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?