The Moon Image Credit: T.Rector/NOAO

Half of Moon is illuminated by Sun and half is dark We see a changing combination of the bright and dark faces as Moon orbits Image credit: Nasa/Galileo spacecraft

Why do we see phases of the Moon? The phases of the Moon occur because light from the Moon is actually reflected sunlight. As the relative positions of the Earth, the Moon, and the Sun change, we see more or less of the illuminated half of the Moon. You may want to do an in-class demonstration of phases by darkening the room, using a lamp to represent the Sun, and giving each student a Styrofoam ball to represent the Moon. If your lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads. Image Credit: Lunar & Planetary Science Institute

} } Phases of the Moon: 29.5-day cycle waxing waning new crescent first quarter gibbous full last quarter } waxing Moon visible in afternoon/evening. Gets “fuller” and rises later each day. } waning Moon visible in late night/morning. Gets “less” and sets later each day.

We see only one side of Moon The moon completes one orbit around the earth every 27.3 days. Synchronous rotation: the Moon rotates exactly once with each orbit Use this tool from the Phases of the Moon tutorial to explain rise and set times for the Moon at various phases. As usual, please encourage your students to try the tutorial for themselves.

Figure 3-4 The Moon’s Rotation These diagrams show the Moon at four points in its orbit as viewed from high above the Earth’s north pole. (a) If the Moon did not rotate, then at various times the red crater would be visible from Earth while at other times the blue crater would be visible. Over a complete orbit, the entire surface of the Moon would be visible. (b) In reality, the Moon rotates on its north-south axis. Because the Moon makes one rotation in exactly the same time that it makes one orbit around the Earth, we see only one face of the Moon.

Nearside as viewed by Galileo Spacecraft Farside as viewed by Clementine Spacecraft Image Credit: Lunar & Planetary Science Institute

What causes eclipses? The Earth and Moon cast shadows. When either passes through the other’s shadow, we have an eclipse. Penumbra – lighter outer part of shadow umbra – darker inner cone This slide starts our discussion of eclipses. Use the figure to explain the umbra/penumbra shadows.

When can lunar eclipses occur? Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total. Use the interactive figure to show the conditions for the 3 types of lunar eclipse.

Figure 3-9 Total Lunar Eclipse This sequence of nine photographs was taken over a 3-hour period during the lunar eclipse of January 20, 2000. The sequence, which runs from right to left, shows the Moon moving through the Earth’s umbra. During the total phase of the eclipse (shown in the center), the Moon has a distinct reddish color. (Fred Espenak, NASA/Goddard Space Flight Center; ©2000 Fred Espenak, MrEclipse.com)

Solar Eclipse Figure 3-11 The Geometry of a Total Solar Eclipse During a total solar eclipse, the tip of the Moon’s umbra reaches the Earth’s surface. As the Earth and Moon move along their orbits, this tip traces an eclipse path across the Earth’s surface. People within the eclipse path see a total solar eclipse as the tip moves over them. Anyone within the penumbra sees only a partial eclipse. The inset photograph was taken from the Mir space station during the August 11, 1999, total solar eclipse (the same eclipse shown in Figure 3-10). The tip of the umbra appears as a black spot on the Earth’s surface. At the time the photograph was taken, this spot was 105 km (65 mi) wide and was crossing the English Channel at 3000 km/h (1900 mi/h). (Photograph by Jean-Pierre Haigneré, Centre National d’Etudes Spatiales, France/GSFS)

When can solar eclipses occur? Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular. Use the interactive figure to show the conditions for the 3 types of solar eclipse.

Why don’t we have an eclipse at every new and full moon? The Moon’s orbit is tilted 5° to ecliptic plane… So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon. Use this pond analogy to explain what we mean by nodes and how we get 2 eclipse seasons each year (roughly). Note: You may wish to demonstrate the Moon’s orbit and eclipse conditions as follows. Keep a model “Sun” on a table in the center of the lecture area; have your left fist represent the Earth, and hold a ball in the other hand to represent the Moon. Then you can show how the Moon orbits your “fist” at an inclination to the ecliptic plane, explaining the meaning of the nodes. You can also show eclipse seasons by “doing” the Moon’s orbit (with fixed nodes) as you walk around your model Sun: the students will see that eclipses are possible only during two periods each year. If you then add in precession of the nodes, students can see why eclipse seasons occur slightly more often than every 6 months.

Summary: Two conditions must be met to have an eclipse: It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).