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Homework #2 • 45.635 degrees is how many degrees, arcminutes, and arcseconds? 45 deg 0.635(60) 38.1arcmin 38 arcmin 0.1(60) 6arcsec • How many degrees, arcminutes, and arcseconds does the moon move across the sky in one hour? (the lunar day is 24 hours and 48 minutes long) 24 hours 48 minutes 24.8 hours The moon moves 360 degrees during that time so : • 360 deg 14.516129 deg/ hour 14o3058.06" 24.8hours The moons diameter is about 30 arcminutes, so find out how long it takes for the moon to travel its diameter. 0.5 deg 0.034 hour 14.516129 deg/ hour (0.034hour ) (60minutes/ho ur ) 2.1 minutes 26 v d/t t d/v Phases of Moon • Half of the Moon is illuminated by the Sun and half is dark. • We see a changing combination of the bright and dark faces as the Moon orbits Earth. How to Simulate Lunar Phases Lunar Motion Phases of the Moon’s 29.5 day cycle • • • • • • • • new crescent first quarter gibbous full gibbous last quarter crescent • Muku • Külua waxing • ÿOlekükahi • Huna • Hoku waning • ÿOlekükahi • Käloakükah i Hilo Hoaka Kükahi Kükolu Küpau ÿOlekükahi ÿOlekükolu ÿOleküpau Külua ÿOlekülua hoÿonui Huna Möhalu Hoku MäheaIani Laÿaukülua Laÿauküpau Hua Akua Kulu Laÿaukükahi poepoe ÿOlekükahi Käloakülua Mauli ÿOlekülua Käloapau Muku ÿOleküpau Käloakükahi Käne Lono ÿemi We see only one side of the Moon Synchronous rotation: The Moon rotates exactly once with each orbit. This is why only one side is visible from Earth. Thought Question It’s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? A. B. C. D. First quarter Waxing gibbous Third quarter Half moon Thought Question It’s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? A. B. C. D. First quarter Waxing gibbous Third quarter Half moon What causes eclipses? • The Earth and Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse. Lunar Eclipse Lunar Eclipse Solar Eclipse Evolution of a Total Solar Eclipse When can eclipses occur? • Lunar eclipses can occur only at full moon. • Lunar eclipses can be penumbral, partial, or total. When can eclipses occur? • Solar eclipses can occur only at new moon. • Solar eclipses can be partial, total, or annular. Why don’t we have an eclipse at every new and full moon? — The Moon’s orbit is tilted 5° to ecliptic plane. — So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon. Summary: Two conditions must be met to have an eclipse: 1. It must be a full moon (for a lunar eclipse) or a new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes). Eclipse Predictions • Eclipses recur in the approx. 18 yr, 11 1/3 day saros cycle • But even then, eclipse location and type (e.g., partial, total) may vary Photo by Olaf Menkens (1999 Mauna Loa) Planets Known in Ancient Times • Mercury Ukalialiÿi – difficult to see; always close to Sun in sky • Venus Höküloa – very bright when visible — morning or evening “star” • Mars Höküula – noticeably red • Jupiter Hoÿomananalo – very bright • Saturn – moderately bright Makulu Why did the ancient Greeks reject the notion that the Earth orbits the sun? • It ran contrary to their senses. • If the Earth rotated, then there should be a “great wind” as we moved through the air. • Greeks knew that we should see stellar parallax if we orbited the Sun – but they could not detect it. What is parallax? You use it every day… Parallax Angle Apparent shift of a star’s position due to the Earth’s orbiting of the Sun Possible reasons why stellar parallax was undetectable: 1. Stars are so far away that stellar parallax is too small for naked eye to notice 2. Earth does not orbit Sun; it is the center of the universe Unfortunately, with notable exceptions like Aristarchus, the Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)… Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems. Scientific Thinking • It is a natural part of human behavior. • We draw conclusions based on our experiences. • Progress is made through “trial and error”. Ancient Astronomy • Many cultures throughout the world practiced astronomy. • They made careful observations of the sky. • Over a period of time, they would notice the cyclic motions of: – – – – Sun Moon planets celestial sphere (stars) Stonehenge (completed 1550 BC) This famous structure in England was used as an observatory. • If you stand in the middle: – the directions of sunrise & sunset on the solstices is marked. – the directions of extreme moon rise & set are marked. • The Aubrey holes are believed to be an analog eclipse computer.