Transcript Scales of the Universe

Cycle of the Sky
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To understand the apparent motions of
celestial objects
To introduce the seasons
So MANY objects….How Do We
Make Sense of it ALL??
The Sun is physically 400 times larger than the Moon.
Why is their angular size is the same?
Angle diameter (in radians) = diameter/distance
Full circle = 2 (in radians)=360 degrees=360o
1o=60 arminutes = 60’
1’ = 60 arcseconds = 60”
Angular Size
Some Examples:
Horizon to zenith (point overhead)
90o
Your fist at arm's length
10o
Your fingernail at arm's length
1o = 60'
Sun or Moon seen from Earth
0.5o = 30'
Smallest detail visible to naked eye
1' = 60''
Smallest detail visible by a single telescope from
Earth's surface:
1''
Angular size of Pluto: 0.15”
Betelgeuse (largest star) seen from Earth 0.004''
Constellations
88 official constellations divide the sky into
areas with clearly boundaries.
The names of constellations are in Latin. But
most bright star names derived from ancient
Arabic.
The original constellations were invented by
farmers over 5000 years ago.
Remember that the constellations are not real!
Projection
Big Dipper ---- an asterism
How long to go all the way
around?
1 day
b.
1 week
c.
1 month
d.
1 year
Are there stars in the sky during the daytime?
There are two ways
to think about the
motion of the skey:
- the C.S. rotates
- or the Earth
rotates while the
sky sits still.
Sometimes one
works better than
the other …
Luminosity and Magnitude
• mV = -2.5*Log(I) + C
• m1 – m2 = -2.5*Log(I1/I2)
• I1/I2 = 10-0.4*(m1 – m2) = 2.512(m1 – m2)
Latitude and
longitude make a
convenient coordinate
system for locating
objects on the Earth.
Altitude and Azimuth
give us the direction
to look on the celestial
sphere.
The altitude of Polaris
above the horizon is
approximately the
same as the observer's
latitude in the
Northern Hemisphere.
Time Zones and Universal Time
Time
The time for many astronomical events is given in
Universal Time (UT), which is (approximately) the
local time for Greenwich, England --- the Greenwich
Mean Time or GMT.
Cycles of the Sky
Earth's rotation causes the sun and stars to rise in the
east and set in the west daily.
But not everywhere!
Near the poles, the sun rises and sets every half year,
and circumpolar constellations such as Ursa Major, never
set.
What other cycles do you know?
The constellations along the ecliptic are called the zodiac
What causes the seasons?
What are the differences between days in
summer and winter?
Longer days
stronger Sun
in summer than in winter
Are the seasons due to the
changing distance from the sun?
Distance doesn't matter here!
(Not to scale)
Seasons are due to the tilt of the earth.
Assigned Reading
• Chapter 2