Chapter 1

Introduction to Astronomy

Observations lead to theories and laws Laws are concise statements that

summaries a large number of observations. Theories are explanations for observed

phenomena. Theories are more powerful than laws

because of there predictive power.

The Scientific Method

Newton’s Law of Gravity: Mathematically describes the gravitational force between two objects.

Keplers 3rd Law: Mathematically describe the relationship between the orbital period of a planet and its distance from the sun.

The Big Bang Theory: Explains how the universe as come to look the way it does today.

Evolutionary Theory: Explains the why life varies in form and distribution.

Examples of Laws & Theories

Earth is average and has no special place in the universe

The earth revolves around the sun with 8 planets and many smaller bodies.

The Sun revolve around the center of the Milky Way Galaxy.

The Milky Way is one of billions of galaxies in the Universe.

The Universe is the totality of all space, time, matter, and energy.

Where are we?

The Milky Way galaxy is about 100,000 light years across.

There are about 3000 stars visible to the naked eye on any clear night. Twice that many if you stay up all night.

The Night Sky

Constellations are groups of stars that have meaning for the observer.

Stars in a constellation are not actually close together.

What are Constellations

There are 88 named constellations in the sky.

12 constellations form the Zodiac. Constellations that are always visible form a

specific location are called circumpolar. Circumpolar constellations circle a point.

Constellations

Stars appear to be located on a rotating sphere surrounding the earth.

The location of constellations are fixed on the sphere.

The Celestial Sphere

The projection of the earth’s equator onto the sphere is called the celestial equator.

A circular plane marked by the celestial equator is called the equatorial plane.

The projection of the north & south poles are called the north & south celestial poles.

The star that marks the point in the sky closest to the celestial pole is called the pole star. Currently the northern pole star is Polaris.

Celestial Sphere Details

The suns path along the celestial sphere is the ecliptic.

the vernal and autumnal equinoxes are the points where the ecliptic intersects the celestial equator.

The Ecliptic

The 12 constellations that lie along the ecliptic are called the zodiac.

The Zodiac

The location of stars on the celestial sphere is measured in angles.

360 degrees of arc in a circle 60 minutes of arc per degree 60 seconds of arc per minute of arc. One fist held at arms length is

approximately 10 degrees. 1 finger at arms length is about 1 degrees.

Angular Measure

Powers of 10

The scale of the universe

Motions of the EarthKeeping the seasons and calendar aligned

A solar day is the time from solar noon on one day to solar noon the next day.

The length of a solar day is the result of the earth’s rotation on its axis and its revolving around the sun. The mean solar day is 24 hours.

A day measured by the stars is a sidereal day. A sidereal day (23h 56m) is about 3.9 minutes

shorter than a mean solar day. The length of a sidereal day depends only on

the rotational motion of the earth.

Solar and Sidereal Days

The Sidereal Day

The meridian is a line on the celestial sphere that passes through the north and south celestial poles and though your zenith.

Solar noon occurs when the sun is on your local meridian.

Each one hour the sun moves 15o to the west.

Time zones are placed every15o to create 24, one-hour time zones.

Time Zones

World Time Zones

Year doesn’t quite have a whole number of solar days in it—leap years take care of this.

Add extra day every 4 years Omit years that are multiples of 100 but

not of 400 Omit years that are multiples of 1000 but

not of 4000 This will work for 20,000 years.

Astronomical Adjustments

1 year is the time it takes the earth to complete one revolution around the sun.

The earth is tilted 23.5o relative to the plane of the orbit.

It’s the tilt of the earth not how close the earth is to the sun that causes the seasons.

Seasons

Seasons

Precession is change in the direction of the earth’s rotational axis.

Precession is a 26000 year cycle. As the axis moves the pole star

Precession

Precession

Because of precession the constellations change relative to the seasons. In 13000 years Orion will be a summer constellation

Consequences of Precession

Lunar Cycles

Moon takes about 29.5 days to go through whole cycle of phases—synodic month

Phases are due to different amounts of sunlit portion being visible from Earth

Time to make full 360° around Earth, sidereal month, is about 2 days shorter

Lunar Phases

Waxing Crescent 1st Quarter Waxing Gibbous Full Waning Gibbous 3rd Quarter Waning Crescent New

Phases of the Moon

A lunar eclipse occurs when the moon passes into the shadow of the earth.

Lunar eclipses only occur during a full moon.

A solar eclipse occurs when the moon passes between the earth and sun.

Solar eclipses only occur during a new moon.

Eclipses

Partial when only part of Moon is in shadow

Total when it’s all in shadow

Lunar eclipse

Eclipses don’t occur every month because the Earth’s and Moon’s orbits are not in the same plane.

Partial when only part of Sun is blocked (penumbra shadow)

Total when it all is blocked (umbra shadow)

Annular when Moon is too far from Earth for total

Solar Eclipse

Earth Coordinate System

Terrestrial Coordinates (North & South)

The earth rotates on an axis that runs from the north pole to the south pole.

Midway between the poles, an imaginary line called the equator divides the earth into equal halves.

Latitude is an angle measured north or south from the equator.

Lines of latitude are also called parallels.

Latitude measure your angular distance from the equator.

Latitude

Important Latitudes The Tropic of Cancer is the circle of latitude on the earth

that marks the northernmost path of the sun (23.5oN). The Tropic of Capricorn is the circle of latitude on the earth

that marks the southernmost path of the sun — the apparent "equator" (23.5oS).

The Arctic Circle marks the point where north of the circle experiences a continuous 24 hours of daylight at least once each year (66.5oN).

The Antarctic Circle marks the point where south of the circle experiences a continuous 24 hours of daylight at least once each year (66.5oS).

Terrestrial Coordinates (East & West)

Longitude is the angle measured east or west around the equator.

The zero point for longitude is a point just south of Greenwich, England.

The line of longitude passing through Greenwich is called the prime meridian or the Greenwich line.

The Prime meridian passes through the Royal Observatory in Greenwich, England.

A brass strip set in the ground marks a portion of the prime meridian.

Celestial NavigationTwo Systems: Alt-Az and Equitorial

The Celestial Sphere The celestial sphere is an imaginary sphere

surrounding the earth to which the stars are attached.

The celestial sphere is divided into to two halves by the celestial equation which is the projection of the earth’s equator onto the sphere.

The top and bottom of the celestial sphere is marked by the celestial poles which are the projections of the earth’s north and south poles.

Also know as the local system Zenith is the point directly overhead. The celestial horizon is the great circle 90o from the

zenith. Altitude is the angular distance above the celestial

horizon. Azimuth is the angular distance measured east

from north. The point opposite the zenith is called the Nardir. The altitude of the north celestial pole is the same

as the observer's latitude. Example: 45o altitude and 270o azimuth would be

45o above the horizon and due west.

Alt-Az System

Azimuth is the angle measured from north to east along the horizon.

The horizon System (alt/az)

EARTH

The Alt/Az System gives different coordinates for each location on earth.

Declination measures the angle above and below the celestial equator.

Right ascension measures east from the vernal equinox.

Right ascension is usually measure in hours, minutes & seconds.

The Equatorial System

The horizon system (alt/az) is easier to use and understand.

The equatorial system gives the same coordinates regardless of your location.

Modern astronomers use software to transform the equatorial coordinate to local coordinates.

Comparing Coordinate Systems

End Chapter 1