AstroLab-2Locating Stars in the Sky

Merav Opher-Fall 2004

Motion of the Night Sky

• Motion of the stars across the night sky (from east to west) (due to the earth rotation from west to east.

Ecliptic-The Path of the Sun along the Sky

• The Earth equator is inclined by 23 1/2 degrees to the plane defined by the Earth’s orbit around the Sun

Ecliptic, Equinoxes and Solstices

• Ecliptic: apparent annual path of the Sun as projected in the Celestial Sphere

• Because of the 23 1/2 tilt the ecliptic intersects the celestial equator in two points called equinoxes.

The Sun’s Daily Path Across the Sky

• The sun rises in the east and set in the west.• During summer in the northern hemisphere, the Sun

reaches its northernmost position at the summer solstice. It reaches its southernmost position at the winter solstice

Celestial sphere

• It’s the apparent sphere of the sky. The celestial equator and poles are projections of the Earth’s equator and axis of rotation out in space.

View from 35 North Latitude

• Viewed from North America the north celestial pole is always above the horizon. From an observer at 35 north latitude (Los Angeles) the north celestial pole is 35 degrees above the horizon

• Stars within 35 south and north rise in the east and set in the west

Celestial Coordinates

• Right ascension and Declination (latitude and longitude)• Right ascension is measured from Vernal Equinox (late March)

eastward• Units: hours and minutes7u!!! (corresponds to the time required

for the celestial sphere to rotate…)

Apparent Solar Time

• Horizon • Meridian• Zenith

Altitude:(height above or Below the horizon)Azimuth: zero at theNorth pole!!!