Chapter 1

Question 1

What is the celestial sphere?

Answer 1

An imaginary spherical surface, centered on the Earth’s center, with a radius immensely larger the Earth’s radius.

Question 2

What is the horizon?

Answer 2

A plane tangent to the idealized, perfectly spherical Earth at the observer’s location (that is, it touches the Earth at the observer’s feet and at no other place).

Question 3

What is the horizon circle?

Answer 3

The horizon intersects the celestial sphere in a great circle(horizon circle). This circle also divides the celestial sphere into two hemispheres; only the hemisphere above the horizon is visible to the observer.

Question 4

What is the zenith?

Answer 4

The point directly above the observer’s head, in the middle of the visible hemisphere of the celestial sphere, denoted by Z.

Question 5

What is the nadir?

Answer 5

The point directly below the observer’s feet, opposite the zenith.

Question 6

Altitude

Answer 6

the angle of a celestial object above the horizon circle

Question 7

Azimuth

Answer 7

the longitude-like coordinate in the horizon coordinate system

Question 8

What is the north celestial pole?

Answer 8

Labeled NCP, it is at the zenith for an observer at the Earth’s north pole; more generally, for an observer at a latitude of l north of the equator, it will be at an altitude of l and a azimuth of 0 degrees.

Question 9

What is the celestial equator?

Answer 9

The projection of the Earth’s equator onto the celestial sphere. The celestial equator passes through the zenith for an observer on the Earth’s equator.

Question 10

What is a point’s declination?

Answer 10

it is a point’s angular distance north or south of the celestial equator, denoted by delta(lc)

Question 11

What is a diurnal circle?

Answer 11

circular paths that are parallel to the celestial equator, fixed angular distance from the CEq, and the declination of the object remains constant

Question 12

Right ascension

Answer 12

the longitude-like coordinate measured eastward from a “Prime Meridian”

Question 13

Equatorial coordinate system

Answer 13

the coordinate system using right ascension and declination

Question 14

Circumpolar stars

Answer 14

Stars that have diurnal circles that do not intersect the horizon plane at all. They will never fall below the observer’s horizon but can be seen to move in counterclockwise circles about the NCP.

Question 15

Retrograde motion

Answer 15

On occasion, planets move westward relative to the stars for a short period.

Question 16

The ecliptic

Answer 16

The great circle along which the Sun moves on the celestial sphere

Question 17

The obliquity of the ecliptic

Answer 17

The tilt of 23.5 degrees between the ecliptic and celestial equator

Question 18

Equinoxes

Answer 18

the two points of intersection involving the ecliptic and the celestial equator

Question 19

Autumnal Equinox

Answer 19

The point where the Sun moves from the northern celestial hemisphere to the southern

Question 20

Vernal Equinox

Answer 20

The point where the Sun moves from the southern celestial hemisphere to the northern

Question 21

Summer Solstice

Answer 21

the point on the ecliptic that is farthest north of celestial equator, has a declination of 23.5

Question 22

Winter Solstice

Answer 22

the point on the ecliptic that is farthest south of the celestial equator, declination of -23.5 degrees.

Question 23

Upper Transits

Answer 23

a celestial object will cross the observer’s meridian twice a day, the upper transit occurs when the object crosses the zenith meridian and the lower transit occurs half a day later

Question 24

Sidereal day

Answer 24

the time between two upper transits of a star; this represents the Earth’s rotation period relative to the distant fixed stars.

Question 25

Solar Day

Answer 25

the time between two upper transits of the Sun

Question 26

Tropical year

Answer 26

the interval of time that elapses between successive passes of the Sun through the vernal equinox

Question 27

Sidereal year

Answer 27

the time it takes the Sun to make a complete circle of the ecliptic relative to the fixed background stars