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.  At the equator. b.  At 45 degrees latitude. c.  At one of the celestial poles. d.  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°