Lunar Observing – Finding ‘moonmarks’

For lunar observing, each phase of the moon provides a different vista. It is easiest to identify convenient landmarks (aka ‘moonmarks’) at first or third quarter of the lunar phase. During these phases,  shadow relief on the moon helps us discern where we are looking on the lunar globe. During our Night Sky Course lunar observing session we join together as a group to determine ‘how-to’ get our ‘moon-legs’ by doing a ‘moon-hop’.

Step 1: Someone found a really distinct feature near the terminator. We were able to identify it using a medium-scale lunar map (courtesy the Hatfield Lunar Atlas). The discovery feature was the Bay of Rainbows, a flooded partial crater (260km wide) extending into Mare Imbrium.

hatfield6annotatedStep 2: As a group, we reasoned out what the other targets could likely be. For example,  was crater Tycho in light or shadow? Well, if we can positively identify Copernicus then we can certainly identify the crater Tycho as it is closer to the terminator.

nightwatchthirdqmoonStep 3: Record your observations even while your are in the field. Here are some ‘chicken scratches’ on our Explore the Universe Lunar Observing form. These details are valuable for later reference, despite the messiness.lunarobs091116-4Here is a scan of the complete (rough) observing report: Lunar Observing Notes

Much of our viewing involved looking at craters in detail. Often we saw central peaks. For a good explanation of what shapes occur from a rebound due to meteoritic impact see

-Courtesy Patrick Moore – On the Moon (marvelous account of lunar explorations)

From First Quarter to Full Moon

Click on the Link to see Kepler Crater Kepler straddled over the terminator .Terminator is the separation of day and night on the moon. The sunlit side here is sunrise.

As the moon continues traveling in its orbit, the sunlight starts to flood the  crater. If for example we are observing  on the third day after first quarter moon, it may be hard to find Crater Kepler which might be still in shadow at first…  but  as sunrise starts to dawn over the face of the crater, we get to observe it . As the shadows on the surface move on, so do the light and shade values change on the lunar features.

Shade and Sunlight

Not only shadows, but full direct sunlight will illuminate very special seleneological features. Observing_and_Understanding_the_Moon’s_Geology_and_Origin

As we approach Full Moon lunar phase, the Williamson Guide for Lunar Observing asks us to re-observe craters Kepler and Copernicus so that we can see the crater ray systems and the crater ejecta blanket – the dark rim around the wall .

Full Moon  is shadowless… but other features become very obvious – such as crater ejecta blankets.An ejecta blanket is a generally symmetrical apron of ejecta that surrounds an impact crater; it is layered thickly at the crater’s rim and thin to discontinuous at the blanket’s outer edge. [1]

Structure of impact craters, showing surrounding ejecta

After an impact event, the falling debris forms an ejecta blanket surrounding the crater. Approximately half the volume of ejecta falls within 1 crater radius of the rim, or 2 radii from the center of the crater. – Courtesy Wikipedia


You can see the dark apron of ejecta material and the ray system

These images, courtesy the Hatfield Lunar Atlas show central crater Tycho before and near Full Moon. When Tycho is in shadow, the ejecta blanket disappears. (It helps to know the day number in the lunar phase … You’ll see a very different picture!)

Step 4: Repeat this exercise after full moon around third quarter to re-enforce your lunar moonmarking skills. What new features can you identify (and check off  your list) ? Here’s some description of what you can see at various days in the lunar cycle. ( Click on the image to take you to  a month of moongazing.)

A Month of Moonwatching