Science 3210 001 : Introduction to Astronomy

Lecture 2 : Visual Astronomy -- Stars and

Planets

Robert Fisher

Items

! Course adds

! The course had been booked to capacity, but I will be adding as many peopleas the room can accommodate, in the order I have received requests.

! People who attended last week’s lecture

! Natasha Shah

! Amanda Mayfield

! Simon Spartalian

! Add/drop day is February 6th. This means today is the last day where I amgenerally available to sign add/drop cards.

! Course webpage has been updated with first week’s lectures, and thefirst reading and homework assignment :

! http://flash.uchicago.edu/~rfisher/saic.html

! Questions -- seek and ye shall find!

Review of Lecture 1

! History of Astronomy

! Ancient Astronomy

! Advent of Natural Philosophy

! Medieval Astronomy in Arab World

! Birth of Modern Science

! Science Overview

! Scales in the Cosmos

! Cosmic Calendar

! Powers of Ten Video

Overview of Lecture 2

! I. The Celestial Sphere

! II. The Stars

! IiI. The Motion of the Planets

Important Lessons to be Learned Today

! Because the stars are very distant, their motion on the sky is well-

described as if they revolved around the Earth

! The motion of the planets is significantly more complex, and

required elaborate geometrical constructions in the ancient

geocentric system due to Ptolemy

! Niklaus Copernicus simplified matters tremendously by putting

the sun at the center of the universe -- even though he lacked the

“smoking gun” evidence to prove his case

Motion of the Stars

! The foundation of all visualastronomy is a simple fact :the Earth is a Sphere

! While common knowledgetoday, determination of theshape of the Earth was asignificant challenge toancient peoples

! The most convincingelementary argument comesfrom the fact that the Earth’sshadow (as seen in lunareclipses) is always circular,as Aristotle correctly deduced

Earth Image, Apollo 17 Crew

Lunar Eclipse

The Earth as the Center of the Universe

! Looking up at the night sky, it appears as if the entire Universe

revolves around the Earth.

Celestial Sphere, Zenith, Nadir, Horizon

! The distant stars appear to lie on a solid sphere, the celestial sphere.

! The zenith is the direction directly upwards.

! The nadir is the direction directly downwards.

! The horizon splits the celestial sphere in half along the zenith-nadir axis.

Zenith and Nadir Depend on Your Location

! The zenith and nadir directions depend on where one stands on the

Earth.

Rotation of the Earth

Motion of the Celestial Sphere

! The rotation of the Earth causes the celestial sphere to appear to

revolve.

! The north/south celestial poles correspond to the north/south poles of the

Earth’s rotational axis.

The Motion of the Sun

! At a given location, the sun rises towards the east and sets towards the

west.

! A sundial gnomon casts a shadow away from the sun, towards the west.

! The invention of the gnomon is attributed to the ancient Greek

philosopher Animaxander, successor to Thales

Determining North from the Sun’s Motion

! At noon, the sun reaches its highest point in the sky, directly north.

! This was a common method used by the ancients to determine North.

Clockwise

Clockwise

! Imagine one wanted to read a sundial. We are facing south.

! In the morning, the sun rises in the east, casting a shadow in the west.

! In the afternoon, the sun begins to set in the west, casting a shadow to the east,following the same circular arc traced in the morning.

! The direction traced by the sun’s shadow in its arc, facing south, is clockwise.

! When mechanical clocks with hands were first made, they were constructed so asto rotate in the same sense as the sundial -- clockwise -- not counter-clockwise.

Describing the Celestial Sphere --

The Great Circle

A great circle on a sphere divides the sphere into two hemispheres.

One can imagine the equator as an example of a great circle, but any

circle dividing the sphere is a great circle.

Great circle

Describing the Celestial Sphere -- Great Circles

Great circles

Any of the circles in the figure above are examples of great circles.

Angles

! Separation between two points on the celestial sphere are

measured in terms of angles -- much like a clock.

! A full circle is 360 degrees.

! Each degree is 60 minutes.

! The full moon is roughly one-half degree in width.

! By remarkable circumstance, the width of the sun is also one-half

degree.

! Each minute is 60 seconds -- sometimes referred to as

arcseconds.

The Meridian

! The great circle on the celestial sphere found by connecting north and south andpassing through the zenith is referred to as the meridian.

! When a celestial body crosses the meridian, it is said to transit.

! When a body transits, it reaches its highest point from the horizon.

! The terms “AM” and “PM” derive their meaning from the meridian :

! AM = Ante-Meridian

! PM = Post-Meridian

The North Celestial Pole and Circumpolar Stars

! Looking north from Chicago at night, one can see the North CelestialPole.

! The North Celestial Pole is the direction along which the Earth’s axis isaligned.

! The stars which immediately surround the pole never set beneath thehorizon. They are called circumpolar stars.

Star Trails Over Mauna Kea, Hawaii

Angles on a Familiar Sphere

! Before describing the celestial sphere in more detail, it helps to recall the

layout of a more familiar sphere -- the Earth.

! On the Earth, angle north or south of the Equator is marked off by

latitude.

! Angle around the Earth from West to East is marked off by longitude.

Daily Motion of the Stars

! The daily motion of the stars Is very simple.

! The celestial sphere makes one full circle about the Earth, once per day.

! The circle is determined by only angle -- the declination, directly

analogous to latitude on the Earth.

Question

! In the Northern hemisphere, the stars rise in the East, set in the

West, and revolve counter-clockwise around the North celestial

pole. In the southern hemisphere the stars rise in the

! A) East, set in the West, and revolve counter-clockwise around the

South celestial pole.

! B) East, set in the West, and revolve clockwise around the South

celestial pole.

! C) West, set in the East, and revolve clockwise around the South

celestial pole.

! D) West, set in the East, and revolve counter-clockwise around the

South celestial pole.

View from North Pole

! At the north pole, the zenith is

the north celestial pole.

! The nadir is the south celestial

pole.

! The horizon is the celestial

equator.

! Precisely half of the celestial

sphere is visible.

! All stars are circumpolar.

View from Equator

! The zenith is the celestial

equator.

! The north celestial pole always

appears directly north.

! The full sky is visible -- each

star rises for 12 hours each day.

View from Chicago

! The altitude of the north celestial

pole is equal to the latitude of

your position on the Earth -

roughly 42 degrees for Chicago.

! Stars within 42 degrees of the

north celestial pole are

circumpolar.

! Stars within 42 degrees of the

south celestial pole are not

visible.

Summary of Celestial Sphere Viewed fom Earth

Question

! The celestial equator is :

! A) The path of the sun compared with the stars.

! B) The path of the moon compared with the stars.

! C) The average path of planets on the sky.

! D) Always directly overhead at the Earth’s equator.

! E) Always along the horizon at the Earth’s equator.

Constellations

! Constellations are the “states” on maps of the celestial sphere.

! Each region of the sky belongs to precisely one constellation.

! Stars within each region are alphabetically named, starting with the

brightest stars, by a greek letter followed by the constellation name -- eg,

Polaris is Alpha Ursae Minoris.

The Ecliptic

! The sun appears to move along a plane in the sky referred to as the

ecliptic.

! The other planets also appear to move close to the ecliptic.

! Physically, the fact that all solar system bodies lie close to the ecliptic is

because the entire solar system lies within a flattened disk.

The Plane of The Ecliptic From Fire Island

The Solstices and Equinoxes

! The solstices occur when the sun reaches a maximum (solstice = solsistere or sun stops in Latin) distance away from the celestial equator --roughly June 21 and December 21.

! The equinoxes occur when the sun intersects the celestial equator --roughly March 21 and September 21. On this day, the sun appearsdirectly above the equator, and every point on earth has equal day andnight.

Earth on Equinoxes

Yearly Sky and Zodiac

! As the sun moves through the ecliptic, different portions of the night sky

become observable.

! The ecliptic falls into 12 constellations over the year -- the zodiac.

Angle of Inclination of Earth

! The ecliptic makes an angle of 23.5 degrees with the celestial equator.

! Physically, this means the Earth’s rotational axis is tilted with respect to

its orbit.

Angle of Inclination

! As the Earth orbits around the sun, the angle of inclination remains the

same.

Origin of Seasons

! The angle of inclination causes seasonal variation on Earth.

Question

! The ecliptic makes its smallest angle with the southern

hemisphere during the

! A) Summer

! B) Autumn

! C) Winter

! D) Spring

Private Universe

Lunar Phases

! The appearance of the moon varies over the course of the month.

Eclipses

! The lunar orbit is inclined by 5 degrees relative to that of the Earth/sun.

! Solar eclipses can occur during the new moon, but only when the sun,

moon, and Earth happen to line up.

! Similarly, lunar eclipses can occur during the full moon, but only when

the sun, Earth, and moon happen to line up.

Lunar Eclipses

! The moon passes through the shadow of the Earth.

! Light is fully blocked in the umbra, and only partially blocked in the

penumbra.

Types of Lunar Eclipses

! Three types of Lunar eclipses.

Lunar Eclipses

Question

! What would a total lunar eclipse look like to an observer standing

on the surface of the moon facing the Earth?

Lunar Eclipses from Moon

Solar Eclipses

! Solar eclipses occur when the

sun’s light is blocked by the

moon.

! In a sense, they are completely

serendipitous : the sun is 400

times larger than the moon, but

is also 400 times further away.

! Hence, the apparent angular

size of both the moon and the

sun are nearly identical.

Solar Eclipses

! Three types of solar eclispes can

occur.

August 11, 1999 Eclipse Viewed from Mir

The Eclipse of May 28, 585 BC

! Thales of Miletus is said to havepredicted a remarkable solar eclipseon May 28, 585 BC.

! Of this occasion, Herodutus writes,

! ‘On one occasion [the Medes and theLydians] had an unexpected battle inthe dark, an event which occurredafter five years of indecisive warfare:the two armies had already engagedand the fight was in progress, whenday was suddenly turned into night.This change from daylight todarkness had been foretold to theIonians by Thales of Miletus, whofixed the date for it within the limits ofthe year in which it did, in fact, takeplace…The Medes and Lydians,when they observed the change,ceased fighting, and were alikeanxious to have terms of peaceagreed on.’

! One must wonder -- how was itpossible for Thales to predict theeclipse?

Solar Eclipses, 1999 - 2020

! Both lunar and solar eclipses recur with a frequency of 18 years, 11 days,

known as the Saros cycle.

! The Saros cycle was known to the ancient Babylonians, and may have

been used by Thales to predict the eclipse of May 28, 585 BC.

Why are Eclipses so Rare?

! For a total lunar or solar eclipse to occur, there must be a precise

alignment of the Sun, Earth, and moon.

! However, because of the inclination of the moon’s orbit with

respect to the plane of the ecliptic, such alignments are rare.

The Planets

The Motion of Planets

! Like the stars, the planets are generally seen to traverse the sky.

! Unlike the stars, occasionally the planets are observed to stop

and move from west-to-east in so-called retrograde motion.

! This behavior gave rise to the ancient greek name -- “planets”

comes from a Greek root meaning “wanderer”.

! A fully satisfactory explanation of this motion was not developed

until Newton.

The Earth as the Center of the Universe

! Looking up at the night sky, it appears as if the entire Universe

revolves around the Earth.

Geocentric Model of the Universe

This observation led the ancients to formulate a geocentric model

of the universe, with the Earth at the center, and the Sun, planets,

and stars all revolving around the Earth along spheres.

Cacophony in the Celestial Harmony --

The Problem of Retrograde Motion

! The geocentric model of the universe works very well for stars,

but there is a major problem for planetary motion.

! Occasionally, the outer planets will appear to slow down, stop,

then reverse their direction on the night sky -- retrograde motion.

Retrograde Motion

! The mystery of retrograde

motion can be simply explained

in a model with the Sun at the

center of the Solar system.

! An inner body (like the Earth) is

moving more rapidly than an

outer body (like Mars), and so

will “pass” it much like a faster

car on the expressway.

! During this passing, the outer

planet will execute retrograde

motion.

Retrograde Motion in the Geocentric Model --

Epicyclic Motion

! Explaining retrograde motion in the geocentric model of the

universe, however, is almost impossible, unless one invents an

additional circular motion which each planet executes, called

epcicyclic motion.

Ptolemaic Model of the Solar System

! The ancient astronomer Ptolemy

(90 - 168 AD) created the most

complex version of the

geocentric model of the system,

which was used for almost one

and a half millenia.

! In the Ptolemaic model, the

moon, sun, and planets all

revolved in circles, which

themselves revolved around

circles around the Earth.

! And in fact, the Earth was not

quite at the center of this model,

either.

Why Did the Ancients Reject a Heliocentric

Model of the Solar System?

! In the heliocentric model, due to

the motion of the Earth about the

sun, the motion of the nearest

stars should appear to vary with

respect to the more distant stars.

! This effect is called parallax.

! The ancients attempted to

measure this effect, but failed. In

fact, because the stars are so

distant, it is only detectable with

telescopic measurements.

The Heliocentric World View

! Niklaus Copernicus was a 16th

century scholar and cleric, who

wrote treatises in a number of

fields.

! He is best remembered today for

his revolutionary astronomical

ideas.

Niklaus Copernicus (1473-1543)

The Copernican Model

! Copernicus summarized his model by the following bold (and remarkablyvalid) assumptions :

! 1.There is no one center of all the celestial circles or spheres.

! 2.The center of the earth is not the center of the universe, but only ofgravity and of the lunar sphere.

! 3.All the spheres revolve about the sun as their mid-point, and therefore thesun is the center of the universe.

! 4….the distance from the earth to the sun is imperceptible in comparison withthe height of the firmament.

! 5.Whatever motion appears in the firmament arises not from any motion ofthe firmament, but from the earth's motion.

! 6.What appear to us as motions of the sun arise not from its motion but fromthe motion of the earth and our sphere, with which we revolve about the sunlike any other planet.

! 7.The apparent retrograde and direct motion of the planets arises not fromtheir motion but from the earth's. The motion of the earth alone, therefore,suffices to explain so many apparent inequalities in the heavens.

Phases of Venus

! In 1610, Galileo used the

telescope to observe the phases

of Venus for the first time from

the Earth.

! The phases only made sense if

Venus orbited the Sun, not the

Earth.

! This proved to be a “smoking

gun” in favor of the heliocentric

model.

Next Week

! I) Planetary Motion

! A) Tycho Brahe and Johannes Kepler

! B) Kepler’s Laws

! II) Physics of Motion

! A) Galileo and the Physics of Kinematics

! B) Newton and Newton’s Laws of Motion

! II) Physics of Matter and Light