Units covered:

Lecture Jan 23: 1-3Lecture Jan 25: 4-8

Textbook: Pathways to Astronomy: third edition

The Scientific Method

• The Scientific Method is the procedure scientists use to construct their ideas about how the Universe works.– Start with a hypothesis – a testable idea

of how something works– Test the hypothesis!– If the test fails, modify or abandon the

hypothesis, and retest.

• Hypotheses that pass rigorous testing become Laws (more mathematical) or Theories (described using both words and equations).

• A Model is a complex description of physical phenomena incorporating many laws and/or theories– Ex: The Celestial Sphere

– Ex: Universal Gravitation

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 we enlarged the nucleus to be the size of a grape, the electrons

would orbit at a distance of slightly less than a football field!– Most “solid” matter, then, is made up of mostly empty space!

Fundamental Forces in Nature

• Gravitational Force– Force between massive

bodies– Infinite in range, but

weakens with distance

• Electromagnetic Force– Force between charged

bodies– 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!

Elementary Particles

• Smallest particles known are quarks, which make up protons and neutrons– Up quarks– Down quarks

• “Up” and “Down” are just labels

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

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

called The Standard Model.

The Celestial Sphere I

• Stars in the universe are located at various distances from Earth, but can be imagined as lying on a sphere, with the Earth at its center.

• This sphere appears to rotate around the Earth, giving the impression that stars rise and set.

• Since earliest times, humans have sought to understand the night sky

• A useful model of the sky is called the Celestial Sphere

• It is not real – it is simply a tool for understanding and prediction

The Celestial Sphere II

• Important Terms– Zenith: The point directly overhead on the celestial

sphere (CS)

– Nadir: The point opposite the zenith on the CS

– North or south celestial pole: The point around which the stars appear to rotate

– Celestial Equator: An extension of the Earth’s equator expanded out to the surface of the CS.

– Horizon: The lower edge of the visible CS

Constellations and Asterisms

• The human mind is very good at recognizing patterns – consequently we have found and named patterns of stars on the celestial sphere

• The names of these patterns have their origins in mythology from all over the globe

• Sometimes very hard to see!

• These patterns are called constellations– 88 internationally recognized

constellations, covering the entire sky

– Star names frequently include the name of the constellation in which they are located

• Some popular patterns are not constellations – these are called asterisms

– Big Dipper

– The Teapot

The Ecliptic

• 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 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!

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

• True, but this accounts for only a minute fraction of the extra heating

The Seasons II

• This tilt 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 little tug

• This tug results in the axis of the Earth rotating, or precessing, with a 26,000 year period

Precession II

• Thanks to precession, Polaris (the North Star) 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

What Time Is It?

• There are many ways to measure time on Earth– Sunrise to sunrise

• Problem – seasons change sunrise times!

– The time between successive crossings of the meridian by the Sun (Solar Day)

• Problem – inaccuracies due to clouds

Length of Daylight Hours

• The number of daylight hours a place has depends on that place’s latitude on the Earth– Regions close to the

northern pole get more daylight hours during the summer, and less in winter

– Within the Arctic Circle (higher than 66.5 degrees latitude), there are some summer days where the Sun never sets!

– Regions close to the equator get close to 12 hours of sunlight all year.

Time Zones

• The globe is divided into 24 time zones, designed such that local noon roughly corresponds to the time when the sun is highest in the sky

• If it is noon on the Prime Meridian in Greenwich, UK, it is midnight on the opposite side of the world. This midnight line is called the International Date Line

The Phases of the Moon

• As the Moon moves around the Earth over its 29.5 day cycle, one half of its surface is always lit by the sun

• From Earth, we see only portions of the illuminated surface, giving the appearance of phases of the Moon– Full Moon: The Earth is between

the Moon and the Sun, so we see all of the illuminated surface

– New Moon: The Moon is between the Earth and the Sun, so we see none of the illuminated surface

Figure 9.04

An ancient Roman calendar showing the first five months of the year. The Roman numerals at the bottom of each column indicate the number of days in each month. The letters A through H indicate the 8 days of the week.

Figure 9.05

A portion of a Mayan calendar which is broken up into 20 day “months.”

The Shape of the Earth

• In addition, he noticed that stars were visible in some southern locations, while not visible in northern locations.

• Again, the Earth must be spherical for this to happen!

• Aristotle concluded from observations of the curved shadow of the Earth on the Moon during a lunar eclipse that the Earth was spherical.

Distance and Size of the Moon

• Aristarchus (~310-230 B.C.E.)– Used the relative sizes of the

Moon and the Earth’s shadow during an eclipse to estimate the size of the Moon

• He estimated that the Moon was 1/3 (0.33) as large as the Earth

• Not too far off! (0.27)

– Also estimated the distance to the Moon by timing how long it took the Moon to pass through the Earth’s shadow during an eclipse

• Estimated a distance of 70 Earth radii

• Pretty close! (~60 Earth radii)

Parallax Preview

• Finally, he postulated that the Earth goes around the Sun, rather than the belief that everything revolves around the Earth

• His critics claimed that if this were true, they would see the positions of the stars change relative to each other.

• This is called parallax– No parallax motion was visible,

so Aristarchus must be wrong!

– Actually, there is parallax (and the Earth does indeed go around the Sun), but the motion was too small for the unaided eye to see – we need telescopes!

Size of the Earth

• Eratosthenes (296-195 b.c.e.) wanted to know the size of the Earth

• He noted that the sun could be seen from the bottom of a well in Syene, so the Sun must be directly overhead

• Then he measured the angle the Sun made with the horizon in Alexandria (7 degrees)

• Calculated a diameter of 13,000 km, almost exactly correct!