Investigating AstronomyTimothy F. Slater, Roger A. Freeman Chapter 1 Predicting the Motions of the Stars, Sun, and Moon
The Scientific Method • Hypothesis: A testable idea • Theory:A description of nature, based on a great deal of data. A theory explains what we see and can never be proven. Exploring the physical world using observation, experiment, logic, and skepticism
Technology in Science • Advances in electronic memory storage • Using the entire electromagnetic spectrum
ConceptCheck: • Which is held in higher regard by professional astronomers, a hypothesis or a theory? • Explain your answer.
Asterisms: Recognizable, dot-to-dot patterns Constellations: One of 88 sections of the sky Constellations and Asterisms
ConceptCheck: • If Jupiter is reported to be in the constellation of Taurus the Bull, does Jupiter need to be within the outline of the bull’s body? Why or why not?
Physical and Angular Distance Physical distance (in km, astronomical units, etc) is important in astronomy. So is angular distance of an object, or between two objects.
All of the observed celestial motions can be described if our planet spins once each day, and orbits around our Sun each year. From the Northern Hemisphere: The Sun, the Moon, and the constellations appear to rise in the East and set in the West, everyday… and, the constellations shift over the course of the year.
Daily Motion and the Earth’s Rotation • Half of the Earth is ALWAYS lit by the Sun. • The Earth spins, changing which part is lit by the Sun.
Yearly Motion and Earth’s Orbit • The Earth orbits the Sun―in an almost-perfect circle. • The side turned toward the Sun sees its light. • The side turned away from the Sun sees a changing pattern of stars.
Imagine a giant “celestial sphere” surrounding Earth. Project the equator and poles into space. • The point directly above you in the sky is the zenith.
Motions of the Celestial Sphere • Objects near the North celestial pole seems to move in a circle, never setting: circumpolar. • Your latitude determines how the stars appear to move.
ConceptCheck • People in which of the following cities in North America experience sunrise first: A. New York B. San Francisco, C. Chicago D. Denver • If the constellation of Cygnus rises along the eastern horizon at sunset (6 PM) , at what time will it be highest above the southern horizon? A. 8 PM B. Midnight C. Sunrise (6AM) D. It stays on the E. Horizon
ConceptCheck • If the Earth suddenly rotated on its axis three times faster than it does now, then how many times would the Sun rise and set each year? A. 122 B. 365 C. 730 D. 1095 • Where would you need to be standing on Earth for the celestial equator to pass through your zenith? A. The North Pole B. San Diego C. Tasmania D. The Equator
The Earth’s spin is tilted to one side. • The Earth’s North Pole is always tilted to point toward the North Star. This will not change in your lifetime. • Earth does not change its tilt “toward” or “away” from the Sun. Remember that Earth orbits the Sun in an almost-perfect circle. Don’t let this picture fool you!
The angle and hours of sunlight change during the year. The Northern Hemisphere’s Winter: The North Pole cannot spin into the sunlight. Light at the Tropic of Cancer is “weak.” The Northern Hemisphere’s Summer: The North Pole cannot spin out of the sunlight. Light at the Tropic of Cancer is “strong.”
The angle and hours of sunlight change during the year. The tilt of the Earth causes sunlight to hit the Earth more directly, and for more hours each day, during Summer. • When the sunlight hits the ground directly, it heats the ground more because it’s spread thicker • When the sunlight hits the ground for longer periods of time, it gets hotter!
The Sun’s Path on the Celestial Sphere • The Sun appears to cover one constellation after another, along the ecliptic. • In reality, the changes we see in the Sun’s position occur because WE are moving. • Bonus: The other seven planets also appear to travel on – or very near - the ecliptic.
Equinoxes and Solstices • When the Sun (which is always on the ecliptic) is also on the celestial equator, day and night are each 12 hours long: the equinox. • When the Sun reaches its most Northern and Southern points in the sky: the solstice.
Over one day, the Sun is nearly stationary on the celestial sphere. But its position changes over the year The Sun appears to rise and set at different locations: • Winter: toward the South • Summer: toward the North • Fall and Spring: due East and West
ConceptCheck • If Earth’s axis was not tilted, but rather was straight up and down compared to the path of Earth’s orbit, would observers near the north pole still observe periods in which the Sun never rises and the Sun never sets? A. Yes, same as today B. Yes, but changing each 3 months C. No, Permanent Darkness D. No Permanent Dim Light • How long does the Sun take to move from being next to a bright star (say, Regulus) all the way around the celestial sphere and back to that same bright star? A. 24 Hours B. One month C. Six Months D. One Year
ConceptCheck • How often (and when) each year does an observer standing on Earth’s equator experience no shadow during the noon-time Sun? A. 365 – Every day B. Twice, June 21 and Dec 22 C. Once, on June 21D. Twice, Mar 21 and Sep 22 • Approximately how many days are there between the northern solstice and the following March equinox? • A. 273 B. 182 C. 30 D. 1
The Moon is lit by sunlight. • Just like the Earth, half of the Moon is lit by sunlight. • The Moon does not produce its own light. This image of the Earth and Moon was taken by the Galileo spacecraft.
Understanding the Moon’s Phases The phase of the Moon is a result of our point of view relative to the direction of the light. The Sun is 400 times farther away than the Moon
The “pictures” of the Moon show what you would see from Earth when the Moon is in that location.
The Moon’s Synchronous Rotation • The Moon makes one orbit around Earth, and spins one time on its axis, in the exact same amount of time. • We always see the same side of the Moon―we never see the “far side.”
Sidereal and Synodic Months The Moon orbits the Earth as Earth orbits the Sun, so the Moon has to make MORE than a full orbit for a cycle • The Moon returns to its place on the background stars: sidereal month(27.3 days). • The Moon completes a full cycle of phases: synodicmonth (29.5 days). Reference is Earth-Sun line.
ConceptCheck • If an observer on Earth sees just a tiny sliver of the crescent moon, how much of the Moon’s total surface is being illuminated by the Sun? A. A tiny fraction B. 20% C. 50% D. 100% • If the Moon appears in its waxing crescent phase, how will it appear in two weeks? • Waxing Crescent B. Waning Crescent C. Full Moon D. Waning Gibbous
ConceptCheck • If astronauts landed on the Moon near the center of the visible surface at full moon, how many Earth days would pass before the astronauts experienced darkness on the Moon? A. 29 B. 14. C. 7 D. ¼ day, or 6 hours • If Earth were orbiting the Sun much faster than it is now, the length of time between full moons would: • Increase B. decrease C. stay the same D. None of these; would be chaotic
Eclipses are about shadows. They occur when the Sun, Moon, and Earth are almost perfectly aligned. The Moon’s orbital plane is tilted just a little from the ecliptic plane.
Lunar Eclipses • When the Moon is opposite the Sun, it can travel through the Earth’s shadow. • The Earth’s shadow is complete in the center (umbra) and partial on the edge (penumbra).
Total Solar Eclipses • The Moon totally covers the face of the Sun. Seen by observers who are inside the umbra. • Those inside of the Moon’s partial shadow (penumbra) see a “bite” taken out of the Sun.
Annular Solar Eclipses • When alignment occurs with the Moon farther than average, the tip of the umbra cone of its shadow doesn’t reach Earth. • The Moon appears to be too small to cover the Sun.
ConceptCheck • Why don’t lunar eclipses occur each time the Moon reaches full moon phase? • Why does the eclipsing Moon spend more time in the penumbral shadow than in the umbral shadow? • Why can total lunar eclipses be seen by people all over the world, whereas total solar eclipses can only been seen from a very limited geographic location? • If you had a chance to observe a total solar eclipse and a total lunar eclipse, in general, how much longer would you expect one type to last than the other?
PartialSolar Eclipses • Frequently, the central point of the eclipse is in space above the north or south pole, but some areas of Earth are in the penumbra. Observers in these areas see the Sun with a “bite” out of it. This is a partial eclipse. • The three types of Solar Eclipses are: Total, Annular, and Partial.
Next Chapter: Chapter 2 Decoding the Hidden Messages in Starlight