9/5/11
Lec 4: 5 Sep 2011
Chapter 2: Daily Motion of the Sky
LAST WEEK - Apparent Sky & Celestial Sphere
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Angular Size and Angular Distance
Altitude-Azimuth; horizon, meridian, zenith
Latitude and Longitude on Earth and on Sky
The Celestial Sphere
TODAY - The Daily Motion of the Sky
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Time (from the Sky)
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Earth’s Rotation
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Apparent Daily Motion of the Sky
WEDNESDAY - Yearly Motion of the Sky
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Synodic v. Siderial Period
The “Ecliptic” and Apparent Solar Motion
Day Time v. Night Time
•  Shine light on a sphere,
and half of it is lit up
•  Half of Earth in daylight;
other half in nighttime
•  Fortunately, we rotate so
that we can share the Sun
(but not equally for all parts of
Earth, as we’ll soon see)
•  Earth ROTATES! How do we
know when we’ve completed one
rotation? Why don’t we “feel” it?
•  1 rotation = 1 day ≈ 24 hours
Rotation of the Earth
•  Spherical Earth “rotates” once in 24 hours
(we will see next time that it’s actually 23h 56m)
•  Earth’s rotation is from west to east (counterclockwise
viewed from above north pole)
•  Sky appears to rotate east to west once in 24 hours
(opposite sense of Earth’s rotation)
•  “Axis” of rotation through north and south poles
–  projects to north celestial pole (NCP) and south celestial pole (SCP)
•  Equatorial plane perpendicular to axis
–  cuts through our equator
–  projects to circle on the sky called the celestial equator (CE)
Apparent Daily Motion of the Sky
•  Sun, Moon, planets, stars, etc. rise in East and
set in West?
–  well, sort of, but not really
–  what do they do in between?
•  Not everything is up for 12 hours! Some things
never rise or set. Some are up for 24 hours.
•  Path across the sky, and how long things are
“up” depends on:
–  your latitude on Earth
–  object’s declination in the sky
The Sky Viewed from the North Pole
•  everything is up for 24 hours!
•  everything moves in counterclockwise circle
(looking up)
•  motion is parallel to ground; elevation angle
never changes
•  NCP at zenith
•  CE along horizon
•  always see the same
stars: never see any stars
in the sky’s southern
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The Sky Viewed from the Equator
•  everything is up for 12 hours!
•  everything moves east to west in “straight lines”
•  rise and set perpendicular to ground (it gets dark in
a hurry!)
The Sky Viewed from Charleston
•  motion depends on where in sky you are looking
•  time from rise to set depends on declination
•  some paths are arcs, some paths are circles
•  NCP at horizon
•  NCP elevation=33o
•  CE overhead: from east
through zenith to west
•  CE from east through
meridian elevated by 57o
to west
•  see all stars in BOTH
hemispheres! •  all stars move in circles
around NCP
•  stars with declination
more than 90 minus your
latitude (57 to 90 degrees
for Charleston) are
“CIRCUMPOLAR”
•  for stars south of this, we
only see a portion of their
circular path, so they appear
to move in an arc
•  notice that stars farther
from pole move farther in
same amount of time
•  see all stars north of
declination -57o The Sky Viewed from Charleston
(what you see depends on declination)
•  Dec +57 to 90: up for 24 hours; circles around NCP
•  Dec 0 to + 57: rise north of east, set north of west,
up for MORE than 12 hours; arc overhead
•  Dec 0 (CE): rise exactly east, set exactly west, up
exactly 12 hours, arc along celestial equator
•  Dec -57 to 0: rise south of east, set south of west, up
for LESS than 12 hours; small arcs facing south
•  Dec -90 to -57: never rise or set; can never see
The Sky From Different Places on Earth
Circumpolar
stars
•  everyone on same longitude sees same
“time” (i.e. Sun same distance east or west of
meridian)
•  everyone on same latitude sees the same part of
the sky (e.g. 33o north latitude sees from -57o to
+90o of declination) at the same local time, but
local time depends on longitude
•  At any time, an observer can see only half of the celestial sphere
•  The other half is below the horizon, hidden by the body of the Earth
–  e.g., at 11 PM Eastern Time in Washington, DC you
see the same thing as someone in Denver, CO (both
are at latitude = 40o N) will see at 11 PM Mountain
time (but that happens 2 hours later)
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Local Solar Time
•  when Sun is “on” the meridian, we call it “noon”
•  one rotation (one day) corresponds to time it
takes Sun to go all the way around and appear
again on the meridian
•  altitude of Sun increases from sunrise to noon
(AM) and decreases from noon to sunset (PM)
•  midnight corresponds to Sun on the meridian on
the other side of the Earth (1/2 rotation or 12
hours after noon)
•  in astronomy, we use local solar time, but it’s not
very practical for society...
•  Earth rotates West to East (Sun rises in the East!)
Solar v. Civil Time
•  Sun “on” meridian at local solar noon
–  only one longitude on Earth has solar noon at any
given instant
•  It would get very confusing if everybody had
different clocks, but –  it would be just as confusing if we all used the same
time: it could be dark at “noon”!!
–  but astronomers, pilots, etc. use “Universal Time”
•  So we split the difference and have 24 time
zones, each about 15o wide (360o/24h=15o/h)
–  Sun (and stars) move 15o each hour = 1 hour of RA
–  “Civil Time” = “Solar Time” ± 1/2 hour
Daylight Savings Time
•  What is “Daylight Savings Time”?
–  has nothing to do with astronomy
–  totally artificial way to make sunset seem later
(at the cost of having a later sunrise)
–  from March to November:
civil time (daylight savings time) = standard time + 1 hour
–  not everybody observes daylight savings time
•  if it’s noon (civil time) everywhere in a time zone...
–  it’s 12:00 PM (solar time) in the middle of the time zone,
–  it’s 12:30 PM (solar time) on the eastern edge, and
–  it’s 11:30 AM (solar time) on the western edge
•  Combining the time zone width with DST ->
civil time can be as much as 1.5 hours off from
solar time
–  depends on longitude w/in time zone
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