This Next Group of Slides

• This next set of slides deals with measurement, Scientific Notation, QWIZDOM unit set-up, lightyears and parsecs, the nature of science.

• Units covered are 3 and part of 4.

The Metric System

English Units (Distance)

12 divisions = 1 inch

12 inches = 1 foot

3 feet = 1 yard

5.5 yards = 1 rod

4 rods = 1 chain

10 chains = 1 furlong

8 furlongs = 1 mile

3 miles = 1 league

Metric Units

10 millimeters = 1 centimeter

100 cm = 1 meter

1000 m = 1 kilometer

1000 milligrams = 1 gram

1000 g = 1 kilogram

• 0.0000001 meters is difficult to read and write; too many zeros.

• Use scientific notation to simplify.

• 0.0000001 m = 110-7

Move the decimal place seven places to the left to determine the power/exponent of ten.– Moving it to the left makes the

power negative.

– Moving it to the right makes the power positive.

Scientific Notation

Commonly used prefixesNumber Scientific

Notation Prefix Abbreviation

1,000,000,000 1 109 giga G

1,000,000 1 106 mega M

1,000 1 103 kilo k

0.011 10-

2 centi c

0.0011 10-

3 milli m

0.0000011 10-

6 micro

0.0000000011 10-

9 nano n

Special Units

• The Light Year (ly)– Distance light travels in 1

year. (1 Earth year.)

– Equivalent to a “look-back” time; it’s how far back in time we are seeing.

– Example: Proxima Centauri is 4.1 ly away. The light we see from it today left the star 4.1 years ago. We see it as it was 4.1 years ago.

• The Parsec (pc)– “PARallax SECond”

– Distance to a body whose parallax motion covers 1 second of arc.

– 1 pc = 3.26 ly

• The Scientific Method is the procedure scientists use to construct their ideas about how nature (everything) works. The hope is to come up with a:

• Scientific Theory/Model/Law– Concerns

observable/measurable things.– Explains observable things.– Must be testable.

– Must be disprovable. – Predictive!

• A Model is a description of physical phenomena that we feel matches well with reality. - The Celestial Sphere, sort of.

- Universal Gravitation

- Electron Orbitals

The Scientific Method

Principle of Uniformity

• The Laws of Science are the same everywhere in the universe.

• The Laws of Science are the same everywhen in time

(starting at some time following the Big Bang).

If these aren’t so, then any model of the universe is possible and, perhaps, equally valid.

This Next Set of Slides

• This part of the lecture covers, matter, fundamental forces, beginnings of astronomy, the path of the sun through the sky throughout the year, the seasons, and precession.

• Units covered: 4 and 6.

The Nature of Matter

• Protons (positively charged) and neutrons (uncharged) make up the nucleus at the center of an atom. Electrons (negatively charged particles) are found relatively far from the nucleus.– If the nucleus were the size of a grape, the closest electrons would

orbit at a distance about the length of a football field.– Most “solid” matter, then, is made up of mostly empty space!

Fundamental Forces in Nature

• Gravitational Force– Force between objects

with mass.– Infinite in range, but

weakens with distance.– Weakest of all forces.

• Electromagnetic Force– Force between charged

bodies.– Like charges repel…– Infinite in range, but

weakens with distance.

• Strong Force– Force that holds atomic

nuclei together– Very short range – 10-15

meters!

• Weak Force– Force responsible for

radioactive decay– Very short range – 10-18

meters!– (Really, related to

electromagnetic force.)

Elementary Particles

The description of the universe and its contents using elementary particles is

called The Standard Model.

• Smaller particles known as quarks, make up protons and neutrons.– Up quarks– Down quarks

• “Up” and “Down” are just labels.

• Other kinds of quarks have names like “strange” and “charm”, and again are just labels.

WHY Astronomy (for ancient peoples)?

* A sense of wonder! The night sky is an amazing and beautiful thing to look at and think about!

* Navigation

* Agriculture (when’s the rain coming?), Calendar

* Time.

* Curiosity.

* Superstition. Guidance. Connection to gods.

The Annual Motion of the Sun

• As the Earth revolves around (orbits) the Sun, the Sun appears to move through 13 constellations on a belt around the celestial sphere called the ecliptic.

• When the sun in the sky is in “front” of a particular constellation, we say that the Sun is “in” that constellation.

• As this motion repeats itself after one year, it is called the Sun’s annual motion.

• The ecliptic ‘belt’ on the celestial sphere is tipped relative to the celestial equator due to the 23.5° inclination of the Earth’s rotational axis.

• In June, the Sun appears north of the celestial equator.

• In December, the Sun appears south of the celestial equator.

• Twice a year, the sun appears on the celestial equator – these times are called the equinoxes.

The Ecliptic

The Seasons I

• The Earth’s inclination is ultimately responsible for the change in seasons.– In June, the

Northern Hemisphere is tilted towards the Sun.

– In December, the Northern Hemisphere is tilted away from the Sun.

• Common Myths:– Summers are warmer because the Earth is closer to

the Sun than in Winter.• Actually, the opposite is true in Northern Hemisphere!

– The tilt of the Earth’s axis brings the Northern Hemisphere closer to the Sun in Summer, and farther from the Sun in Winter creating the seasons.

• Geometry is true, but this accounts for only a minute fraction of the extra heating in summer.

The Seasons II

• This tilt of the Earth has two important effects:– In Summer, the Sun

spends more time above the horizon – days are longer, resulting in more heating.

– In Summer, light from the Sun strikes the ground more directly, concentrating the Sun’s energy.

• Summers are therefore warmer than winters.

Precession I

• The Earth spins about its axis like a top, but the Sun’s gravity adds a pull that causes it to “wobble”.

• This wobble means the axis of the Earth is rotating, or precessing, with a 26,000 year period.

Precession II

• Because of this precession, Polaris (the North Star) has not always been and will not always be “The North Star”.

• 6000 years ago, the North Star was Thuban, a star in the constellation Draco.

• In 12,000 years, the Earth’s axis will point toward Vega, a bright star in Lyra.