Bellringer

Name as many Modern constellations in the night sky as

you are able.

Bellringer – List of Modern Constellations

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Chapter 1

Charting the Heavens

Our Place in Space

• Earth is average—we don’t occupy any special place in the universe

• Universe: totality of all space, time, matter, and energy

Our Place in Space

• Astronomy: study of the universe

• Scales are very large: measure in light-years, the distance light travels in a year—about 10 trillion miles

Scientific Theory and the Scientific Method

Developed by Aristole

Scientific theories:

• Must be testable

• Must be continually tested

• Should be simple

• Should be elegant

Scientific theories can be proven wrong, but they can never be proven right with 100% certainty.

Scientific Theory and the Scientific Method

• Observation leads to theory explaining it

• Theory leads to predictions consistent with previous observations

• Predictions of new phenomena are observed.

•If observations agree with prediction, more predictions can be made.

•If not, a new theory can be made.

The “Obvious” View

Simplest observation:

~3000 stars visible at any one time

•distributed randomly

•human brain tends to find patterns

The “Obvious” View

Group stars – Constellations• Figures having meaning to those doing the grouping•Useful: Polaris, which is almost due north•Not so useful: Astrology•Makes predictions about individuals based on the star patterns at their birth

Orion’s Belt

The “Obvious” View

Stars that appear close in the sky may not actually be close in space

The “Obvious” View

Celestial sphere:

•Spherical coordinates (similar to latitude and longitude) to locate sky objects

•Zenith – Point directly above your head

Angular Measurements

• Full circle contains 360° (degrees)

• Each degree contains 60′ (arc-minutes)

• Each arc-minute contains 60′′ (arc-seconds)

•1/3600th of a degree

•Angular size of an object depends on actual size and distance away

Angular Measurements Examples

• Fingernail on Index Finger = 1 arc second

• Thumbnail = 2 arc seconds

• 1 arc second = viewing a dime @ 2 kilometers

• 10 Degrees = your clenched fist

• Big Dipper = 20 degrees

• Pluto = 0.1 arc second

• Equivalent ping pong ball (40mm) @ 80 Km

Celestial Coordinates

• Declination: degrees north or south of celestial equator

• Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox

•Vernal Equinox – position of sun at equator on or near 3/21 Annually

1.4 Earth’s Orbital Motion

• Solar Day - noon to noon

•Sidereal Day – Earth’s true rotation period

•Time required to return to same orientation relative to distant stars

•24 hr period – earth rotates 361o

•Earth also moves in orbit ~ 1o

•Solar Day is 4 Minutes longer than Sidereal Day

Earth’s Orbital Motion

Seasonal changes to night sky are due to Earth’s motion around Sun

Earth’s Orbital Motion12 constellations Sun moves through during the year are called the zodiac; path is ecliptic

Earth’s Orbital Motion• Ecliptic is plane of Earth’s path around Sun; at 23.5° to celestial equator

• Northernmost point (above celestial equator) is summer solstice; southernmost is winter solstice; points where path cross celestial equator are vernal and autumnal equinoxes

• Combination of day length and sunlight angle gives seasons

• Time from one vernal equinox to next is tropical year

Earth’s Orbital Motion

Precession: rotation of Earth’s axis itself; makes one complete circle in about 26,000 years

Earth’s Orbital Motion

Time for Earth to orbit once around Sun, relative to fixed stars, is sidereal year

Tropical year follows seasons; sidereal year follows constellations—

In 13,000 years July and August will still be summer, but Orion will be a summer constellation

Astronomical Timekeeping

Solar noon: when Sun is at its highest point for the day

Drawbacks: length of solar day varies during year; noon is different at different locations

Astronomical Timekeeping

Mean (average) solar day —what clocks measure

Takes care of length variation

24 time zones around Earth

Takes care of noon variation.

1.5 Astronomical TimekeepingWorld time zones

Astronomical Timekeeping

Lunar month- complete lunar cycle

Doesn’t have whole number of solar days

Tropical year doesn’t have whole number of months

Current calendar have months close to lunar cycle & adjusted so there are 12 of them in a year

Astronomical Timekeeping

Synodic Month – time from one New Moon to the next

Sidereal Month – time taken for the moon to rotate exactly 360o

Sidereal month is 2 days shorter than Synodic Month

Astronomical Timekeeping

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.

Motion of the Moon

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

Phases - 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

Motion of the Moon

Eclipses occur when Earth, Moon, and Sun form a straight line

Motion of the Moon

Lunar eclipse:

• Earth is between Moon and Sun

• Partial when only part of Moon is in shadow

• Total when it all is in shadow

1.6 Motion of the Moon

Solar eclipse: Moon is between Earth and Sun

• Partial when only part of Sun is blocked

•Total when it all is blocked

• Annular when Moon is too far from Earth for total

Motion of the Moon

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

Measurement of Distance

Triangulation: Measure baseline and angles, can calculate distance

Use of simple geometry & trigonometry to estimate distances

Measurement of Distance

Parallax: Apparent motion of an object against distant background

Similar to triangulation

Try with your outstretched arm & thumb and an object in the room. Simply alternate closing eyes

Apparent displacement is Parallax

Measurement of Distance

•Measuring that angle and the distance between the cities gives the radius

•Simple Geometry

Earth’s radius:•Eratosthenes completed 2300 years ago•Noticed that when Sun was directly overhead in one city,it was at an angle inanother

More Precisely

Measuring Distances with Geometry

Converting baselines and parallaxes into distances

More Precisely 1-3:

Measuring Distances with Geometry

Converting angular diameter and distance into size