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Moon Astronomy 1-1 Lecture 11-1 Vital Statistics Mean Distance from Earth Sidereal Period Rotation Period Inclination of Axis Inclination of Orbit Mass Mean Density Albedo Astronomy 1-1 384,401 km 27 days, 7 hours, 43 minutes 27.32 days ~ 7 degrees 5 degrees 7.35 x 1022 kg 3.34 g/cm3 0.07 Lecture 11-2 Synchronous Rotation Time it takes for the Moon to rotate about its own axis is the same as the time to make one orbit about the Earth The Moon always presents the same surface to the Earth In actuality about 59% of the Moon's surface is seen from Earth due to the Moon's wobbling which is known as libration Astronomy 1-1 Lecture 11-3 The Phases of the Moon During the lunar month, the Moon's appearance changes – phases of the moon 1) New Moon 3) First Quarter Moon 5) Full Moon 7) Last Quarter Moon 2) Waxing Crescent Moon 4) Waxing Gibbous Moon 6) Waning Gibbous Moon 8) Waning Crescent Moon Each successive phase rises later in the evening Astronomy 1-1 Lecture 11-4 The Phases of the Moon Astronomy 1-1 Lecture 11-5 Lunar Month The Moon takes longer to go through its phases than it takes to go around the Earth Sidereal Month True orbital period for the Moon's motion about the Earth. ~27.3 days Synodic Month Time to go from one full moon to the next. ~29.5 days Both of these are averages Astronomy 1-1 Lecture 11-6 Shadows A region of darkness where light is blocked Type of shadow produced is dependent upon whether the source is A Point Source An Extended Source A Point Source illuminating an object produces only a one section shadow behind the object The dark part of the shadow is called the umbra Astronomy 1-1 Lecture 11-7 Shadows Extended Source illuminating an object Two sections to the shadow behind the object Penumbra The lighter, outer portion of the shadow Area is partially illuminated Umbra The dark, inner portion of the shadow Area is not illuminated Astronomy 1-1 Lecture 11-8 Eclipses Caused by either Moon coming between Earth and Sun (Solar Eclipse) or Earth coming between Moon and Sun (Lunar Eclipse) Naively, both of types of eclipses should occur every month BUT THEY DO NOT! Astronomy 1-1 Lecture 11-9 Eclipses Plane of Moon's orbit is tilted with respect to plane of Earth's orbit by 7 degrees Moon can therefore be above or below plane of Earth's orbit and the respective shadows will not fall on anything Astronomy 1-1 Lecture 11-10 Eclipses Eclipses can only occur when a straight line can be drawn through the Earth, Sun, and Moon system This can only occur when the Moon crosses through the Earth's orbital plane The points were the Moon's orbital path intersects the Earth's orbital plane are called NODES The line joining these two points is called the LINE of NODES Eclipses then occur when the Line of Nodes is aligned with the Sun Astronomy 1-1 Lecture 11-11 Eclipses By this reasoning both Solar and Lunar eclipses should each occur twice every year But they do NOT! The Moon's orbit is affected by the gravitational pull of the Sun The orbit of the Moon precesses This causes the line of nodes to also precess So eclipses on average occur only 2 to 5 times a year Astronomy 1-1 Lecture 11-12 Eclipse Types The type of eclipse is dependent upon what part of the shadow, the shadowed body goes through Penumbral Eclipse Occurs when the shadowed body only goes through the penumbra part of the shadow. Partial Eclipse Occurs when the shadowed body goes partly through the penumbra and partly through the umbra. Total Eclipse Occurs when the shadowed body goes totally through the umbra. Astronomy 1-1 Lecture 11-13 Lunar Eclipses Occur in all three ways Penumbral Eclipse Partial 1/4 Astronomy 1-1 1/2 3/4 Lecture 11-14 Total Lunar Eclipse During a total eclipse the Moon is not completely dark but has a reddish tinge The redness that is seen across the Moon’s face is due to refraction and filtering of sunlight through the Earth's atmosphere Astronomy 1-1 Lecture 11-15 Solar Eclipses Solar eclipses also occur in the same three ways as lunar eclipses penumbral, partial, and total There is one more type of eclipse that can only occur with respect to a solar eclipse - an Annular Eclipse Average distance of the Moon from the Earth is about 384,400km Earth-Moon distance fluctuates by about 13% from 363,300km (perigee) to about 406,000km (apogee) Accordingly, the apparent size of the Moon in the sky changes by the same amount Astronomy 1-1 Lecture 11-16 Annular Solar Eclipse When the Moon is at apogee, the umbral part of the Moon's shadow falls short of the Earth's surface The Earth is in the part of the Moon’s shadow that is known as the antumbra (negative shadow) Astronomy 1-1 Lecture 11-17 The Diamond Effect and Totality Just before totality, sunlight shines through the lunar valleys Diamond Effect Last blast of sunlight before totality Totality Solar Corona Astronomy 1-1 Total solar eclipse allows the viewing of the solar corona which ordinarily is outshined by the surface of the Sun Lecture 11-18 Lunar Features Features on the Moon there for at least 3 billion years Prominent features that can be seen from the Earth are: Craters Highlands Maria Rilles Astronomy 1-1 Lecture 11-19 Lunar Features Differences between Near Side and Far Side Near Side Highlands and Maria Astronomy 1-1 Far Side Basically has no Maria Lecture 11-20 Lunar Features Astronomy 1-1 Lecture 11-21 Craters Results of bombardment by meteorites Surface of the moon is scarred with millions of impact craters No erosion and little geologic activity to wear away these craters Range in size up to many hundreds of kilometers Most enormous craters have been flooded by lava, and only parts of the outline are visible Low elevation maria (seas) have fewer craters than other areas These areas formed more recently, and have had less time to be hit Many lines of mountainous cliffs or scarps on the lunar surface These are remnants of the rims of ancient craters Astronomy 1-1 Lecture 11-22 Craters Astronomy 1-1 Lecture 11-23 Types of Impact Craters The type of impact crater is dependent upon the size, mass, speed, and angle of the falling object Small, slow objects have a low energy impact and cause small, simple craters Moltke Crater Large, fast objects release a lot of energy and form large, complex craters Astronomy 1-1 Euler Crater Lecture 11-24 Types of Impact Craters Very large impacts can even cause secondary cratering, as ejected material falls back to the ground, forming new, smaller craters, or a series of craters Secondary craters tend to cluster in a ring around the main crater The greatest number lying a bit more than one crater diameter away Tycho Crater Material falling near to the main crater tends to be moving too slowly to form craters but instead piles up as an ejecta blanket Astronomy 1-1 Lecture 11-25 Highlands Are at an elevation generally higher than the maria Differ chemically from the maria Age is approximately 4 billion years Saturated with craters Appears lighter than the maria Older surface than the maria Astronomy 1-1 Lecture 11-26 Maria Relatively smooth areas Covers ~31% of near side surface Vast solidified polls of basaltic lava Estimated to be a few 100 meters thick Lack of impact craters implies younger than highlands Age is approximately 3.5 billion years Round shapes of maria probably formed due to giant impacts Astronomy 1-1 Lecture 11-27 Rilles Narrow trench like features There are three types of rilles on the lunar surface Straight rilles Sinuous rilles Arcuate rilles Astronomy 1-1 Lecture 11-28 Straight Rilles Long, linear paths Section of the crust that has sunk between two parallel faults Readily identified when they pass through craters or mountain ranges Astronomy 1-1 Lecture 11-29 Sinuous Rilles Meander in a curved path like a river Thought to be the remains of collapsed lava tubes or extinct lava flows Usually begin at an extinct volcano Astronomy 1-1 Lecture 11-30 Arcuate Rilles Smooth curved Found on the edges of the dark lunar maria Believed to form when the lava flows that created a mare cools, contracts, and sinks Astronomy 1-1 Lecture 11-31 Lunar Origins Various models put forth: Fission Hypothesis Capture Theory Accretion Hypothesis Giant Impact Hypothesis Astronomy 1-1 Lecture 11-32 Fission Hypothesis Moon broke off from the Earth while the Earth was cooling down Equatorial bulge was spun off from the Earth by the Earth's high rotation rate Problems: 1. If spin off occurred while Earth was in molten state, then why the low iron content of Moon compared to Earth. 2. For the bulge to have been thrown off, a much higher total angular momentum for the Earth, Moon system would have required. 3. If the Moon was thrown off from the Earth, why is not the Moon's orbital plane in the equatorial plane of The Earth? Astronomy 1-1 Lecture 11-33 Capture Hypothesis Moon was an object that wandered by the Earth and was captured in the Earth's gravitational field Accounts for high angular momentum of the Moon and for the difference in chemical composition Problems: 1. The gravitational interaction between the Earth and Moon would cause the Moon to speed up, not slow down. 2. The chemical composition of the Moon is too different from other planets or moons. Astronomy 1-1 Lecture 11-34 Accretion Hypothesis Earth and Moon formed at the same time Problem: The chemical composition of the Moon should be similar to that of the Earth But the chemical compositions are different Astronomy 1-1 Lecture 11-35 Giant Impact Hypothesis A large planetesimal impacted Earth at a glancing angle when the Earth had partially solidified Collision crushed and vaporized the surfaces of the colliding bodies Material forms a disk around the Earth and this material then formed the Moon Requires an object about the size of Mars Astronomy 1-1 Lecture 11-36 Giant Impact Hypothesis Astronomy 1-1 Lecture 11-37 Lunar History Radioactive dating of lunar rock samples show that the Moon is about 4.5 billion years old, about the same age as the Earth. After the initial formation, the Moon began to cool with the attendant differentiation of the heavier materials into the core region. The Moon was then heavily peppered by meteorites which caused the cratering that is now seen in the highlands and on the far side of the Moon. In places, the impacts probably fractured the crustal surface. Then came the lava flows which filled in many of the cratered areas. The magma came from depths of about 400km. Eventually the lithosphere became thick enough to halt the magma from breaking through. Astronomy 1-1 Lecture 11-38 Lunar History Then some additional peppering by meteorites occurred which produced the cratering that is seen in the maria. Since then the Moon has been relatively quiet. Astronomy 1-1 Lecture 11-39 Lunar Structure Instruments left by the Apollo astronauts have yielded valuable information about the Moon. The instruments confirmed the fact that the Moon has no magnetic field. This implies that there is no molten segment to the core. Lunar rocks have indicated that at one time the Moon did have a weak magnetic field. Seismometers that were left behind indicate that there is seismic activity on the Moon but not at a rate comparable to that on Earth. There are on average about 3000 moonquakes per year On Earth the number of earthquakes is in the hundreds of thousands. The moonquakes are far weaker than those occurring on Earth. Astronomy 1-1 Lecture 11-40 Internal Structure of the Moon Analysis of the seismic waves that occur indicate the following structure for the Moon: A solid core which is probably rich in iron. The core extends from the center out to a radius of about 500 km. An asthenosphere extending from 500 km from the center to a distance of about 1000 km from the center of the Moon. Like on Earth the asthenosphere is plasticized. A lithosphere extending from 1000 km to 1650 km from the center. This is a solidified region. The lunar crust being about 80 km thick. The lunar crust is covered by a layer of loose fragments of rocks, crystals, and glass. The particles range in size from fine dust particles to large boulders. This covering layer is called regolith. Astronomy 1-1 Lecture 11-41 Internal Structure of the Moon Astronomy 1-1 Lecture 11-42 Quakes and Impacts The moonquakes originate about 600 to 800km below the surface, much deeper than on Earth. The moonquakes are thought to occur at or near the boundary between the Moon's lithosphere and asthenosphere. The seismometers also indicate that the Moon is still being impacted by meteorites at the rate of 80 - 150 per year. Astronomy 1-1 Lecture 11-43 Astronomy 1-1 Lecture 11-44