Download Motions of the Celestial Sphere

MOTIONS OF SKY
Goals
• To identify the different parts of the celestial
sphere model
• To understand how to express the location of
objects in the sky
• To understand and explain the motion of celestial
objects
Celestial Sphere
• Imaginary sphere with the Earth at its center.
Celestial Sphere
• Sky looks like a dome with stars painted on the
inside.
Parts of the Celestial Sphere
Zenith
•Point directly above the observer
•Any point on horizon is 90° from zenith
Parts of the Celestial Sphere
Nadir
• Point directly below the observer
Parts of the Celestial Sphere
Horizon
 The great circle on the celestial sphere that is 90
degrees from the zenith
 Lowest part of the sky the observer can see
Definitions
 Hour circle: The great circle through the
position of a celestial body and the celestial
poles
 Meridian: The hour circle that passes
through the zenith and both celestial poles
Parts of Celestial Sphere
Ecliptic
• The apparent path of the Sun across the sky.
Celestial Equator and
Celestial Poles
In the celestial
coordinate system
the North and South
Celestial Poles are
determined by
projecting the rotation
axis of the Earth to
intersect the celestial
sphere
Right Ascension and Declination
• The right ascension (R.A.) and declination (dec) of an object on
the celestial sphere specify its position uniquely, just as the
latitude and longitude of an object on the Earth's surface define a
unique location. Thus, for example, the star Sirius has celestial
coordinates 6 hr 45 min R.A. and -16 degrees 43 minutes
declination,.
Equatorial Coordinate System
Right Ascension (RA)
•Celestial longitude
•Measured in hours
• 24 hr = 360°
• 1 hr = 15° of arc
Term: right ascension
• Right ascension (RA) is like longitude. It locates where a
star is along the celestial equator. The zero point for right
ascension is the vernal equinox. To find the right ascension
of a star follow an hour circle "straight down" from the star
to the celestial equator. The angle from the vernal equinox
eastward to the foot of that hour circle is the star's right
ascension.
• There is one oddity in right ascension: the unit used to
report the angle. Right ascensions are always recorded in
terms of hours, minutes, and seconds. One hour of right
ascension (1h) is 15°. Since 24x15°=360°, there are
24h of right ascension around the celestial equator. The
reason for this oddity is that the celestial sphere makes
one full rotation (24h of RA) in one day (24 hours of time).
Thus the celestial sphere advances about 1h of RA in an
hour of time.
Equatorial Coordinate System
Declination (dec)
•Celestial latitude
•Measured in degrees
• 1° = 60 arcmin
• 1 arcmin = 60 arcsec
Term: declination
• Declination is like latitude.
It reports how far a star is
from the celestial equator.
To find the declination of a
star follow an hour circle
"straight down" from the
star to the celestial
equator. The angle from
the star to the celestial
equator along the hour
circle is the star's
declination.
Altitude and Azimuth
• The altitude is the distance an object appears to be
above the horizon. The angle is measured up from the
closest point on the horizon.
• The azimuth of an object is the angular distance along
the horizon to the location of the object. By convention,
azimuth is measured from north towards the east along
the horizon
Directions on the Local Sky
 Altitude: The minimum angular distance between the
position of a celestial body and the horizon
 Azimuth: The angular bearing of an object, measured
from
 North (0 degrees) through East (90 degrees)
 South (180 degrees)
 West (270 degrees)
 back to North (360 degrees)
The Earth Rotates
Earth’s Rotation
• Spin axis of Earth defines points on the celestial
sphere (using north and south poles)
• North Celestial Pole
• South Celestial Pole
• Celestial Equator
• Sky appears to rotate east to west about the
celestial poles because Earth rotates west to
east.
Rotating Celestial Sphere
Definitions
 Celestial Sphere: An imaginary sphere
where celestial objects are projected on the
basis of their direction from Earth
 Celestial Poles: The two points where the
spin axis of the Earth’s spin axis intersects
the celestial sphere
 Celestial Equator: The projection of the
Earth’s equator onto the celestial sphere
 Great Circle: A circle on the sphere's
surface whose center is the same as the
sphere’s center, and divides the sphere
into two equal hemispheres
Celestial Coordinates
Motion Depends on Declination
The Sky at the North Pole
• At the North Pole, the
North Celestial Pole
is at the zenith
• Stars never rise or
set
• Planets, Moon, and
Sun do rise and
set…Why?
Stars Rise and Set at the Equator
The Sky at Our Latitude
The Sky at Southern Latitudes
Diurnal Motion
Path of Stars
•Stars “attached” to
celestial sphere
•Path is a circle
• (like latitude circle)
•Called diurnal circle
• (diurnal = daily)
Circumpolar Constellations
• Circumpolar constellations never set.
• Circumpolar constellations change with latitude… sky
changes with latitude
Motion of the Stars
• Altitude of pole = latitude of observer
• Circumpolar zone and appearance of stars
depends on latitude
A space ship lands on an unknown planet.
The occupants notice that the stars never rise
or set but appear to move in circles parallel
to the horizon.
Where on the planet did the space ship land?
a.
b.
c.
d.
At the equator.
At 45 degrees latitude.
At one of the celestial poles.
Cannot be determine.
Atlanta is located at a latitude of about 33°N.
The star Polaris is very close to the CNP.
What is the altitude of Polaris when viewed from
Atlanta?
a.33°
b.45°
c.57°
d.90°