Transcript Motions of the Night Sky - d_smith.lhseducators.com

Motions of the Night Sky
Stars, Sun, Moon, Planets
Motions of the Stars
 Like all objects in the sky, the stars rise
in the east at an angle of 50o with the
horizon, move across the sky, and set in
the west (also at an angle of 50o with
the horizon, here in Lancaster.)
 This ordinary motion that we see every
clear night is known as daily or diurnal
motion.
Motions of the Stars (2)
 The stars appear to rotate around the
North Celestial Pole, earth’s axis of
rotation projected out into space.
 Here’s a surprising observation: for a
given star to travel from a set spot in the
sky (let’s say on the meridian) to the
exact same spot the next night doesn’t
take 24 hours, but only 23 hours and 56
minutes. What happened to those 4
minutes?
Motions of the Stars (3)
 The earth rotates on its axis in 23h 56m,
but must travel 1o further in its orbit for
the sun to appear to travel from the
meridian back to the meridian (local
solar noon). This extra travel takes an
extra 4 minutes.
 One day by the sun is the solar day (24
hours). One “day” by the stars (23h
56m) is the sidereal day.
Click the link below for an animation of the
difference between a sidereal & a solar day.
http://csep10.phys.utk.edu/astr161/lect/time/timekeeping.html
Other motions of the sun
 Besides the normal diurnal or rise/set
motion of the sun, the points of sunrise
and sunset change with the seasons, as
does the altitude or elevation of the
noon-time sun.
 During the summer, the sun rises far to
the north of east, sets far to the north of
west, has a noon-time altitude of 73.5o,
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
Other motions of the sun (2)
 On the equinoxes, the sun rises due
east, spends 12 hours above the
horizon, and sets due west.
 In the winter, the sun rises far to the
south of east, spends as few as 9 hours
above the horizon, and sets far to the
south of west.
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 hyperlink below
illustrates further.
http://deepimpact.jpl.nasa.gov/disczone/braintwist-comet1a.html
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 (2)
 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 (3)
 2nd Observation: the moon doesn’t orbit the
earth on the ecliptic (the plane of the earth’s
orbit and the apparent path of the sun against
the background of stars), but rather spends
half its time above and half its time below the
ecliptic.
 From this, we can conclude that the moon’s
orbit is tilted or inclined (5o with respect to
the ecliptic.)
http://www.mmscrusaders.com/newscirocks/eclipse/eclipses.htm
Motions of the Moon (4)
 Later, we’ll use the inclination of the
moon’s orbit as a clue to its origin. The
tilt is evidence that the moon was NOT
formed at the same time and in the
same region of space as the earth was
formed, but that another process
caused the origin of the moon.
Motions of the Moon (5)
 3rd Observation: The moon moves
eastward against the background of
stars, ~0.5o (the width of the moon) per
hour, 13o per day. This causes the
moon to lag behind the stars, rising 53
minutes later every evening.
 The orbital motion results in the
changing lunar phases.
http://www.sumanasinc.com/webcontent/anisamples/astronomy/moonphase.html
The hyperlink above illustrates the changing
lunar phases as the moon orbits the earth.
Motions of the Moon (6)
 The 4th observation is that 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.
 Later, this synchronous rotation will give
us a clue about the construction of the
moon’s interior.
http://www.sckans.edu/~gangwere/LAS170a2/_27.html
Synchronous Rotation of the Moon
Motions of the Planets
 Like all objects, 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 (2)
 Once or more each year, however, 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.
East or Prograde
West or Retrograde
Photographs of Mars, several days apart, over about a
6 month period.
Motions of the Planets (3)
 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.
Motions of the Planets (4)
 The inferior planets, Mercury & Venus,
are never seen far from the sun.
Mercury is never farther than 27o and
Venus never farther than 47o from the
sun.
 These planets appear to move back and
forth from one side of the sun to the
other, alternately appearing at sunrise,
then at sunset.
Motions of the Planets (5)
 Not only are the 2 inferior planets
always found near the sun, but they
also exhibit phases.
 Today, we interpret these motions to
indicate that the planets are orbiting the
sun. Ancient astronomers had some
less straight-forward explanations.
http://lpc1.clpccd.cc.ca.us/lpc/harpell/astro20/a20lectures/ahistlec.htm
Motions of the Planets (6)
 The superior planets, Mars, Jupiter,
and Saturn (Uranus, Neptune, and Pluto
aren’t visible to the naked eye) also
exhibit retrograde motion, but somewhat
differently.
 These outer planets form loops or Scurves in the sky.
http://www.astro.uiuc.edu/projects/data/Retrograde/
The hyperlink above will take you to a retrograde motion animation.
Motions of the Planets (7)
 Today, we realize that the retrograde
motion of a superior planet results from
the earth “lapping” or passing the
planet. The line of sight from the earth
to the planet angles backward against
the very distant stars, causing the
planet to appear to be moving
backward.
Motion of the Planets (8)
 Consider an analogy – 2 race cars
moving around two concentric circular
tracks at the same speed. The car that
is on the inner track will periodically lap
the car on the outer track. Both cars
are constantly moving forward, but from
the point of view of the inner car, the
outer car appears to move backward.
The car on the inner track laps the outer cars.
To the inner driver, the outer cars appear to
move backward, while actually moving forward.
The Next Step
 In the next lessons, we’ll see how the
motions of the objects through the sky,
especially those of the planets, affected
the concept of how the solar system
functions, and the development of
astronomy as a science.