EDUCATOR’S PAGE
1999). Several large craters, from impacts suffered during the late-heavy bombardment period of the solar system’s forma- tion, excavated the huge basins in which we find the maria. Interestingly, though, the basalts that fill these large craters are younger. The explanation for the discrepancy relates to a later stage of interior heating in the moon, probably result- ing from radioactive decay and subsequent melting of mantle material that flowed upward into the basins through fractures from the initial impacts (Spudis, 2003). There are no signs of interior heat today, leaving the moon an essentially geologi- cally dead body, with the notable exceptions of surface changes due to micrometeorite, and occasionally larger, impacts, and space weathering (e.g., solar wind effects).
Phasing In and Out
Have you ever seen the moon during the day? Earth’s moon is visible at night, or during the day, depending on the particular phase it is in. Perhaps the reason many people erroneously believe the moon is only seen at night stems from the predomi- nant phases at this time -- the more conspicuous full, or nearly full (half and gibbous), moon. Rest assured, though, the moon can be seen during the day, particularly in the crescent and quarter moon phases, with most phases overlapping periods of both darkness and light on Earth. In our northern hemisphere, it’s easy to know whether we are approaching full moon, or have passed it, just by looking at which half of the moon is lit up from our vantage point: a waxing moon, or one becoming progressively more visible with each passing day until full, is seen illuminated on the right side; a waning moon, or one becoming progressively less visible with each passing day after a full moon, is seen illuminated on the left side.
Too many non-scientists have a misunderstanding of what accounts for the phases the moon exhibits each month. The real explanation deals with the orientation in space of the three bodies involved: sun, moon, and Earth. Light from the sun is reflected off the moon, and unless eclipsed, fifty percent of the moon is always illuminated by the sun. See Figure 1. As the moon journeys around the Earth in its monthly orbit it shows different portions of this lit side to our planet. In fact, the word month derives from “moonth.” A complete cycle of phases – from full moon back to full moon – takes about a month (29.5 days), with one complete orbit taking just under one month (27.3 days).1 During each month we see every phase of the moon at least once and everybody on Earth sees the same phase of the moon in any given 24-hour period (though, south of the equator the illuminated side will be flipped). The expression “once in a blue moon” refers to the relative rarity of an occurrence, and also more specifically to an infrequent second full moon in any given month. [In what month will you never see two full moons? Why?]
We see a specific named phase by virtue of Earth’s location relative to the sun and moon, which determines how much of the illuminated half of the moon is visible. When the moon is
Figure 1. Illustration of moon phases at relative locations in its orbit around Earth. In
order, 1-8 are new moon, waxing crescent, first quarter, waxing gibbous, full, waning gib- bous, third quarter, waning crescent. Letters a, b, and c show the phases seen from the perspective of Earth.
directly opposite the sun, with the Earth in between, we see the entirety of the illuminated half – and so we describe the phase as a full moon. When the moon forms a ninety degree angle to the sun, with the Earth at the vertex, we see half of the moon’s illuminated face. We appropriately refer to it as either a first or third quarter moon.2
When looking at Figure 1, a natural question to consider may be: why don’t we have an eclipse whenever the moon is in the new or full phase? After all, it appears that the moon is directly between the Earth and sun at new moon, and directly behind the Earth from the sun at full moon. The reason is because the moon does not orbit the Earth in the same plane that the Earth orbits the sun; instead, the moon’s orbit is tilted about 5 degrees from the plane of the Earth-sun (i.e., ecliptic). The result of the offset orbit typically places the moon either above or below the plane of the Earth during the new and full phases. See Figure 2 on the following page.
Falling into Shadow
Every month the moon crosses the plane of the Earth-sun, but only every so often will these crossings occur at exactly the right moon phase. In order to have a solar eclipse, when the sun and its light are blocked out by the moon, the moon must be in a new moon phase and concurrently passing through the Earth’s orbital plane. A solar eclipse only happens about 2-4 times a year, with a total solar eclipse (all of the sun blocked) occurring only about every 18 months (Debebe , n.d.a). Because the shadow of the moon is smaller than the Earth, only certain portions of the planet see the eclipse. See Figure 3. Who sees the eclipse is determined by where the Earth is in its daily rotation for the observer. A new moon phase coincides with day (not night), and so only countries experiencing daytime at the moments of crossing have a chance to see the eclipse.
1. This certainly seems to defy logic, shouldn’t they be the same? We must remember that while the moon is orbiting the Earth, the Earth is simultaneously orbiting the sun. After a month, the Earth is 1/12 further along in its orbit, displacing the sun from where it was when the cycle of phases began. One complete orbit is defined by position in space, and one complete cycle of phases is defined by the moon’s position relative to the sun’s location. See the diagram at the bottom of the reference (Pogge, 2008) for an illustration of this confusing issue, or search “synodic vs. sidereal month” online.
2. Why “quarters” you ask? The moon is about a fourth of the way through its monthly orbit around the Earth –and consequently one- quarter through its cycle of phases – at first-quarter phase, and three-quarters through its orbit at third quarter phase.
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