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Astronomy Picture of the Day
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What properties of physical objects would you
expect to be important in Astronomy?
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What properties of physical objects would you
expect to be important in Astronomy?

• Mass
• Size
• Distance from Earth
• Temperature
• Speed and Velocity
• Brightness (Luminosity)?
• Age
• Chemical composition

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Properties of Matter
Mass (How much stuff there is)? 1 kilogram
(kg) 1000 grams (g) 2.2 lbs We tend to use
mass and weight interchangeably, but weight
depends on gravity.
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Massive
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Difference between Mass and Weight

• Mass of cat on Earth Mass of cat on Moon
• BUT,
• Weight of cat on Earth 18lbs
• Weight of cat on Moon 18/6 lbs 3lbs

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Distance 1 meter (m) 100 centimeters (cm)?
39.4 inches
(slightly longer than a yard)? A way to measure
the distance to far astronomical objects is using
ANGULAR measures parallax
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Volume (How much space something takes up)? 1
cm3 0.06 cubic inches (about the size of a
sugar cube)?
2 cm x 1cm x 2 cm 4 cm3
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Density (How much stuff is concentrated in
an object) Density Mass (g /
cm3)? Volume
Densities of Common Substances Balsa Wood
0.13 g / cm3 Gasoline
0.7 Water 1.0 Polystyrene
1.05 Average Rock 2.4 Glass
2.6 Ceramic 4.6 Iron
7.9 Lead
11.3 Gold 19.3
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Temperature (How hot something is)?

• The Kelvin Scale
• T(K) T(oC) 273 oC
• "Absolute zero" 0 K -273 oC
• At what temperature (in Kelvin) does water
  freeze?
• Boil?

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Speed and Velocity
Speed how fast something is moving Velocity
speed direction a vector quantity - an
object can have the same speed but by changing
its direction have a different velocity.
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Angular Measures
90o
20o
360o, or 360 degrees, or 360 "arc degrees" in a
circle.
1o 60' or arcminutes 1' 60" or arcseconds
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Angular Measure
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Scientific Notation
(A shorthand way of writing large numbers, which
occur often in astronomy). We use powers, or
exponents, of 10
100 102 ( 10 x 10) 1000
103 ( 10 x 10 x 10)? 1,000,000
106 10 101 1
100 0.1
10 -1 0.0001 10 -4 0.007
7 x 10 -3
Add the exponents
(4 x 103) x (2 x 10 -3)? 8 x 100 8
4000 x 0.002
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Powers of Ten Video
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Question

• Why is looking at a distant astronomical object
  like looking into its past?

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Scale of the Universe

• Speed of light
• 186,000 miles/sec
• 300,000 km/sec
• Light-year (LY)?
• Distance light travels in one year
• 63,200 AU
• 6 trillion miles!

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The Constellations

• Northern constellations named for figures in
  Greek myth
• Attempt to understand heavens
• Astronomical Landmarks

• Are the stars in a constellation all at about the
  same distance from the Earth?

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The Constellation Orion
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The Celestial Sphere
An ancient concept, as if all objects at same
distance. But to find things on sky, don't need
to know distance, so still useful today.
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The "Solar Day" and the "Sidereal Day"
Solar Day How long it takes for the Sun to
return to the same position in the sky (24
hours). Sidereal Day How long it takes
for the Earth to rotate 360o on its axis. These
are not the same! Which is longer? Why? (Demo)?
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One solar day later, the Earth has rotated
slightly more than 360o . A solar day is longer
than a sidereal day by 3.9 minutes (24 hours
vs. 23 hours 56 minutes 4.091
seconds). Difference due to rotation and
revolution of Earth!
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What causes the seasons?
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Seasons
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Summer
Winter
In winter, sunlight is spread out more thinly
across the ground gt each bit of ground receives
less radiation gt cooler Seasons caused by tilt
of Earths axis of rotation. Earth is farthest
from sun during summer!
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Precession
What causes precession? How long does one cycle
take?
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Precession
The Earth has a bulge. The Moon "pulls down" on
the side of the bulge closest to it, causing the
Earth to wobble on its axis.
Earth
Moon

Vega

Polaris
Spin axis
Precession Period 26,000 years!