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Unformatted text preview: THE SKY Questions
How do we orient objects in the sky?
How do we find objects on the moving sky?
What b t th
Wh t about the moving objects?
bj t ?
How is the Earth (and humans) affected by the sky? Observations and Theories
The Ancient Greeks and Egyptians developed theories to explain their
observations. However, they maintained several postulates (e.g., circular
motion), which prevented a truly impartial theory.
A. Geocentric Cosmology
1 Earth appears to be flat.
2. Celestial Sphere appears to be above the Earth.
3. Celestial Sphere moves around the Earth.
Stars are fixed, except for meteors.
Stars rise in East, set in the West. Observations and Theories
B. The Sun’s Motion
Sun travels on the ecliptic. There is a 4 minute difference per day
of the Sun s position against the stars. The angle between the
Earth’s orbit and the Earth’s rotation axis is 23o (obliquity).
C Wandering Stars
Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn
D. Zodiac along the Ecliptic
Constellations are groups of stars. Today, there 88. Distances
Here is an example. The distance from my house in Atlanta to my parents’
house north of Nashville is about 17,740,000 inches the number is correct
but it is completely meaningless because we do not have a real-world feel for
numbers that large.
I could state that the distance is about 280 miles. The number 280 is just on
the verge of our ability to comprehend its magnitude.
An even better way of relating the distance between the houses is to say it
takes me 4 hours to drive from one to the other. In expressing it that way, two
things have happened: (1) the quantity of 4 hours is easily comprehensible and
(2) the measurement units were switched from distance to time. Hopefully by
i h df
H f ll b
this change in measurement units, you get a better feel for the distance
between these two homes. What About Angles on the Sky?
Full Moon = Sun
= 30 arcmin
= 1,800 arcsec
,800 a csec Measuring Angles
360o in Circumference x
-------- = -------360o
2R Angular Measurements
tan = D / d
[Diameter / distance] = D / d (radians) Local Coordinates
Relative to the Observer
Z ith i a point directly over-head.
i t di tl
Meridian is a line through the Poles
Azimuth is angle from North
Altitude is angle from Horizon Celestial Coordinates
Celestial Poles are above the North &
Celestial Equator is directly above the
Longitude = Right Ascension ()
Latitude = Declination () Motions
Rotation The i i
R t ti – Th spinning of a body
f b d
around its axis (one day).
Revolution – The orbital motion of
a body around another due to
Gravity (one year). Diurnal Motion
Effects Due to Rotation
Gives us Night and Day.
Ni ht d D
Causes the Sun and Stars to rise in
the East and set in the West.
If there was no Revolution, then
each night sky would be the same. Seasonal Changes
Effects Due to Revolution
Gives us the Year.
Has a role in producing the Seasons,
which are not due to a change in
distance. Path of the Sun Sun’s Path is the Ecliptic. The Earth’s tilt (obliquity) of
23 5o is what causes the Sun s
path not to be on the Celestial
Equator. Yearly Solar Motion Yearly Solar Motion The Day
If there was only the Rotation of the
Earth then the day would be
= 24h 00m 00s
Think about it – this would be the time
from one crossing of the Meridian to the
next crossing by the Sun. (am & pm) The Day (cont.)
But the Earth is also Revolving while
it is Rotating so with respect to the
background stars, one day
= 23h 56m 04s
Think about it – there are 360o in a
circle and there are 365.25 days in a
year. The Day (Cont.)
Mean Solar Day
= 24h 00m 00s
Think about it – Does a clock really
tell accurate time? Is the day really
based on the Sun? Time Zones
24 zones x 15o/zone = 360o
International Dateline The Year
Julian Calendar Problem is that 46 BC
1 year = 365 2500 d
365.2500 days 1 year = 365 2422 days
365.2422 1 leap day is added if the year is
evenly divisible by 4.
Think about it – Should the year and
day be nicely linked? (a difference of 11m 14s) Think about it – Should this
difference cause a problem? The Year (cont)
Easter’s D t h d hift d
E t ’ Date had shifted
Removed 10 days
Oct 4 Oct 15, 1582 The Leap Year
If the year is evenly divisible by 4,
If the year is evenly divisible by 100,
If the year is evenly divisible by 400,
Yes Key Individuals
Pythagoras [ d. 497 BC, Italy]
Aristotle [384 - 322 BC]
Aristarchus of Samos [310 - 230 BC]
Eratosthenes [276 - 195 BC, Alexandria, Egypt]
Hipparchus [160 - 127 BC, Alexandria, Egypt]
Claudius Ptolemy (or Ptolemaeus) [AD 140] Hipparchus [160 - 127 BC]
He cataloged 850 stars by
position and by magnitude. Hipparchus (cont.)
He noted that the North Celestial Pole
d h h
h l i l l
had precessed (changed position) over
the previous 150 years.
Precession is due to the gravitational
pull of the Moon and Sun on the Earth.
It takes 26,000 years for one cycle.
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This note was uploaded on 03/04/2012 for the course PHYS 2022 taught by Professor Jarrio during the Spring '12 term at Central GA Tech.
- Spring '12