astronomy chapter 2

the study of the  positions of objects in the sky and how these positions change

groupings of stars

daily motion of stars

you will find that it rises approximately 4 minutes earlier each night or 2 hours earlier each month

imaginary object that has no basis in physical reality. nonetheless the sphere remains a useful too fo positional astronomy.

divides the sky into northern and southern hemispheres

the two celestial poles where Earth's axis of rotation intersects the celestial sphere.

the star Polaris is less than 1 degree away from the north celestial pole which is why it is called the North Star

point in the sky directly overhead an observer anywhere on Earth

stars that are never rising or setting

the plane of Earth's orbit around the sun

the ecliptic and the celestial equator intersect at only two points which are exactly opposite each other on the celestial sphere. 

when sun appears at either of these points day and night are each about 12 hours long

On about March 21 of each year the Sun passes northward across the celestial equator. This marks the beginning of spring in northern hemisphere

On September 22 the Sun moves southward across the celestial equator at the autumnal  equinox, marking the moment when fall begins in the northern hemisphere.

between the vernal and autumnal equinoxes lie. the point on the ecliptic farthest north of the celestial equator is called the summer solstice.

at the beginning of the northern hemisphere's winter the sun is farthest south of the celestial equator

Between the two tropics at 23.5 degrees south lat and the latter 23.5 degrees north, the Sun is at zenith.

caused by the gravitational pull of th eSun and the Moon on Earth's equatorial bulge. It causes the positions of the equinoxes and celestial poles to shift slowly.

The moons path varies somewhat from one month to the next but always remains wtihin the band called zodiac which extends 8 degrees on either side of the ecliptic.

The intersections of the celestial equator and the ecliptic define the equinoxes so these key locations in the sky also shift slowly from year to year

At the north pole all stars are circumpolar. Stars on the celestial equator would make a 360 path following the horizon. From the equator all stars rise and set. The 90 degree difference in latitude is responsible for the difference.

The equinoxes are the points where the ecliptic intersects the equator and the solstices are 90 degrees from the equinoxes along the ecliptic