Astronomy Midterm Problem Set 1

Describe the apparent motion of the stars as seen from the equator

An observer on the equator sees the NCP on the horizon due north and the SCP on the horizon due south. The celestial equator intersects the horizon due east crosses the meridian at the zenith and intersects the horizon due west. No stars are always above or below, all of them rise and set. Stars make clockwise around NCP and counter clockwise around NCP.

Describe the apparent motion of the stars as seen from the North Pole.

At the north pole the NCP is directly overhead at the zenith. The SCP will be at the nadir directly underfoot. The celestial equator is on the horizon. At the poles, unlike other spots on earth, the zenith is always in the same location so objects visible from the NP follow circular paths that have the same angle above the horizon. Objects closer to the zenith appear to follow small circles. While objects near the horizon appear to follow larger circles. Stars north of the celestial equator (in the northern celestial hemisphere) will always be above the horizon. Stars south of the celestial equator will always be below the horizon. The poles are special because nothing rises or sets each day as the earth turns. you will see the same half of the celestial sphere. At the north pole stars appear to move counterclockwise, at the south pole they appear to move clockwise.

North Celestial Pole

Directly about earth's north pole is the NCP. It is the point on the celestial sphere defined by the projection of the earth's axis of motion onto the sky due north. Just as the north pole is 90 degrees away from the equator, the NCP is 90 degrees away from the celestial equator.

Meridian

Imaginary north-south line. It is the arc of the great circle which intersects the horizon due south, passes through the zenith and intersects the horizon due north. Separates the sky into an eastern and western half.

Describe what is meant by the statement: North and south are fixed directions in space, east and west are not.

In planetary terms north means in the direction of the NCP, south means in the direction of SCP east is in the direction of earths rotation and west means opposite to the direction of earth's rotation. North and South are fixed because they are defined by pointing along the earth's fixed axis of rotation. For east and west, two observers on opposite sides of the earth who are both facing east will both be facing in the direction of earths rotation but will be facing opposite physical directions in space.

Does the location of the observer matter for NCP, SCP, Celestial Equator, ecliptic, zenith, nadir, horizon or meridian?

NCP, SCP and celestial equator and ecliptic are defined by the physical motion of the earth in space and are the same for al observers. The Zenith nadir horizon and meridian depend on the location of the observer.

Zenith

Point in the sky directly overhead for any observer

Celestial Equator

Projection of the plane of the earth's equator onto the sky.

Describe the Rotation of the earth. What is the right hand rule?

The earth rotates about an axis which is fixed in space. The sense of rotation about the axis is given by the right hand rule. Place the thumb of your right hand along the axis of rotation with your thumb pointing north. The fingers of your right hand curl in the direction of earths rotation.

Horizon

The great circle on the celestial sphere defined by the projection of the plane tangent to the surface of the earth of the observer onto the sky. The horizon separates the half of the sky that is visible to the observer (above the horizon) from the half not visible (below the horizon).

Does location matter for the apparent daily motion of the stars?

The path each objects takes across the sky is called the apparent daily motion. The apparent daily motion of the stars depend on where you are. They are very different in a northern location like Alaska from a tropical location like Hawaii. The earth's rotation about its axis and revolution around the sun affect our perception of celestial motions as seen from different places on earth. The daily rotation of the earth on its axis causes the apparent daily motion of the sun, moon and stars. Out location on earth and earth's location in the orbit around the sun determine which stars we can see.

Ecliptic

The path of the sun in the sky throughout the year. It is also the great circle on the celestial sphere defined by the projection of the earth's orbital plane onto the sky. The sun moves against the background stars throughout the year due to the orbit of the earth around the sun. The constellations the sun passes through during the year are the signs of the zodiac.

South Celestial Pole

The point on the celestial sphere defined by the projection of the earth's axis of motion onto the sky due south.

Nadir

The point on the sky directly underfoot for any observer

Describe the apparent motion of the stars as seen from away from a pole.

Your latitude is the angle between a line from the center of the earth to your location and the line from the center of the earth to a point on the equator closest to you. *NP is at 90 degrees north, equator is at 0 degrees, SP is at 90 degrees south.* Your latitude determines the part of the sky you can see throughout the year. At a latitude of 60 degrees north, your horizon is tilted 60 degrees from the north celestial pole. You will see the NCP from an altitude of 60 degrees above the northern horizon and the celestial equator 30 degrees above the southern horizon. The SCP will be 60 degrees below the horizon due south. The celestial equator intersects the horizon due east crosses the meridian at 30 degrees above the horizon due south and intersects the horizon due west. Stars within 60 degrees of the NCP will always be above the horizon, stars within 60 degrees of SCP will always be below. Stars within 30 degrees of celestial equator rise and set. Stars on celestial equator rise east and set west. Stars in the northern celestial hemisphere but within 30 degrees of celestial equator rise north of east and set north of west. (same true for south) All stars appear to make counterclockwise circles around NCP and clockwise around SCP.