The Moon

Earth’s Lone Satellite

• Fission theory

Theories about the Origin of the Moon

• The first moon-origin theory to gain a solid foothold was put forth in 1878. That year, George Howard Darwin, son of the famous evolutionist, proposed that Earth spun so rapidly in its early years that the sun's gravity eventually yanked off a chunk of an increasingly elongated Earth; that chunk became the moon.

• Earth was spinning so fast the Moon split off.

• Four years later, the geologist Osmond Fisher added a juicy addendum: The Pacific ocean basin marks the scar left behind where our satellite ripped away. The so-called "fission" theory became the accepted wisdom well into the 20th century.

Theories about the Origin of the Moon

• In 1909, an astronomer with the all-American name of Thomas Jefferson Jackson See proposed that the moon was a wandering planet that had been snared by Earth's gravity, like a fly in a spider web.

• Earth's gravitational field captured the Moon.

• Capture Theory

Theories about the Origin of the Moon

Theories about the Origin of the Moon

• The third theory, advocated by the astronomer Edouard Roche among others, was coaccretion.

• In this model, the Earth and the moon formed independently, side by side as it were, from the same material that formed all the planets of our solar system.

• The Moon formed right alongside Earth.

• Coaccretion / Double Planet

Theories about the Origin of the Moon

• Some clever scientist eventually dubbed the Big Three "daughter" (fission), "spouse" (capture), and "sister" (coaccretion).

Criticisms of the theory:

• Fission theory: (a) The Moon would likely spin off from the Earth's equator, so why is the Moon's orbit inclined the Earth's equator? (b) The Earth's rotation would have to be about once every 4 hours. Why was it so fast?

• Binary Accretion theory: If they formed together, how could it be that the Moon would lack much of an iron core?

• Capture theory: (a) Capture is difficult (how to slow down the Moon enough!). (b) If Moon formed elsewhere, why do Moon and Earth have such similar compositions?

Theories about the Origin of the Moon

• There was a collision between a Mars sized body and the Earth.

• The collision knocked a lot of molten rock debris into orbit around the Earth.

• The debris eventually collected together to form the Moon we now have.

• Giant Impact theory:

Giant Impact theory is the favored model for the following reasons:

• 1. Composition of Earth and Moon are similar because the Moon formed out of debris from the Earth's mantle.

• 2. Differences in compositions can be explained because the original impacting body formed elsewhere.

• 3. There is no reason to think the impact would be at the Earth's equator, so the inclination of the Moon's orbit is not surprising. Interesting the Moon's orbit is little inclined to the ecliptic, which is the most likely place for the original impacting body to have existed.

• 4. The original body could have formed in an iron poor environment, and if the Earth's iron had already settled to its core, then this would explain why the Moon has little iron in its core.

Facts about the Moon

Distance 384,000 km

Diameter 3746 km

Revolution 27.3 days

Rotation 27.3 days

Gravity 1/6th the gravity of the earth

Eccentricity of Orbit 5o

Direction of Revolution East to West

To complete the Phases

29.5 days

Moon’s Orbit

• Perigee – the moon is nearest the earth in its orbit.

• Apogee – the moon is farthest from the earth.

Phases of the moon• New Moon – Moon is faintly

visible.• Waxing Crescent – Horn

away from the sun.• First Quarter – half lighted• Waxing Gibbous – humped-

phase of the moon. • Full Moon – Well lighted

side faces us.• Waning Gibbous – • Last Quarter – • Waning Crescent – horn is

directed towards the sun.

Phases of the moon

Eclipses

• Lunar Eclipse – 1. Penumbral– 2. Partial– 3. Total

• Solar Eclipse– 1. Total Solar– 2. Partial Solar– 3. Annular eclipse

Ecliptic Plane

Lunar Eclipse• A Lunar eclipse occurs when the Sun casts Earth's

shadow onto the Moon.  • For this to happen, the Earth must be physically

between the Sun and Moon with all three bodies lying on the same plane of orbit. 

• A lunar eclipse can only occur during a Full Moon and when the Moon passes through all or a portion of Earth's shadow.

• The outer portion of the shadow cast from Earth is known as the penumbral shadow, which is an area where Earth obstructs only a part of the Sun's light from reaching the Moon. 

• The umbral shadow is the "inner" shadow, which is the area where Earth blocks all direct sunlight from reaching the Moon. 

• A penumbral lunar eclipse is subtle and very difficult to observe.  A partial lunar eclipse is when a portion of the Moon passes through the Earth's umbral shadow. 

• Finally, a total lunar eclipse is when the entire Moon passes into the Earth's umbral shadow.  During a total lunar eclipse, the sequence of eclipses are penumbral, partial, total, partial and back to penumbral.  

Lunar Eclipse

Conditions for Total Lunar Eclipse to occur

• A total lunar eclipse occurs when the Sun, Earth and Moon form a near-straight line in space, so that the full Moon passes through Earth’s shadow.

• Unlike a solar eclipse, which requires special equipment to observe safely, you can watch a lunar eclipse with the naked eye.

Solar Eclipses• Partial Solar Eclipse - A partial solar eclipse is when the Moon

only covers part of the solar disc.

• Total Solar Eclipse - A total solar eclipse is when the Moon appears to cover the entire solar disc. Total solar eclipses are only visible from a very small area on Earth, a narrow track that moves across the Earth's surface (as the Earth rotates). The partial phase of a total eclipse lasts about an hour. In any one place, totality (when the solar disc is entirely covered) lasts no more than 8 minutes. During totality, the sky is dark enough to see stars in the sky.

• Annular Eclipse - During an annular eclipse, the sun looks like an "annulus" or ring. The ring is visible when the Moon does not entirely cover the disk of the Sun during a solar eclipse. This type of eclipse happens when the Sun is at perihelion (closest to the Earth, making the solar disc appear larger) and the Moon is at apogee (farthest from the Earth, making it look smaller).

Total Solar Eclipse

Annular

Annular Solar

Hybrid / Annular

Caution!

• DO NOT observe a solar eclipse with the naked eye.  Serious eye damage can result. 

• Use approved solar filters (camera film negatives do not count) or cut a pin hole in a shoe box and watch the Sun's light cast through the pin hole onto a smooth surface such as cardboard.

How eclipses happen?

Tides

• The regular rise and fall in the level of sea water.

• Two kinds of tides:– Spring - When the gravitational effects of

the Sun and the Moon combine– Neap - when the Sun and Moon are at right

angles to each other and their tidal influences partially cancel each other out

Spring and Neap TidesNeap tides occur during quarter moons

Spring tides occur during the full moon and the new moon.

Tide TypesDuring spring tide, the high tides are very high and the low tides are very low

Blue moon

• The second full moon within a month.• The rare phenomenon of two blue moon

occurring in the same year happens approximately once every 19 years.

• 1999 was the last time, in January and March. The months of double blue moon are almost always Jan and March, due to short month that falls bet them, February, is the key to once every nineteen year phenomenon.

• End of Presentation!

• Exam next meeting.