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

Constellations and Asterisms

Asterisms: Recognizable, dot-to-dot patternsConstellations: One of 88 sections of the sky

Using Asterisms to Navigatethe Sky

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.

The azimuth and elevation (or altitude) system is an observer centered coordinate System for locating objects from a specific location and time.

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.

The projection of the longitude and latitude lines from Earth’s surface onto the sky and fixed to the stars is called the Right Ascension (R.A.) and Declination (Dec.)

Declination ranges from +/- 90 degrees at the poles but Right Ascension ranges from 0 to 24 hours allowing the positions of stars to act like a clock.

Motions of the Celestial Sphere

• Objects near the North celestial pole seems to move in a circle, never setting: circumpolar.

• Your latitude impacts how the stars appear to move.

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.

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

North– Summer: toward the

South– Fall and Spring: due

East and West

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.”

Eclipses occur when the Sun, Moon, and Earth are perfectly aligned.

The Moon’s orbital plane is just a little off of 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, 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.

Path of Aug 21 2017 total 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.