Chapter 1 Flashcards
What is the celestial sphere?
An imaginary spherical surface, centered on the Earth’s center, with a radius immensely larger the Earth’s radius.
What is the horizon?
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).
What is the horizon circle?
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.
What is the zenith?
The point directly above the observer’s head, in the middle of the visible hemisphere of the celestial sphere, denoted by Z.
What is the nadir?
The point directly below the observer’s feet, opposite the zenith.
Altitude
the angle of a celestial object above the horizon circle
Azimuth
the longitude-like coordinate in the horizon coordinate system
What is the north celestial pole?
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.
What is the celestial equator?
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.
What is a point’s declination?
it is a point’s angular distance north or south of the celestial equator, denoted by delta(lc)
What is a diurnal circle?
circular paths that are parallel to the celestial equator, fixed angular distance from the CEq, and the declination of the object remains constant
Right ascension
the longitude-like coordinate measured eastward from a “Prime Meridian”
Equatorial coordinate system
the coordinate system using right ascension and declination
Circumpolar stars
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.
Retrograde motion
On occasion, planets move westward relative to the stars for a short period.
The ecliptic
The great circle along which the Sun moves on the celestial sphere
The obliquity of the ecliptic
The tilt of 23.5 degrees between the ecliptic and celestial equator
Equinoxes
the two points of intersection involving the ecliptic and the celestial equator
Autumnal Equinox
The point where the Sun moves from the northern celestial hemisphere to the southern
Vernal Equinox
The point where the Sun moves from the southern celestial hemisphere to the northern
Summer Solstice
the point on the ecliptic that is farthest north of celestial equator, has a declination of 23.5
Winter Solstice
the point on the ecliptic that is farthest south of the celestial equator, declination of -23.5 degrees.
Upper Transits
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
Sidereal day
the time between two upper transits of a star; this represents the Earth’s rotation period relative to the distant fixed stars.
Solar Day
the time between two upper transits of the Sun
Tropical year
the interval of time that elapses between successive passes of the Sun through the vernal equinox
Sidereal year
the time it takes the Sun to make a complete circle of the ecliptic relative to the fixed background stars
