Investigating Astronomy Timothy F. Slater, Roger A. Freeman Chapter 1 Predicting the Motions of the Stars, Sun, and Moon

The Scientific Method Exploring the physical world using observation, logic, and skepticism Hypothesis: A testable idea Theory: A description of nature, based on a great deal of data. A theory explains what we see.

Technology in Science Advances in electronic memory storage Using the entire electromagnetic spectrum

A Quick Guide to Objects in the Sky

ConceptCheck: Which is held in higher regard by professional astronomers, a hypothesis or a theory? Explain your answer.

Constellations and Asterisms Asterisms: Recognizable, dot-to-dot patterns Constellations: One of 88 sections of the sky

Using Asterisms to Navigate the Sky

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?

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 seem to move in a circle, never setting: circumpolar. Your latitude impacts how the stars appear to move.

ConceptCheck People in which of the following cities in North America experience sunrise first: New York, San Francisco, Chicago, or Denver? If the constellation of Cygnus rises along the eastern horizon at sunset, at what time will it be highest above the southern horizon? 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? Where would you need to be standing on Earth for the celestial equator to pass through your zenith?

The Earth is tilted on its side. The Earth’s North Pole is always tilted 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 a longer periods of time, during Summer. When the sunlight hits the ground directly, it heats the ground more efficiently. 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 eight planets also appear to travel on the ecliptic.

Equinoxes and Solstices When the ecliptic and celestial equator intersect, 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.

The Sun appears to move on the celestial sphere. 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 were not tilted, but rather was straight up and down compared to the path of Earth’s orbit, would observers at Earth’s North Pole still observe periods in which the Sun never rises and the Sun never sets? How long does the Sun take to move from being next to a bright star all the way around the celestial sphere and back to that same bright star? How often each year does an observer standing on Earth’s equator experience no shadow during the noon-time Sun? Approximately how many days are there between the northern solstice and the March equinox?

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.

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―not the “dark side,” but the “far side.”

Sidereal and Synodic Months The Moon orbits the Earth as Earth orbits the Sun, so the Moon has to “catch up.” The Moon returns to its place on the background stars: sidereal month. The Moon completes a full cycle of phases: synodic month.

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? If the Moon appears in its waxing crescent phase, how will it appear in two weeks? 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? If Earth were orbiting the Sun much faster than it is now, would the length of time between full moons increase, decrease, or stay the same?

Eclipses occur when the Sun, Moon, and Earth are perfectly aligned. The Moon’s orbital plane is just a little off the ecliptic.

The Sun, Moon, and Earth rarely line up.

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 and partial on the edge.

Total Lunar Eclipse from January 20, 2000

Total Solar Eclipses The Moon totally covers the face of the Sun. From inside the darkest part of the Moon’s shadow. Those inside of the Moon’s partial shadow see a partial eclipse.

Spectacular, Rare Total Eclipses

Annular Solar Eclipses When the Moon is at its farthest position, the 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?

Chapter 2 Decoding the Hidden Messages in Starlight Next Chapter: Chapter 2 Decoding the Hidden Messages in Starlight