Transcript motionofobjects

Motions of the Night Sky
Stars, Sun, Moon, Planets
Motions of the Stars
• Objects rise in the east move across the sky,
and set in the west.
• Motion is known as daily or diurnal motion.
Motions of the Stars (2)
• Stars appear to rotate around the North
Celestial Pole, earth’s axis of rotation
projected out into space.
• The points of sunrise and sunset change
with the seasons
• During the summer, the sun rises far to the
north of east, sets far to the north of west,
and spends up to 15 hours above the
horizon.
Compare these
3 sunrise points for
the summer solstice,
fall equinox, and
winter solstice.
Photos by Rick Pirko
http://analyzer.depaul.edu/paperplate/Sunrise%20Sunset.htm
This change in the sun’s apparent behavior
is due to the 23.5o tilt of the earth’s axis,
and is also the cause of our seasonal
weather changes. The hyperlinks below
illustrates further.
http://www.scienceu.com/observatory/articles/seasons/seasons.html
http://www.suntrek.org/earth-beyond/spinning-orbiting-earth/what-causes-seasons/watch-tilt.shtml
Motions of the Moon
• You can make 4 observations if you watch
the moon move through the sky over
several months:
• First, the apparent size of the full moon
varies from month to month.
Motions of the Moon
• From the different sizes of the full moon,
we can conclude that the moon’s orbit is
quite elliptical.
• In fact it varies by 13.3% of the average
distance.
• Perigee or closest approach is 356,000 km.
Apogee or farthest approach is 407,000 km,
with an average of 384,000 km.
Motions of the Moon
2nd Observation
• The moon doesn’t orbit the earth on the ecliptic, but
rather spends half its time above and half its time
below the ecliptic.
• The moon’s orbit is tilted or inclined
http://www.mmscrusaders.com/newscirocks/eclipse/eclipses.htm
Motions of the Moon
3rd Observation
• The moon moves eastward against the
background of stars and the moon lags behind
the stars, rising 53 minutes later every
evening.
• The orbital motion results in the changing
lunar phases.
http://www.sumanasinc.com/webchttp://www.sumanasinc.com/webcontent/animations/content/moonphase.htmlontent/anisamples/astronomy/moonphase.html
The hyperlink above illustrates the changing
lunar phases as the moon orbits the earth.
Motions of the Moon
The 4th observation
• The same side of the moon always faces
the earth. This means that the moon
rotates on its axis at exactly the same rate
at which it orbits the earth. We call this
“locked” rotation synchronous rotation.
http://www.sckans.edu/~gangwere/LAS170a2/_27.html
Synchronous Rotation of the Moon
Retrograde Motion
You’re going the wrong way!
Motions of the Planets
• The planets rise on the eastern horizon and
set in the west, due to the earth’s rotation.
However, like the moon, the planets usually
move eastward against the background of
stars.
• How fast they move depends on their
distance from the earth and their orbital
distance from the sun.
Motions of the Planets
• Once or more each year the planets do
something strange. They pause in their
normal eastward (or prograde) motion, and
begin moving westward (or retrograde) for a
few weeks, pause again, then resume their
normal eastward prograde motion.
History of Retrograde Motion
• Ancient Greeks noticed that certain celestial
objects changed their locations from time to
time
• They called them “wanderers”; ‘planet’ comes
from that Greek word
• Greeks charted stars and planets, noticed that
planets seemed to stop and change direction
sometimes
Retrograde Motion
• On Earth, planets appear to move from east to west
due to our rotation (prograde motion)
• Once in a while, planets appear to stop and move
west to east (retrograde motion)
Time lapse of Mars
Retrograde Motion
• For planets further from the Sun than Earth
– Earth orbit is faster than these planets
– Earth catches up to a planet in its orbit
– Planet appears to stop and move backwards
(West to East)
• YouTube “Retrograde Motion and the
Opposition of Mars”
http://www.youtube.com/watch?v=72FrZz_zJFU
Motions of the Planets
• More than any other motion of objects in
space, this retrograde motion of the planets
confused the ancient peoples and seriously
affected the development of astronomy as a
science.