Celestial Sphere

Earthly Sphere

• Latitude measures the number of degrees north or south of the equator.– DeKalb at 41° 55’ N

• Longitude measures degrees east or west of the prime meridian.– 0° at Greenwich

Observatory, London

– DeKalb at 88° 45’ W

Projection

• The sky above us is measured as a projection outward from the earth.

• The celestial poles (P, Q) are directly out from the north and south poles.

• The celestial equator (A-B) is directly out from Earth’s equator.

P

Q

BA O

Declination

• Declination is the celestial equivalent of latitude.– Angle compared to

equator

– North positive

– South negative

P

Q

X BA O

Rotation

• The earth rotates, so the stars appear to rotate.– Over same latitude but changing longitude

– North and south celestial poles P, Q fixed

• East-west orientation is defined by the sun’s position ϒ at the vernal equinox.– Crosses equator from S to N

– First day of spring, March 21

Right Ascension

• Right ascension is the celestial equivalent of longitude.– Angle

• Stellar coordinates use right ascension and declination. – X(,)

P

Q

OX

ϒ

E

Heavenly Time

• Right ascension is not measured in degrees.• Degrees are converted to time.

– 24 hours = 360°– 1h = 15° 1° = 4m– 1m = 15' 1' = 4s– 1s = 15'' 1'' = 1/15 s

Local View

• From a point on the earth the stars appear to be fixed on a sphere that rotates.

• The point directly above the observer is the zenith.

• The point directly below through the earth is the nadir.

Horizon

• The horizon is the line of the ground for an observer.

• Altitude is the angle up from the horizon.

• Azimuth is the angle east from north.