Cycles of The Moon Chapter 3
The Motion of The Moon • What direction does the moon move in the sky? • East… about 13º each day • Result of moon’s orbit around Earth • Do you see the same side or different sides of the moon? • Same side… the markings on the face don’t change
Phases • Sunlight illuminates ½ of the moon… just like Earth • The moon appears to go through phases because we see different amounts of the sunlit side… depending on where it is in its orbit. • Full: our side fully lit, far side dark, opposite Sun in the sky • New: our side dark, far side fully lit, close to Sun in the sky
Phases Continued… • Full – disc facing Earth lit (50% of the moon) • Waxing – right side of disc facing earth is lit • Waning – left side of disc facing earth is lit • Gibbous – more light than shadow • Crescent – more shadow than light
More Phases… • 1st Quarter – the right ½ of the disc facing Earth is lit (1/4 of the moon) • 3rd Quarter – the left ½ of the disc facing Earth is lit (1/4 of the moon)
Phases In Order • Full • Waxing Gibbous • 1st Quarter • Waxing Crescent • New • Waning Crescent • 3rd Quarter • Waning Gibbous… then back to Full
Moon During The Day?? • You can see the moon during the day depending on where it is in its orbit around Earth. • It appears pale because the Sun is so bright.
Orbital Period vs Phase Cycle • Sidereal Period – time it takes the moon to circle the sky once, 27.3 days • Synodic Period – one complete cycle of lunar phases, 29.5 days
At New Moon, the moon is near the Sun in the sky. • After 27.3 days, the moon has returned to its same position in the sky but the Sun has moved along the ecliptic. We don’t “see” a New Moon… • Wait another 2 days and the moon has caught up with the Sun… New Moon
Lunar Eclipse • Occurs at Full Moon • Moon moves through Earth’s shadow
Earth’s Shadow • Umbra: total shadow, about 1.4 million km long, points directly away from sun • Penumbra: partial shadow
Total Lunar Eclipse • The entire moon passes through Earth’s umbra. • The moon appears red in color because of sunlight bent by our atmosphere.
Interesting Tid-Bits • Aristotle concluded that Earth is a sphere because of the shadow projected causing a lunar eclipse. • A total lunar eclipse takes about 6 hours.
Partial Lunar Eclipse • The moon is tilted about 5° to the plane of Earth. • It may not pass through the center of Earth’s umbra and only be partially eclipsed. • A penumbral eclipse occurs when the moon passes through the penumbra but misses the umbra entirely.
Solar Eclipse • Occurs at New Moon • Moon moves between Earth and the Sun
Why? • The moon and the sun have the same angular diameter. • This allows the moon to cover the sun in our sky.
The Math… Small Angle Formula • Angular Diameter = Linear Diameter 206,265 Distance ** We use 206,265 because that is the # of seconds of arc in 1 radian… you don’t have to remember that! ** Make sure linear diameter and distance are in the same unit.