An<br />
eclipse can only occur when <strong>the</strong> Moon is<br />
crossing <strong>the</strong> Earth’s orbital plane <strong>and</strong> <strong>the</strong><br />
Moon, Earth <strong>and</strong> Sun are lined up. For<br />
more information, see our companion<br />
brochure, <strong>Observing</strong> <strong>and</strong> <strong>Underst<strong>and</strong>ing</strong> Lunar<br />
Eclipses.<br />
A related phenomenon is <strong>the</strong> solar eclipse<br />
when <strong>the</strong> Sun is covered because <strong>the</strong> Moon<br />
is passing between <strong>the</strong> Sun <strong>and</strong> <strong>the</strong> Earth.<br />
This event takes place only on select New<br />
Moons.<br />
Tides<br />
The tides on <strong>the</strong> Earth are primarily due to<br />
<strong>the</strong> change in intensity of <strong>the</strong> <strong>Moon's</strong><br />
gravitational pull from one side of <strong>the</strong> Earth<br />
to <strong>the</strong> o<strong>the</strong>r. This forms two tidal bulges,<br />
one on ei<strong>the</strong>r side of <strong>the</strong> Earth.<br />
Full Moon Effect<br />
Some people blame <strong>the</strong> Moon for affecting<br />
people’s moods, but careful scientific studies<br />
over <strong>the</strong> years have found no measurable<br />
effect.<br />
Summary<br />
The Moon presents an ever-changing face to<br />
us because of its movement through our day<br />
<strong>and</strong> night skies <strong>and</strong> is an ideal way for a<br />
beginner to enjoy astronomy as well as for<br />
experts to ponder its movements <strong>and</strong> origin.<br />
More information on <strong>the</strong> topics covered in<br />
this brochure is available in <strong>the</strong> Observer’s<br />
H<strong>and</strong>book, which is included with<br />
membership in <strong>the</strong> Royal Astronomical<br />
Society of Canada.<br />
Find out more<br />
To learn more about <strong>the</strong> Royal Astronomical<br />
Society of Canada, or membership in <strong>the</strong><br />
Society, contact your local <strong>Centre</strong> or <strong>the</strong><br />
Society’s National Office:<br />
Website:<br />
www.rasc.ca<br />
E-mail:<br />
Nationaloffice@rasc.ca<br />
Telephone:<br />
Phone: (416) 924-7973<br />
Fax: (416) 924-2911<br />
Mailing Address:<br />
The Royal Astronomical Society of Canada<br />
203 - 4920 Dundas St W<br />
Toronto ON M9A 1B7