Phases of the Moon

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Slide 1

Phases of the Moon

New 0% Illuminated

Waxing Crescent 25% Illuminated on RHS

First Quarter 50% Illuminated on RHS

Waxing Gibbous 75% Illuminated on RHS

Full 100% Illuminated

Waning Gibbous 75% Illuminated on LHS

Third (Last) Quarter 50% Illuminated on LHS

Waning Crescent 25% Illuminated on LHS

Slide 2

Rising/Meridian/Setting Times

*Assuming the moon is always on the celestial equator and above the horizon 12 hours per day.

Slide 3

Lunar Periods

Sidereal Period – 27.3 days

The time between successive alignments of the earth, moon, and a star

The true orbital period of the moon

Synodic Period – 29.5 days

The time between successive alignments of the earth, moon, and the sun

The period of the cycle of lunar phases

Slide 4

Lunar Eclipses

Total Lunar Eclipse

The moon is entirely inside the earth’s umbra

Partial Lunar Eclipse

The moon is partially inside the earth’s umbra

Penumbral Lunar Eclipse

The moon is inside the Earth’s penumbra

These happen regularly and no one notices. The brightness of the moon only decreases by 10% to 20%

Slide 5

Solar Eclipses

Total Solar Eclipse

The moon completely obscures the sun for an observer at this location on the Earth

Partial Solar Eclipse

The moon partially obscures the sun for an observer at this location on the Earth

Annular Solar Eclipse

The moon is near apogee and its angular diameter is insufficient to completely cover the sun. An observer for whom the moon and sun are aligned will see a ring of the sun around the moon

Slide 6

Tides

- caused by differential gravitational forces exerted by the moon (the moon pulls more on one side of the earth than the other)

Even though the sun’s gravitational force is much larger than the moon’s, its differential force is smaller (since the size of the earth is small compared to the earth-sun distance)

This moon’s differential force causes “water bulges” on the earth on both sides of the line to the moon. Tides are caused by the Earth’s rotation carrying observers through a water bulge. Thus, typically high tide occurs twice a day.

Neap Tides – weak tides which occur when the moon’s and sun’s pull are perpendicular

Spring Tides – strong tides when the moon and sun are aligned.