The Horizon System
Altitude and Azimuth
 Altitude and azimuth describe the location of an object in the sky as viewed
from a particular location at a particular time.
Altitude
 The distance an object appears to be above the horizon
 The angle is measured up from the closest point on the horizon
Azimuth
 The angular distance along the horizon to the location of the object
 Measured from north towards the east along the horizon
The Changing Night Sky
 At any location on Earth, the
altitude of the NCP equals the
latitude.
 Ithaca is located at 42 degrees
North latitude
 So as seen in Ithaca, the altitude of
the North Celestial Pole (NCP) is
42 degrees.
 Like the Sun, stars rise in the east
and set in the west.
 Through the night, a star moves
across the sky as the Earth rotates.
 As seen in Ithaca, some of the
stars close to the North Celestial
Pole never set = circumpolar
stars
 The path that the Sun follows
changes from day to day because
of the Earth's tilt on its axis.
The Sky as Seen at Different Locations of Earth
 At different latitudes, different portions of the celestial sphere are visible.
 Northern latitudes - cannot
ever see stars around the
South Celestial Pole.
 Southern latitudes - cannot
ever see stars around the
North Celestial Pole.
 At the North Pole, all of
the stars are circumpolar.
 Only half of the stars are
above the horizon, but
they are always above it.
 At the equator, all of the
stars are visible but only
for half of the time.
 Hawaii lies at 19 degrees north latitude.
 Can you figure out what Theta (in the diagram below) is?