Lunar Features
Because the moon spans 3476 kilometers, about a quarter the diameter of Earth, and lies only a
quarter million miles away, it exhibits a wealth of detail in a small telescopes and binoculars. Both will
reveal the Moon's desolate landscape punctuated by bright highlands, dark plains, and rayed craters.
At every phase except when full, you'll notice that the lunar globe is divided by the terminator, the
line separating the Moon's bright, sunlit side from the side hidden in shadow. Here is where surface
features stand out best. Seen in a small telescope or high-power binoculars, the landscape near the
terminator stands out in bold contrast and detail. The terrain looks very rough near the terminator
because here the Sun is near the lunar horizon. Thus every low hill casts a long, black shadow that
creates an exaggerated impression of height.
The term 'meteorite impact' is used to describe the process of surface bombardment by cosmic
objects. The objects themselves are variously referred to as impactors or 'projectiles'. The impact
process is explosive, impacting the surface at more than 20 km/sec (45,000 mi/hour). Upon impact,
the impactor vaporizes and the planetary or lunar material is compressed and is tossed out of the
target area, piling up around the hole with the bottom of the crater lower than the original ground
surface with the piled up material on the rim higher.
PART I: SIZE OF LUNAR FEATURES
To determine the size of any lunar feature you must first determine the scale of the photograph.
Using a ruler, measure the diameter of the Moon (Image #1) to the nearest millimeter. The moon’s
actual diameter is known to be 3476 km. Determine the scale of your photograph in km/mm.
Diameter of lunar image = __________ mm
Scale of Image #1 = __________ km/mm
To determine the magnification of Image #2 measure the distance from the centers of the craters
Plato and Cassini on Image #1 and Image #2. From these measurements determine the magnification
of Image #2.
Distance Image #1 = __________ mm
Distance Image #2 = __________ mm
Magnification = ________ __ times
The surface features on the full image are blurry making accurate measurements difficult. By knowing
the scale of the lunar image and the magnification of the inset (Image #2), you can now calculate the
diameters of the two craters, Plato and Cassini with greater accuracy as they appear in Image #1.
Compare your results with the known diameters of the two craters (Google it!)
Crater
Measured Diameter
Image #2 (mm)
Calculated Diameter
Image #1 (mm)
Calculated
Diameter (km)
Actual
Diameter (km)
Plato
Cassini
5
Height of Lunar Features
The height of a certain lunar feature such as mountains or craters can be calculated by analyzing the
length of their shadows. In the figure below you are viewing the moon from above one of its poles.
S
M
sunlight
T
B
C
In this diagram MB represents the height of a
surface feature, such as a mountain, BC is the
moon’s radius, MS is the length of the shadow as
seen from Earth, and BT is the distance of the
mountain from the terminator. Notice that each of
the two triangles are right triangles so that the
ratios of the sides are equal. So that:
MB
BT
=
MS
(Equation 1)
BC
Since we are interested in the height of the surface feature, rearranging results in:
(MS) (BT)
MB =
(Equation 2)
BC
Remembering your algebra, we must know all of the terms on the right hand side of the equation to
determine the height of the crater. BC is the Moon’s radius, which is known (from Part I) and BT, the
distance of the mountain from the terminator can be measured from the full lunar photograph
(Image #1). The length of the shadows (MS) is difficult to measure from the full lunar photograph,
but can be determined by measuring the shadow on the enlarged image and scaling it to the size of
Image #1. Now you have all the needed information to calculate the height of a lunar feature.
1. The mountains Mons Piton and Mons Pico have been identified on the magnified lunar
photograph. Measure the length of their shadows on the magnified image (Image #2) and from this
determine the length of the shadows (MS) on the full image (Image #1) using the scale of the
enlarged photo.
2. Measure the distance from the mountain’s center to the terminator from Image #1. (BT)
3. Use Equation 2 to calculate the height of the mountain (in millimeters) as it appears in Image #1.
Use the scale of Image #1 to determine the calculated height of the mountain (in kilometers).
4. Compare your answers with the known heights (Google it!)
Feature
Distance to
Terminator
(BT) (mm)
Moon’s
Radius (BC)
(mm)
Length of
Shadow (MS)
(mm)
Height (MB)
(mm)
Calculated
Height (MB)
(km)
Known Height
(km)
Mons Pico
Mons
Piton
6
Image #1:
Lunar Surface at 1st Quarter
Image #2: Magnified Portion
Crater Plato
Mons Pico
Crater Cassini
Mons Piton
7