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Astronomy
Understanding the night sky
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We measure the sky using
angles.
Pinky= 1 degree
3 finger (index, middle and ring finger)= 5 degree
closed fist= 10 degree
Index and pinky= 15 degree
thumb and pinky= 25 degree
Full circle =
360 degree
=
2 pi radians
Angles in degrees=
360 degrees
Angles in Arcminutes=
21600' minutes
Angles in Arcseconds=
1296000" seconds
Whose Idea was angular measurement = 360 degrees?
The ancient Sumerians
around
3000BC.
An objects's angular size appears
smaller
if it is farther away.
A
proper motion
is how much an object moves (in angle on the sky compared to the stars) per unit time
Angular speed
is an object moving with a velocity will have a proper motion because its physical position changes with time.
Angular resolution
is the angle at which 2 points of light can be distinguished
Anatomy of the milky way
A)
Globular clusters
B)
Disc
C)
Stellar halo
D)
sun
E)
bulge
5
Stars at different distances all appear to lie on the
celestial
sphere
North
celestial pole is directly above the Earth's North pole.
South
celestial pole is directly above the Earth's South pole.
Celestial equator
is a projection of Earth's equator onto sky.
Ecliptic
is the sun's apparent path through the celestial sphere.
Zenith
: the point directly overhead.
Horizon
: all points 90 degrees away from zenith.
Meridian
: line passing through the zenith and connecting N and S points on the horizon.
A)
Zenith
B)
Horizon
2
Latitude
: position north and south of the equator.
Longitude
: position east and west of prime meridian.
The sky varies with
latitude
but not
longitude.
Stars near the north celestial pole are
circumpolar
and never set.
We cannot see stars near the
south
celestial pole.
All other stars (sun, moon, planets) rise in
east
and set in
west.
As the Earth orbits the sun, the sun appears to move
eastward
along the ecliptic.
At midnight, the stars on our meridian are
opposite
the sun in the sky.
Why do the constellations we see depend on latitude and time of year?
Depends on latitude because your position on earth determines which constellations remain below the
horizon.
Depends on time because Earth's orbit changes the apparent location of the
sun
among the
stars.
Seasons depend on how Earth’s
axis
affects the directness of
sunlight.
Summer solstice
- June
Winter solstice
- December
Spring equinox
- March
fall equinox
- September
Sun's path across the sky:
Summer Solstice:
highest
path; rise and set at most extreme
north
of due east.
Equinoxes: sun rises due
east
and sets due
west.
Winter solstice:
lowest
path; rise and set at most extreme
south
of due east.
Seasonal changes are more extreme at
high latitudes
Earth's axis points in the same direction (to
Polaris
) all year round, so its orientation relative to the sun changes as Earth orbits the Sun.
Summer
occurs in your hemisphere when sunlight hits it more directly;
winter
occurs when sunlight is less direct.
Axis tilt
is the key to the seasons.
How does the orientation of Earth's axis change with time?
Earth has a
26000
year
precession cycle
that slowly and subtly changes the orientation of Earth's axis.
Sidereal
day
:
Earth
rotates once on its axis in 23 hours, 56 minutes, 4.07 seconds.
Solar
day
: the
sun
makes one circuit around the sky in 24 hours.
A solar day is
longer
than a sidereal day by about 1/360 because Earth moves about
1
degree in orbit each day.
The phases of the moon are a
29.5
-day cycle.
Phases of the moon:
A)
first quarter
B)
gibbous
C)
full
D)
gibbous
E)
last quarter
F)
crescent
G)
crescent
H)
Waxing
I)
waning
9
Waxing: Moon visible in
afternoon
/
evening
; gets
fuller
and
rises
later each day
Waning: moon visible in late
night
/
morning
; gets
less
full
and
sets
later each day.
Sidereal month: moon orbits Earth in
27.3 days.
Earth and Moon travel 30 degrees around the sun during that time.
Synodic month: a cycle of
lunar phases
; takes
29.5
days ( which is 1/12 longer than a sidereal)
We only see one side of the moon because of
synchronous rotation-
the moon rotates exactly once with each orbit.
Eclipse
: earth and moon cast shadows. The moon is in front of the sun.
Solar eclipses can occur only at
new moon
;
Moon's
shadow on
Earth
solar eclipse can be
partial
,
total
, or
annular
Partial solar eclipse
: The Moon partially obscures the Sun.
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