Reports - 2014
Astronomy Benalla Meeting Presentations - Wednesday 16th July 2014
Presenter: Patrick Watson
MOON MATTERS
MOONLIGHT
Light that reaches Earth from the Moon consisting mostly of reflected Sunlight, with some reflected starlight, and some
reflected Earthlight (”Earth shine”)/
PHASES OF THE MOON
Phases refer to The proportion of the visible disc of the Moon that is illuminated - as seen from Earth
e.g.
New Moon: The Phase when none of its illuminated side is visible from Earth.
Full Moon: The Phase of the Moon when it is fully illuminated as seen from Earth.
TERMS USED WITH PHASES OF THE MOON
Waxing: The Phase is increasing from New to Full.
First Quarter: The Phase of the Waxing Moon that occurs midway between New Moon and Full Moon. Half the
Moon is illuminated.
Waning: The Phase of the Moon is decreasing from Full to New.
Last Quarter: The Phase of the Waning Moon that occurs midway between Full Moon and New Moon. Half the
Moon is illuminated [Also known as the third quarter].
Can we tell ,just by looking at the Moon, if the Phase is at First or Last Quarter?
Here in the southern hemisphere the Moon is seen to the north.
When facing the Moon, east is on your right and west on your left.
The sunlit part of the Moon “moves” from your left to your right.
SYNCHRONOUS ROTATION
Synchronous: In this sense, the circumstances in which a satellite spins on its axis in the same time as it takes to orbit
a planet thereby keeping the same face turned towards the planet at all times.
The Moon is in synchronous rotation with Earth.
The far side of the Moon is the hemisphere of the Moon that always faces away from Earth.
The near side of the Moon is the lunar hemisphere that is permanently turned towards the Earth.
In the strict sense of the statement - the Dark side of the Moon is the side that is opposite to the Sun.
HOW MUCH OF THE MOON CAN WE SEE?
We knpw that the Moon keeps the same side turned towards Earth. However, the Moon’s speed along its elliptical (not
circular) orbit varies with its distance from Earth while its rotation remains constant, and the Moon appears to swing
from side-to-side.
The Moon also appears to nod from north to south because its axis of rotation is not perpendicular to its orbital plane.
These motions are called librations and together allow us to see up to 59% of the Moon’s surface.
A third libration occurs because we view the Moon from different sides of the Earth at moonrise and moonset, another
about 1%.
What we see is an apparent oscillation by which parts near the edge of the Moon’s disc are alternately visible and
invisible.
DID YOU KNOW THAT THE MOON DOES NOT ORBIT THE EXACT CENTRE OF THE EARTH?
The Moon orbits a point on a line between the centres of the Earth and the Moon, approximately 1,710 km below the
surface of the Earth. Both the Earth and the Moon orbit this point.
Also, It is this point which produces the orbital path of the Earth around the Sun, not the Earth’s centre - as indicated in
most diagrams.
DIRECTIONS ON THE MOON
The convention is that "east" on the Moon is the direction toward Mare Crisium, and "west" is in the direction towards
Grimaldi – a walled plain.
At two points in it’s orbit, the Moon is referred to as being at:
PERIGEE: The point in an elliptical orbit that is nearest the Earth’s centre.
APOGEE: The point in an elliptical orbit around the Earth that is farthest from the Earth’s centre
APSIDES
The two points in an elliptical orbit that lie closest to or farthest from the centre of the body being orbited.
Singular: apsis or apse.
The straight line joining these two points is the line if apsides and is the same as the major axis of the orbit.
In the case of the Moon’s orbit around the Earth, the apsides are ...... PERIGEE and APOGEE.
LIMB – The rim of the visible disc of a celestial body as seen from Earth.
TERMINATOR – The dividing line between the sunlit and night portions of a planet or satellite.
