Star Points for May, 1998; by Curtis Roelle
Eye on the Moon
Tonight (Sunday, May 3) a beautiful half-full 1st quarter moon will
be hanging in the southern sky after sunset. From our distant
vantage point on earth the moon appears round but its true shape is
that of a ball. At any given time exactly one-half of the
spherical moon is illuminated by the sun's light. (There is an
exception however: during a total lunar eclipse.) So why do the
moon's "phases" change from night to night?
Orbiting earth at an average distance of nearly one quarter million
miles, the moon completes its cycle of phases in 29.5 days. This
period is called the "synodic" month.
The moon is called "new" when it is on the same side of the earth
as the sun. At this time we cannot see the moon because the side
facing us is not illuminated by sunlight. The new moon rises and
sets with the sun. New moon was one week ago on April 26.
When the moon has gone 1/4 of the way around the sun the right side
appears half full to people in earth's northern half. (In the
southern hemisphere the moon is "upside down" so the illuminated
half is on the left.) The "first quarter" moon rises around noon
and sets approximately at midnight.
Having completed half of its orbit around the earth the moon is
"full". This month full moon is on May 11. The moon rises at
sunset and sets near sunrise.
When the moon has completed 3/4 of its orbit it again appears half
full, but now the left half is illuminated. The "third quarter"
moon occurs on May 19, rising near midnight and setting around
noon. The new moon returns on May 25.
When is the best time to look at the moon with a telescope or
binoculars? Tonight or any night within a day or two of 1st
quarter. There are several reasons for this.
1. The moon is up as soon as it gets dark. You don't need to set
the alarm and get up in the middle of the night to see it.
2. The "terminator" crosses the center of the moon and is hence
seen straight on. The terminator is the line that separates day
and night on the moon. Along this line objects such as craters,
mountains, and valleys are much more visible because they have
shadows that are long and dramatic.
3. Even the unlit portion of the moon can appear dimly illuminated
by "earth shine". Earth shine is light reflected from earth's
oceans which illuminates the darkened lunar landscape. Earth shine
seen from the U.S. during the evening is caused by sunlight
reflected off the Pacific Ocean. Earth shine in the morning before
sunrise is caused by light scattering off the Atlantic Ocean.
The moon is an object which never disappoints no matter how small
a telescope is used to view it. Passing a pair of binoculars
between family members or friends can be a memorable experience
leading to a lifelong passion of star gazing.
For web surfers there are several good guides for learning more
about and observing the moon. "The Hitchhiker's Guide to the Moon"
and "The Lunar Website"
are two of the best around.