Astonishing Astronomy 101With Doctor Bones (Don R. Mueller,

Ph.D.)

EducatorEntertainer

JU

G G LE

RPLANETARY

Scientist

ScienceExplorer

Chapter 1 - Charting the Heavens

Our Planetary Neighborhood

Sun

The Earth• The Earth is a planet, which

orbits a star (Sun)

• Radius: 6371 km (3909 miles)

• Mass: 6 billion trillion tons

• Actual value:

5,970,000,000,000,000,000,000,000 kg

Use 5.97 1027 kg, instead!

The Moon

• The Moon is a satellite, a body orbiting the Earth– Rocky world, littered with

craters• Bombarded by meteors• Where are the Earth’s

craters?– Smaller than the Earth

Less than 1/80 the mass

¼ the diameter of Earth– Small, so it cooled quickly!– Cold, airless and lifeless

The Planets

• Wide variety of planets in the Solar System– Rocky, hot and airless worlds– Gas giants and ringed wonders– Cold planets of blue methane

The Sun

• The Sun is a star, a ball of gas held together by gravity and generating light via nuclear fusion reactions: Converting

Hydrogen to Helium“Burning H to form He”• Source of all energy in

the Solar System.• 100x wider than the

Earth and 300,000x as massive.

Creating Helium via the fusion of Deuterium and Tritium.

DeuteriumTritium

Helium

The Solar System

• Planets, asteroids, comets and dust all held together by the Sun’s gravity• Everything goes around the Sun on elliptical paths called orbits• All orbits lie in the same plane, like peas rolling around on a dinner plate• Too big to describe using meters – we need something more convenient

The Scientific Method

• The Scientific Method is the procedure we use to construct ideas about how the Universe works.– Starting with a hypothesis – a

testable idea of how something works.

– Testing the hypothesis!– If the test fails, modify or abandon

the hypothesis and then retest.• Hypotheses that pass the many

years of testing become Laws or Theories .

• A Model can be a simple to complex description of physical phenomena incorporating many laws and/or theories:– Ex: The Celestial

Sphere– Ex: Universal

Gravitation

The Celestial Spherehttp://www.youtube.com/watch?v=3r4x5jRca20 Hoberman Sphere

• 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 once each day, 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 (CS)

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

The Celestial Sphere (CS)

• 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 that are not

constellations – are called Asterisms– Big Dipper– The Teapot

Skywatching

• Under dark skies, you can see thousands of stars. There are some stars and constellations, however, that you can only see from northern or southern latitudes

• In the northern hemisphere, constellations that never set (but simply circle around the North Celestial pole) are called

circumpolar constellations• Skywatchers at your latitude in the

southern hemisphere never see your circumpolar stars!

Finding Our Way Around

• Finding your way around the celestial sphere (CS) is as easy as finding your way around on Earth– The CS is divided by: – Lines of declination (running

North-South) – Lines of right ascension (running

East-West)– Lines of declination are

comparable to lines of latitude – Lines of right ascension are

comparable to lines of longitude.

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’s glare blocks a particular constellation from view, 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

The ecliptic is tipped relative to the celestial equator due to a 23.5° inclination of the Earth’s rotational axis.

The Seasons I

• The Earth’s inclination is 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:• Northern 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.• This accounts for only a minute fraction of the extra heating.

Please insert figure 6.3B

The Seasons II

• This tilt of the Earth has two important effects:

• In Summer, the Sun spends more time above the horizon, the 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.

Please insert figure 6.3 A

The Tilt of the Ecliptic

Solstices and Equinoxes

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 sunriseProblem – seasons

change sunrise times.– The time between

successive crossings of the meridian by the Sun (Solar Day).

Problem – inaccuracies due to clouds.

Sidereal Time

• One Solar Day is 24 hours – the sun returning to the same spot in the sky on successive days.

• One sidereal day is 23 hours, 56 minutes:

• A sidereal day (23 hours, 56 minutes, 4.091 seconds) corresponds to the time taken for the Earth to complete one rotation relative to the vernal equinox.

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 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.

Please insert figure 7.3

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 line is known as the International Date Line.

Please insert figure7.4

The Phases of the Moon

• As the Moon moves around the Earth over its 29.5 day cycle (a sidereal month), 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.

Phases of the Moon – the Big Picture

Please insert figure 8.2

Solar Eclipses

• At New Moon, the Moon is between the Earth and the Sun. Sometimes, the alignment is just right, allowing the Moon to block the light from the Sun, creating an eclipse.

Solar Eclipse – the Shadow of the Moon

• In a solar eclipse, the Moon casts a shadow on the surface of the Earth. People within the shadow see the eclipse, and those outside the shadow do not.

• The Moon’s umbra is the darkest part of the shadow, directly behind the body of the Moon. Within the umbra, the Sun appears completely eclipsed (total eclipse).

• The penumbra of the Moon is the part of the shadow where the light from the Sun is only partially blocked (partial eclipse).

A solar eclipse seen from space

Regions of visible total solar eclipses

Lunar Eclipses

• As the Moon passes behind the Earth, the Earth can cast a shadow on the surface of the Moon, creating a lunar eclipse.

• The reddish glow of a fully eclipsed Moon is light that has been refracted through the Earth’s atmosphere and bounced back to Earth. – It is, in essence, the light of every sunrise and sunset on Earth reflected off the Moon.

Lunar Eclipse – the Shadow of the Earth

• In a lunar eclipse, the Earth casts a shadow on the surface of the Moon. In its orbit, the Moon passes through the penumbra and umbra of the Earth

• The penumbra of the Earth is the part of the shadow where the light from the Sun is only partially blocked. The Moon dims a little as it passes into the penumbra (a penumbral eclipse).

• The Earth’s umbra is the darkest part of the shadow, directly behind the body of the Earth. After the Moon moves into the umbra, its surface becomes very dark. This is a total lunar eclipse.

Why don’t eclipses happen all the time?

• In order for an eclipse to occur, the Moon must lie directly between the Earth and the Sun (solar eclipse), or the Earth must lie directly between the Moon and the Sun (lunar eclipse).

• The orbit of the Moon around the earth is inclined slightly to the plane of the ecliptic (the plane in which the Earth’s orbit lies).

Of course, most of the time, the Moon’s shadow misses the Earth or the Earth’s shadow misses the Moon!

• For an eclipse to occur, the Moon must be crossing the ecliptic at the same time it passes either in front of (solar eclipse) or behind (lunar eclipse) the Earth (B & D).

• Otherwise, no eclipses are possible (A & C)

A

B

C

D

The Solar System

Planets:

MercuryVenusEarthMarsJupiterSaturnUranusNeptunePluto (?)Dwarf Planet

Beyond the Solar System - The Milky Way

Our Neighborhood

• The Universe is “clumpy” – galaxies tend to pull together by gravity:–Our immediate neighborhood is called the Local

Group, a cluster of around 4 dozen galaxies (3 million light years across): Andromeda is the largest and the Milky Way is second largest.

– The Local Group is part of the Virgo Cluster, a large collection of smaller clusters and groups of galaxies.

– Superclusters: collection of larger clusters– The Universe – simply everything!

• The Milky Way is a gravitationally-bound collection of several hundred billion stars. The Sun is one of these stars and is located ~ 24,000 light years (or 8000 parsecs) from the center of our the Milky Way.

1 parsec = 3.26 light-years. It is defined as the length of the adjacent side of an imaginary right triangle in space. The two dimensions that specify this triangle are the parallax angle (defined as 1 arc second) and the opposite side (defined as 1 astronomical unit (AU). Given these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found.

Parsec unit

1 parsec = 3.26 light-years

Nearby Stars

Proxima-Centauri(4.23 ly)

Alpha-Centauri A, B(4.32 ly)

Barnard’s Star(5.9 ly)

Sirius A, B(8.60 ly)

Ross 128(10.9 ly)

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 http://www.youtube.com/watch?v=fE5WHZW3taM

3:00 minute mark

• 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. See link above.• 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)

Distance and Size of the Sun

• Aristarchus then went on to estimate the distance of the sun, using his calculated distance to the Moon.

• Estimated a distance of 20 times the Earth-Moon distance.

• Using the relative distances of the Moon and Sun, and his calculated value for the size of the Moon, he calculated the size of the Sun to be 7x bigger than the Earth

• The Sun is 100x bigger!

Parallax Preview

• 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 http://www.youtube.com/watch?v=G8cbIWMv0rI

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

• 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 is almost correct!

Measuring Angular Diameter

• In Astronomy, we will frequently estimate the sizes of planets, etc.

• To do this, we measure the angle that the object makes in the sky.

• We say that an object subtends an angle (A) in the sky.

• For example, the moon subtends 0.5 degrees.

• The Sun also subtends 0.5 degrees, which is why solar eclipses are so beautiful!

Measuring Linear Diameter

• If we measure the angle subtended by an object in the sky (A), and we know the distance to it (d), then we can calculate its actual, linear diameter (L).