Homework #1 has been posted It is due by Friday, Jan. 28, 4 p.m. Answers can be submitted at any time

Phobos: Largest of two Martian moons, is expected to break up and crash into Mars within the next 105 years. Russian Phobos-Grunt robotic spacecraft is scheduled to launch toward Phobos later this year and return surface samples in 2014.

Understanding motions in the sky

Celestial Sphere Large imaginary spherical surface centered on the Earth. Stars and other celestial objects “fixed” on its surface. Conceptual Model, not a physical model

The North Celestial Pole (NCP) and the South Celestial Pole (SCP) are located at the intersection of the earth’s rotation axis with the celestial sphere. The Celestial Equator is the extension of the Earth’s equator onto the celestial sphere.

The celestial sphere is “fixed” in space. We view it in the context of the local sky

Cardinal directions – NSEW Zenith – point directly overhead Nadir - point directly opposite zenith Horizon – half way between zenith and nadir Meridian – line from north point on horizon, through zenith, to south point on horizon (divides sky into eastern and western halves)

Local sky positions remain fixed: horizon, zenith, cardinal locations, meridian, are always in same location. An object’s altitude is the angle between its direction and the horizon

As you move around on the Earth, the portion of the celestial sphere visible in your local sky changes.

At what location on the Earth will the North Celestial Pole appear directly overhead?

Where on Earth is the Celestial Equator directly overhead?

How much of the Celestial Sphere can you see at any given time?

Half of the sky is always visible above the horizon.

Moving north or south on the Earth (changing your latitude), leads to viewing a different portion of the celestial sphere.

The Earth rotates west to east . This causes the (fixed) celestial sphere to appear to rotate east to west The apparent motion of the stars is referred to as “diurnal motion”.

If you are located in the northern hemisphere, what is the point in the sky around which objects on the Celestial Sphere appear to rotate over the course of a day? Do they appear to rotate clockwise or counter-clockwise?

The star named “Polaris” is very near the North Celestial Pole (within one degree). It is also referred to as the “North Star”.

It is helpful to think of the local sky with the zenith towards the top

Half of the Celestial Sphere is visible in the local sky, the other half is below the horizon. Rotation of the Earth causes the portion of the celestial sphere visible in the local sky to change with time.

The apparent motion of the stars due to the Earth’s rotation is referred to as “diurnal motion”.

No matter where you are on Earth, the Celestial Equator ALWAYS intersects the East and West points on the horizon.

In general, most stars rise in the eastern half of the sky (i.e., east of the meridian) increase their altitude (angular distance from the horizon) until they cross the meridian set in the western half of the sky (i.e., west of the meridian).

Some stars, called “circumpolar stars”, are always above the horizon.

Half of the celestial sphere is always visible above the horizon. As you move around on the Earth, the portion of the celestial sphere visible in your local sky changes. Half of the celestial sphere is always visible above the horizon. Moving north or south (changing your latitude), leads to viewing a different portion of the celestial sphere.

How does the altitude of the North Celestial Pole change with the latitude of an observer?

Location: Fairbanks, AL (Figures from Nick Strobel’s “AstronomyNotes” website - www.astronomynotes.com)

Location: Seattle, WA

Location: Los Angeles

Location: Equator

IMPORTANT POINTS!! 1. The altitude of the celestial pole is equal to the latitude of the observer 2. The Celestial Equator ALWAYS intersects the East and West points on the horizon.

What is the “Meridian”? The longitude passing through Greenwich England The point directly over an observer’s head A great circle half way between the observer’s zenith and nadir An imaginary line in the sky that passes from the north point on horizon, through the zenith, to the south point on horizon

What is the “Meridian”? The longitude passing through Greenwich England The point directly over an observer’s head A great circle half way between the observer’s zenith and nadir An imaginary line in the sky that passes from the north point on horizon, through the zenith, to the south point on horizon

ConceptTest You are in Bloomington and observe a star rising directly to the east. When this star reaches its highest point above the horizon, where will it be? (a) high in the northern sky (b) high in the eastern sky (c) high in the southern sky (d) high in the western sky (e) at the zenith

ConceptTest You are in Bloomington and observe a star rising directly to the east. When this star reaches its highest point above the horizon, where will it be? (a) high in the northern sky (b) high in the eastern sky (c) high in the southern sky (d) high in the western sky (e) at the zenith

ConceptTest Where are circumpolar stars at their lowest point in the sky? In the western sky In the eastern sky In the southern sky On the meridian

ConceptTest Where are circumpolar stars at their lowest point in the sky? In the western sky In the eastern sky In the southern sky On the meridian

In Bloomington, where is the North Celestial Pole? On the Celestial Equator On the meridian at an altitude of approximately 40 degrees. Near the zenith At the north point on the horizon

In Bloomington, where is the North Celestial Pole? On the Celestial Equator On the meridian at an altitude of approximately 40 degrees. Near the zenith At the north point on the horizon

You are in Bloomington and observe a star rising directly to the east You are in Bloomington and observe a star rising directly to the east. Where does this star lie on the Celestial Sphere?

ConceptTest As you move from the Earth's equator toward the North Pole, the number of circumpolar stars: increases decreases stays the same

ConceptTest As you move from the Earth's equator toward the North Pole, the number of circumpolar stars: increases b. decreases c. stays the same