105 - The Moon

Phases of the Moon O swear not by the moon, the inconstant moon, that monthly changes in her circled orb, lest that thy love prove likewise variable ---Romeo and Juliet, Act 2 scene 2 Overview ● Understand the relationship between the Moon, Earth, and Sun, that causes the Moon's phases. ● Understand when the different Moon phases are visible. ● Link the observed phases with the Moon's rise and set times. Introduction How does the relation between the Moon, Earth, and Sun cause the phases that we see? What is the relation between the Moon's phases, its monthly orbit, and its rise and set times? The central point to understanding the phases of the Moon is that exactly half of the Moon is illuminated at all times, which is the half facing the Sun. The phases result from our viewing angle of the lit side, which depends on the Moon's position in its orbit around the Earth, relative to the Sun. For example, if the Moon is on the opposite side of the Earth from the Sun, we can see the entire lit side, and it is a full moon (Figure 1). If the Moon and Sun are aligned in the same direction, then the lit side is the far side of the Moon, and we see nothing. This is a new Moon. It takes 29.5 days for the Moon to go through a full cycle of its phases. Figure 1 : Top-down view of the Moon's orbit, showing the Moon in various positions, along with the phase that we see from Earth. The lit side always faces the Sun, and from the Earth's viewpoint, we see different fractions of the lit side. As the Earth turns, we experience noon when the observer is facing the Sun, and midnight when the observer is standing diametrically opposite the Sun. Note that East is always toward the leading side of the Earth's rotation. For example, the person in the Figure is experiencing sunset, with the Sun on the west horizon. Page 1 of 7

Figure 1 shows that as the Moon moves from new to full, we see an increasing fraction of the lit portion, so the Moon's phase is said to be waxing . The opposite happens as it moves from full to new, when it is waning . When the Moon's position is exactly between full and new, it looks like a half-moon. However, as is apparent in Figure 1, we can see one quarter of its surface, so it is formally called the first quarter for a waxing Moon, and last quarter or third quarter for waning. In between the new and quarter phases, the Moon is a crescent . Between the quarter and full, the Moon is in a gibbous phase. The Moon's rise and set times are linked to the phases. For example, the new moon is aligned toward the Sun, so it will rise and set with the Sun. The full moon is opposite the Sun, so it will rise at sunset, and set at sunrise. This allows basically continuous light on the Earth during this phase; for example, the "Harvest Moon" provides light to continue harvesting the crops near the autumnal equinox. In Figure 1, the person is standing at sunset, with the Sun on the west horizon. If the Moon is in the full phase, it will just be rising in the east. If the the Moon is in the first quarter phase, it is at its highest point (crossing the meridian) at this time of day. For the first quarter to appear on the east horizon, the person must be standing at noon, with the Sun overhead; and for the first quarter to appear on the west horizon, the person must be standing at midnight. So we see that the first quarter moon rises at noon, is highest at sunset, and sets at midnight. Looking at Figure 1, you might expect that at every new moon, the Moon will actually block out the Sun. This does happen sometimes, resulting in a solar eclipse , but not at every new moon. The Moon’s orbit is tilted about 5º to the ecliptic, so usually the Moon passes above or below the Sun in the sky. Similarly at full Moon, the Moon usually passes above or below Earth’s shadow, avoiding a lunar eclipse . There are usually several eclipses per year, including both solar and lunar eclipses. However, the Moon's shadow on the earth is only a few miles wide, so it's difficult to be in exactly the right place, at the right time, to observe a solar eclipse. Figure 2: The gegenschein. More information on the gegenschein is at http://apod.nasa.gov/apod/ap080507.html . Original image by Yuri Beletsky. In very dark skies, you might also be able to observe the gegenschein , the point directly opposite the Sun in the sky. Interplanetary dust on the ecliptic will reflect sunlight, and create a very faint glow in the sky at the point exactly opposite the Sun. Although very rare in real life, the planetarium makes use of this effect in order to show the point directly opposite the Sun. Additional Resources ● Dates of the Major Moon Phases 2005 - 2015 from the USNO: http://aa.usno.navy.mil/data/docs/MoonPhase.php ● Images and current Moon phase: http://tycho.usno.navy.mil/vphase.html ● Chris Dolan's Moon Phase java applet: http://www.astro.wisc.edu/~dolan/java/MoonPhase.html Last update: 11/05/14 MSO Page 2 of 7