Astronomy
Celestial Observations
The Celestial Sphere
All celestial objects, (sun, stars, moon,
planets), from Earth appear to be on
the surface of an imaginary sphere
surrounding Earth.
This imaginary sphere is called the
Celestial Sphere.
Celestial Observation Terms
Horizon = where celestial sphere meets Earth.
Zenith = point on celestial sphere directly
above observers position.
Altitude = objects distance, in degrees, above
horizon.
Azimuth = objects distance, in degrees,
measured clockwise from due north position
(due north is 00 azimuth).
Star Paths
All celestial objects appear to move from east
to west across the sky in an arc. Why?
Circumpolar stars move in
counterclockwise circles between the
northern horizon and Polaris and never set.
Rotation
The apparent daily motion of celestial objects
is due to Earth’s rotation.
Rotation is the turning of an object on its axis.
Celestial objects are not actually moving from
east to west across the sky, rather the earth
is rotating under them.
Rotations
Revolution
Revolution is the movement of one
celestial object around another.
The path along which an object travels
during a revolution is called an orbit.
Earth’s Revolution
Orbits
What are Constellations?
Constellations are stars that appear to be
grouped in patterns that form the outlines
of “things” (people, animals, objects).
The night sky is divided into 88 constellations.
Circumpolar constellations are those
constellations that are always visible as they
never set below the horizon.
Star Shifts
Constellations shift, rise and set, (similar to
the rise and set of the Sun) during a night
due to the rotation of the Earth.
Constellations in the sky slowly change from
one night to the next as a result of the
revolution of Earth around the sun.
Constellations rise approximately 4 minutes
later each night.
Planetary Motions
• On a daily basis planets and stars
appear to move from east to west.
• Over long periods of time the planets
appear to move eastward relative to the
stars behind them.
Celestial Models
Geocentric
Heliocentric
Theory: this
Theory: this
theory, put
theory, put
forward by the
forward by the
ancient
Polish scientist
Egyptian
Copernicus,
astronomer
states that the
Ptolemy, states
Sun is the
that the Earth is
center of the
the center of
universe.
the universe.
Ptolemy and Geocentric Theory
Copernicus and Heliocentric Theory
Orbital Forces
Johann Kepler developed the “Laws of
Planetary Motion” which states that planets
orbit in ellipses.
Unlike a circle the center of an ellipse consists
of two fixed points called foci.
Kepler’s First Law:
The orbits of the planets around the Sun are
ellipses, with the Sun at one of the foci.
Kepler’s Laws
Kepler’s second law states that:
An imaginary line joining a planet to the
Sun will sweep over equal areas in equal
periods of time.
Perihelion = when the planet is closest to
the Sun (moving the fastest).
Aphelion = when the planet is farthest
from the Sun (moving the slowest).
Evidence of Earth’s Rotation
• Foucault Pendulum: the greater the
latitude the greater the hourly change
in direction.
• Coriolis Effect: is the tendency of
matter moving across the Earth’s
surface to be deflected from a straightline path.
• Other evidence includes the day night
cycle, star paths and satellite photos.
Apparent Motions of the Sun
• Due to the rotation of the Earth, the
Sun appears to move in an arc across
the sky from east to west.
• The tilt of the Earth’s axis, 23.50,
causes the location of the direct rays
of the Sun to change in a cyclic
pattern.
Change of Seasons
• Three factors cause the seasonal
changes:
– Earth’s revolution around the sun
– Tilt of the Earth’s axis
– Parallelism of the axis
Time and Earth Motions
• Sidereal day is the time period for Earth to
make one complete rotation on it’s axis (23
hrs, 56 min, 4 sec.).
• Apparent solar day is about 4 min longer
(based on solar noon to solar noon) to
account for the additional rotation needed as
a result of Earth’s revolution around the
sun.
• Since our orbital speed varies, therefore the
apparent solar day varies, the mean solar
day (24 hrs) is used for daily timekeeping.
Moon Motions
• Moon orbits earth once every 27 1/3 days.
• The barycenter is the gravitational center
the Earth and Moon revolve around
(1700km beneath surface of Earth).
• Sidereal month (27 1/3 days) is time for
the Moon to revolve around the Earth.
• Synodic month (29 ½ days) is the time for
the Moon to complete a cycle of phases.
Phases of the Moon
• The illuminated portion of the moon that
faces Earth varies in a cyclic pattern called
phases.
• New moon = when the entire illuminated
portion is facing away from Earth.
• Waxing period = when right hand portion
seems to grow more lighted.
• Full moon = when the entire illuminated
portion is facing toward Earth.
• Waning period = when the left hand portion
shrinks in illumination.
Eclipses
• Solar eclipse occurs when the Moon passes
between the Earth and the Sun.
– Umbra is the part of the Moon’s shadow in
total darkness
– Penumbra is the part of the Moon’s shadow in
partial darkness.
– Annular eclipse occurs when the Moon is at
apogee and the outer edge of the Sun is visible.
• Lunar eclipse occurs when the moon passes
through the Earth’s shadow.
Solar and Lunar Eclipse