Investigating Astronomy Timothy F. Slater, Roger A. Freeman

Chapter 1Predicting 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.

Exploring the physical world using observation, logic, and skepticism

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 patternsConstellations: One of 88 sections of the sky

Using Asterisms to Navigatethe 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• 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.

• 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 tilt of the Earth causes sunlight to hit the Earth more directly, and for a longer periods of time, during Summer.

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.