Astronomy 101Astronomy 101Section 4Section 4

Lecture 2Lecture 2

Ancient to Modern: Knowing the HeavensAncient to Modern: Knowing the Heavens

John T. McGraw, ProfessorJohn T. McGraw, ProfessorLaurel Ladwig, Planetarium Laurel Ladwig, Planetarium

ManagerManager

All ancient cultures looked to the sky for a All ancient cultures looked to the sky for a variety of reasons - the ancients were variety of reasons - the ancients were

excellent astronomers!excellent astronomers!

The Anasazi culture The Anasazi culture practiced astronomy to practiced astronomy to stabilize their society. stabilize their society.

They measured the same They measured the same things in the sky that we things in the sky that we

do today.do today.

Pueblo Bonito - Pueblo Bonito - Chaco CanyonChaco Canyon

Sun pillarsSun pillars Shadow patternsShadow patterns WindowsWindows All to measure angles, all to measure with precision.All to measure angles, all to measure with precision.

Pointing to any place on the dome of the sky Pointing to any place on the dome of the sky requires specifying two angles.requires specifying two angles.

This means two motions are necessary.This means two motions are necessary.

Telescope mounts Telescope mounts allow selection of allow selection of two angles.two angles.

How Things in the Sky You’ve Never How Things in the Sky You’ve Never Really Noticed Actually Work.Really Noticed Actually Work.

Day, Night and the Motion of the Day, Night and the Motion of the Stars - The Diurnal (Daily) MotionStars - The Diurnal (Daily) Motion

The appearance of the sky changes The appearance of the sky changes during the course of the night and during the course of the night and

from one night to the nextfrom one night to the next

Stars appear to rise Stars appear to rise in the east, slowly in the east, slowly rotate about the rotate about the earth and set in the earth and set in the west.west.

This diurnal or daily This diurnal or daily motion of the stars motion of the stars is actually caused is actually caused by the 24-hour by the 24-hour rotation of the earth.rotation of the earth.

Why the Why the Constellations Constellations Change During the Change During the YearYear

Annual MotionAnnual Motion The stars also appear to The stars also appear to

slowly shift in position slowly shift in position throughout the yearthroughout the year

This is due to the orbit of the This is due to the orbit of the earth around the sunearth around the sun

If you follow a particular star If you follow a particular star on successive evenings, you on successive evenings, you will find that it rises will find that it rises approximately 4 minutes approximately 4 minutes earlier each night, or 2 hours earlier each night, or 2 hours earlier each month earlier each month

Eighty-eight constellations Eighty-eight constellations cover the entire sky cover the entire sky

Ancient peoples looked Ancient peoples looked at the stars and at the stars and imagined groupings imagined groupings made pictures in the made pictures in the sky sky

We still refer to many of We still refer to many of these groupingsthese groupings

Astronomers call them Astronomers call them constellations constellations (from (from the Latin for “group of the Latin for “group of stars”)stars”)

Modern ConstellationsModern Constellations On modern star charts, On modern star charts,

the entire sky is divided the entire sky is divided into 88 regionsinto 88 regions

Each is a constellationEach is a constellation Most stars in a Most stars in a

constellation are constellation are nowhere near one nowhere near one anotheranother

They only They only appear appear to be to be close together because close together because they are in nearly the they are in nearly the same direction as seen same direction as seen from Earth from Earth

The Reasons for SeasonsThe Reasons for Seasons

The seasons are caused by the The seasons are caused by the tilt of Earth’s axis of rotationtilt of Earth’s axis of rotation

The Earth’s axis of rotation is not The Earth’s axis of rotation is not perpendicular to the plane of the Earth’s perpendicular to the plane of the Earth’s orbitorbit

It is tilted about 23½° away from the It is tilted about 23½° away from the perpendicularperpendicular

The Earth maintains this tilt as it orbits The Earth maintains this tilt as it orbits the Sun, with the Earth’s north pole the Sun, with the Earth’s north pole pointing toward the north celestial pole pointing toward the north celestial pole

Solar Energy: Designing Your HouseSolar Energy: Designing Your House

The Ecliptic, Equinoxes and SolsticesThe Ecliptic, Equinoxes and Solstices

That Winter/Summer Thing AgainThat Winter/Summer Thing Again Some snappy circles!Some snappy circles!

Time Zones Aid the Human (Rat?) Time Zones Aid the Human (Rat?) Race!Race!

It is convenient to imagine that the stars are It is convenient to imagine that the stars are located on a celestial spherelocated on a celestial sphere

The celestial sphere is an The celestial sphere is an imaginary imaginary object that has object that has no basis in physical realityno basis in physical reality

However it is still a model However it is still a model that remains a useful tool that remains a useful tool of positional astronomyof positional astronomy

Landmarks on the celestial Landmarks on the celestial sphere are projections of sphere are projections of those on the Earththose on the Earth

Celestial equatorCelestial equator divides the sky into divides the sky into northern and southern northern and southern hemisphereshemispheres

Celestial polesCelestial poles are are where the Earth’s axis where the Earth’s axis of rotation would of rotation would intersect the celestial intersect the celestial sphere sphere

Polaris is less than 1° Polaris is less than 1° away from the north away from the north celestial pole, which is celestial pole, which is why it is called the why it is called the North StarNorth Star or the Pole or the Pole Star.Star.

Point in the sky directly Point in the sky directly overhead an observer overhead an observer anywhere on Earth is anywhere on Earth is called that observer’s called that observer’s zenith.zenith.

Astronomers use angles to denote the Astronomers use angles to denote the positions and apparent sizes of objects in positions and apparent sizes of objects in

the skythe sky

The basic unit of angular measure is the The basic unit of angular measure is the degree (degree (°).°). Astronomers use angular measure to describe the apparent size of a Astronomers use angular measure to describe the apparent size of a

celestial object—what fraction of the sky that object seems to covercelestial object—what fraction of the sky that object seems to cover The The angular diameter angular diameter (or (or angular sizeangular size) of the Moon is ½° or the ) of the Moon is ½° or the

Moon Moon subtends subtends an angle of ½°. an angle of ½°.

If you draw lines from your eye to each of two If you draw lines from your eye to each of two stars, the angle between these lines is the stars, the angle between these lines is the angular distance angular distance between these two stars between these two stars

The adult human hand held at arm’s length The adult human hand held at arm’s length provides a means of estimating angles provides a means of estimating angles

AngularAngular MeasurementsMeasurements Subdivide one degree into 60 Subdivide one degree into 60 arcminutesarcminutes

– minutes of arcminutes of arc– abbreviated as 60 arcmin or 60´abbreviated as 60 arcmin or 60´

Subdivide one arcminute into 60 one arcminute into 60 arcsecondsarcseconds– seconds of arcseconds of arc– abbreviated as 60 arcsec or 60”abbreviated as 60 arcsec or 60”

1° = 60 arcmin = 60´1° = 60 arcmin = 60´1´ = 60 arcsec = 60”1´ = 60 arcsec = 60”