The Moon Image Credit: T.Rector/NOAO

The moon completes one orbit around the earth every 27.3 days. We see only one side because it has a synchronous rotation (it rotates exactly once with each orbit) 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

Half of Moon is illuminated by Sun and half is dark Image credit: Nasa/Galileo spacecraft

Why do we see phases of the Moon? The phases occur because as the Moon orbits the earth, we see more or less of the illuminated side. 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.

Sidereal Month A sidereal month is the time the Moon takes to complete one full revolution around the Earth with respect to the background stars. A sidereal month lasts 27.322 days

Synodic month Because the Earth is constantly moving along its orbit about the Sun, the Moon must travel slightly more than 360° to get from one new moon to the next. Thus, the synodic month, or lunar month, lasts 29.531 days.

Moon Animation http://www.sumanasinc.com/webcontent/animations/content/sidereal.html

What is an eclipse? When either the Earth or the Moon 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.

Lunar eclipses can occur only at full moon. They 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)

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.

The Moon’s orbit is tilted 5° to ecliptic plane… So we have about two eclipse seasons each year, with a lunar eclipse at full moon and solar eclipse at new moon.

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).