The Main Point. Phases and Motions of the Moon. Lecture #5: Earth, Moon, & Sky II. Lunar Phases and Motions. Tides. Eclipses.

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1 Lecture #5: Earth, Moon, & Sky II Lunar Phases and Motions. Tides. Eclipses. The Main Point The Moon s size and orbit lead to many interesting phenomena: changing phases, tides, and eclipses. Astro 102/104 1 Astro 102/104 2 Phases and Motions of the Moon The Moon orbits the Earth in an inclined (~5 ) and slightly elliptical (0.055) orbit about 60 Earth radii away. The Moon's appearance in the sky (its phase) changes considerably as it orbits the Earth. The result of changing illumination by the Sun. Astro 102/104 3 Astro 102/

2 Sidereal and Synodic Periods The sidereal period is measured wrt the background stars. The Moon's sidereal orbital period is 27.3 days. But because of the Earth's motion around the Sun, it takes ~29.5 days (a lunar month or synodic month) for the Moon's phases to repeat. Astro 102/104 5 Is the Moon Spinning? Yes! But then why do we see the same features all the time? Try the following experiment to prove it to yourself: Without spin With spin Astro 102/104 6 Day and Night on the Moon The Moon is in synchronous rotation with the Earth: Spin period = Orbital period. One lunar "day" = One lunar month. Natural end state of tidal evolution (as we shall see). Common Misconception: dark side of the Moon. The Moon spins, just like the Earth, so every place on the Moon experiences night and day. More accurate to speak of the Near Side and the Far Side of the Moon: the near side is the dark side at New Moon. the far side is the dark side at Full Moon. Astro 102/104 7 Tides Gravitational forces that distort the shape of an object are called tides. The Sun and Moon exert tides on the Earth. But the Earth is not a point; some places closer to the Moon than others. Astro 102/

3 The difference in the Moon's gravitational pull at various locations on Earth causes two tidal bulges. The Moon raises tides in the oceans of a few meters; the Sun also raises tides, about half as strong. The net tides vary with lunar phase. The ocean acts like a flexible shell covering the Earth. The shell is pulled towards the Moon by tidal forces, distorting it into an ellipsoidal shape. Tidal Effects This model of tides on the Earth is oversimplified: Earth not completely covered by oceans. The solid Earth distorts and bulges from tides as well. The Earth distorts the Moon into an ellipsoid. Tidal forces change the Earth-Moon system: Tidal Friction is slowing the Earth's spin (2 msec/century). Conservation of angular momentum: energy goes into Moon's orbit: Moon is receding from Earth (3.8 cm/year). Tides very important for controlling the spin and internal heating of many other solar system bodies. Astro 102/104 9 Astro 102/ Tidal Effects Response of Earth is not instantaneous; tidal bulge leads. By conservation of angular momentum, length of day on Earth increases, and Moon recedes from Earth. Astro 102/ Eclipses It is an amazing cosmic coincidence that the angular size of the Sun and Moon in the sky are almost the same! The Sun is about 400 times larger than the Moon. But the Sun is about 400 times farther away than the Moon. An eclipse occurs whenever any part of the Earth or Moon encounters the shadow of the other. Solar Eclipse: when the Moon passes in front of the Earth along a line between the Earth and Sun and its shadow falls on the Earth. Lunar Eclipse: when the Moon passes behind the Earth along a line between the Earth and Sun and Earth's shadow falls on the Moon. Eclipses are among the most spectacular events in nature! Eclipses allowed the Sun's corona to be studied. Astro 102/

4 Solar Eclipse: The Moon's shadow passes across the Earth's surface (only a few minutes of totality). Solar (not to scale) Lunar (not to scale) Astro 102/ Astro 102/ Lunar Eclipse: The Earth's shadow passes across the Moon's surface (takes hours). Upcoming Eclipses Why does the Moon appear to turn red while it is in shadow? Astro 102/ Total Lunar Eclipses Feb 21, 2008 Dec 21, 2010 June 15, 2011 Dec 10, 2011 Astro 102/

5 Summary The Moon's phase changes as it orbits the Earth. The Moon spins on its axis at the same rate that it orbits the Earth (synchronous). The Moon and the Sun raise oceanic and solid body tides on the Earth. Tidal friction modifies the orbits and spins of planets and satellites (as we shall see...) Eclipses occur when the Earth or Moon pass through each other's shadow. Astro 102/ Astro 102/ Next Lecture... Making Measurements of the Solar System. Different forms of electromagnetic radiation. Spectroscopy. Readings for this week: Chapters 2.2, S1, 2.3, 5. Astro 102/