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Bellringer
Name as many Modern constellations in the night sky as
you are able.
Bellringer – List of Modern Constellations
TURN IN SAFETY CONTRACTS IF YOU HAVE THEM, PLEASE & THANK YOU
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