Exploring the Earth: Atmosphere, Seasons, Eclipses, and the Moon - Prof. James Higdon, Study notes of Astronomy

Download Exploring the Earth: Atmosphere, Seasons, Eclipses, and the Moon - Prof. James Higdon and more Study notes Astronomy in PDF only on Docsity! 1 0Lect. 4: Earth Astronomy of the Solar System ASTR 1010 Lecture 5: The Earth as a Planet -2 1 Lect. 5: Earth Today: Planet Earth (Part-2) • Atmosphere (finish) • Seasons • Eclipses • Tides • The Moon’s Origin Textbook: 3.1-3.2, 3.5-3.7, 8.4 2 2 The Earth’s Magnetic Field Eddy currents in the molten metal core of the Earth create a weak magnetic field that extends far into space. (Rotation of Earth also helps generate this field.) When Earth’s core cools and solidifies in the distant future this field will mostly disappear. The North & South magnetic poles “wander” 15-miles during a given year. Every 200,000 years or so the North & South poles actually switch! (nothing much appears to happen to living things, though…) Birds, turtles & some insects use the Earth’s magnetic field to navigate. 3Lect. 4: Earth 4 The Magnetosphere The Magnetosphere is the Earth’s “magnetic atmosphere”. It extends far from the Earth’s surface and deflects the “solar wind”, a high-speed stream of charged particles emitted by the sun (and saving our bacon!). 5 8Lect. 4: Earth The Seasons … Why are some months colder than others? Why is there a pattern of cold and hot months? Why is July hot in Savannah but cold in Sydney Australia? 9 Things visible in the night sky … The Moon • Earth’s companion. • Completes an orbit of Earth in 29.5 days. ! (“moon-th”) • Rotation period same as orbital period. ! always presents same side to Earth. • Coincidence: the moon covers same area of the sky as the sun. Image of Earth & Moon taken by Galileo spacecraft from 3.9 million miles away in 1992 as it heads to Jupiter. Lect. 5: Earth 6 10 Where did the Moon come from? Contending theories of the Moon’s origin: 1) Capture Theory: moon once freely roamed the young Solar System until Earth’s gravitational field captured it. 2) “Fission” Theory: rapidly spinning Earth tossed Moon from Pacific Ocean! 3) “Sister” Theory: moon formed along side the Earth out of same material. 4) “Big Whack” Theory: Earth & Mars sized planet collide 4-billion years ago. Moon forms out of debris. Amazingly, the “Big Whack” theory explains the data best, especially the moon’s composition and density, based on analysis of rocks brought back by the Apollo missions. 11 Phases of the Moon … The Moon undergoes a regular cycle of changing appearance, called phases. During this 29.5-day cycle the fraction of the Moon directly illuminated by the Sun (the “bright” part) changes from 100% (full) to 0% (new). Lect. 5: Earth 7 12 Phases of the Moon … The moon orbits the Earth. At any given time, half of its surface is illuminated by the Sun, and half is in Darkness. However, it’s constantly changing as it moves. From the Earth’s surface, we see changing light & dark aspect of the Moon’s surface. Lect. 5: Earth 13 Solar eclipses … The Sun and Moon have the same angular size. Occasionally, the Moon can pass between the Sun and Earth, covering up the Sun in the sky. This is a Solar Eclipse… Lect. 5: Earth 10 18 Tides are due to gravitational force ( FG ) on Earth from Moon (mainly) & Sun. 1) FG on near side of Earth is greater than FG on far side of Earth. 2) Water can flow freely in response. Tides 3) The “bulge” of water follows the moon as it orbits the Earth 4) The Earth rotates under the “bulge” of water. !Water’s level rises & falls ~ twice a day. Lect. 5: Earth 19 Spring Tides The strongest tides occur when the Moon’s tidal forces are helped by the Sun’s (weaker) – when they are all aligned. ! Spring Tides • Spring Tides happen at New & Full moon (i.e., every ~2 weeks). • Have nothing to do with the Spring. Lect. 5: Earth 11 20 Neap Tides The weakest tides occur when the Moon’s tidal forces are Partially cancelled by the Sun’s – when they are at 90o. ! Neap Tides Lect. 5: Earth 21 Summary Greenhouse Effect: CO2 & H2O trap additional energy near Earth’s surface: 60 o F warmer. Global Warming: we’re dumping lots of CO2 into atmosphere, changing the climate. Blue light from the sun scatters off of molecules in atmosphere much more than red light. As a result it appears to diffuse through the sky, making it look blue. Eddy currents in Earth’s molten iron core generate a giant (but weak) magnetic field. This field is also called the Magnetosphere. It deflects most of the Solar Wind; creates Aurora. Seasons on Earth (or any planet) are a direct consequence of the planet’s tilted axis. At any given time, one hemisphere will receive less/more solar energy per area than the other, and be colder/hotter. No axis tilt, no seasons. Eclipses are simple: Earth passes into Moon’s shadow (Solar Eclipse) or Moon passes into Earth’s shadow (Lunar Eclipse). Tides are a consequence of gravity’s sharp decrease in strength with distance. Lect. 5: Earth