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Astronomy Picture of the Day

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

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

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.

“Massive”

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

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

Volume (How much space something takes up)

1 cm3 = 0.06 cubic inches(about the size of a sugar cube)

2 cm1 cm

2 cm

2 cm x 1cm x 2 cm = 4 cm3

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 0.7Water 1.0Polystyrene 1.05Average Rock 2.4Glass 2.6Ceramic 4.6Iron 7.9Lead 11.3Gold 19.3

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?

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.

Angular Measures

90o

20o

360o, or 360 degrees, or 360 "arc degrees" in a circle.

1o = 60' or arcminutes1' = 60" or arcseconds

Angular Measure

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

4000 x 0.002 = (4 x 103) x (2 x 10 -3)= 8 x 100 = 8

Add the exponents

Powers of Ten Video

Question

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

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!

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?

The Constellation Orion

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.

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)

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!

What causes the seasons?

Seasons

Summer Winter

In winter, sunlight is spread out more thinly across the ground => each bit of ground receives less radiation => cooler

Seasons caused by tilt of Earth’s axis of rotation.

Earth is farthest from sun during summer!

Precession

What causes precession? How long does one cycle take?

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

Spin axis

**Vega Polaris

Precession Period 26,000 years!