The document discusses several key facts about the Earth: 1) Estimates of the Earth's age come from radioactive dating of rocks and minerals, with the oldest being approximately 4.2 billion years old. The Earth formed around 4.5 billion years ago from collisions of planetesimals. 2) Multiple lines of evidence show that the Earth is approximately spherical in shape, including observations of ships disappearing over the horizon and photographs from space. While spherical, the Earth is actually an oblate spheroid due to spinning, being flattened at the poles. 3) Measurements of the positions of stars like Polaris in the northern night sky provide evidence that the Earth must be approximately spherical, as these observations could only

1. Astronomical Studies about the Earth
1. AGE OF THE EARTH
 Estimated age for the Earth and the rest of the solar system is about 4.55 billion years comes from Lead isotope
measurements.
 The oldest Earth rocks: 3.8 to 3.9 billion years
 Oldest Earth minerals (zircons): 4.2 billion years
 Oldest Moon rocks: 4.44 billion years
Early Earth Timeline
• About 5.5-6 billion years ago (BYA), the solar nebula begins to collapse
• About 4.6 BYA, Sun begins fusion
• About 4.5-4.56 BYA, Proto-Earth formed from planetesimals.
• 4.44+ BYA, Earth-Moon formed by giant impact. Earth melts, Magma Ocean.
• 4.2 BYA, Earth was completely differentiated.
• 4 BYA, earliest oceans formed, thick atmosphere exists
• 3.8 BYA, life develops
• 2.5-3 BYA, photosynthesis leads to O2 in ocean
• 2 BYA, O2 hits atmosphere
2. THE SIZE AND SHAPE OF THE EARTH
A. Evidence that Earth is Spherical
1. The mast of a ship is the 1st part to appear over the horizon & the last to disappear.
2. Photographs of Earth from space
3. An eclipse of the moon occurs when Earth's shadow falls on the moon
The shadow is always an arc of a circle.
4. Ships sailing north or south observed changes in the nighttime sky.
 North Star appears higher in the sky as a ship travels north
 Ships sailing south lose sight of the Big Dipper & begin to see the Southern Cross
B. Oblate Spheroid
 Sphere that is slightly flattened at the poles & bulged at the equator
 Caused by Earth's rotation
II. Earth’s Density
A. Density – The amount of matter (mass) in a given space (volume)
 Average density = 5.5 g/cm3
III. Measuring Earth's Circumference
 Eratosthenes (er uh Tos thuh neez) Believed the sun's rays hit the Earth as parallel lines
 Eratosthenes figured Scene was due south of Alexandria (He was slightly off on this one)
 Distance between Syene & Alexandria (925km) slightly off
 Thought both cities were on the same line of longitude (off on this by 30)
 Believed the sun's rays hit the Earth as parallel lines
 Eratosthenes figured Syene was due south of Alexandria (He was slightly off on this one)
 Distance between Syene & Alexandria (925km) slightly off
 Thought both cities were on the same line of longitude (off on this by 30)
IV. Types of Data
 Qualitative – data that is relative but not numeric.
– Ex. The color of an object or stating someone is tall or taller than another person
 Quantitative – data where a numeric measurement or a count of objects has been made
3. PROOFS ABOUT THE SHAPE OF THE EARTH
What is Earth’s shape?

2.  Is it flat like a coin or spherical like a ball? Perhaps it’s like a tin can, flat on the ends and round in the middle?
 Describe the evidence that best supports you answer.
Spherical Earth?
 Scientists claim that Earth is much like a slightly flattened ball - an oblate spheroid.
 How do they know or do the merely have faith in the claim?
 What is the evidence that supports a scientific belief in a roughly spherical Earth?
Ships appear to sink gradually below horizon from bottom up.
Polaris, the North Star
 Is located practically over the North Pole of planet Earth.
 If you stood at the North Pole, Polaris would be almost directly overhead.
Why do observations of Polaris help determine Earth’s shape?
 The North Star gradually appears lower and lower in the sky as you travel toward the equator.
 When the North Star is visible at the horizon, you are located at the equator.
 South of the equator the North Star is not visible at all.
 These observations are possible only for a roughly spherical Earth.
Many other indications…
 Circumnavigation of the globe.
 Existence of time zones.
 Earth-orbiting satellites.
 Photographs from space.
 Eyewitness reports of astronauts.
 Other objects - sun, moon, planets - are roughly spherical.
Is Earth perfectly spherical?
 The Earth is NOT a perfect sphere
 It is flattened at the poles and bulges at the equator due to its spinning.
 Earth is very slightly out of round or OBLATE.
 Saturn is an even better example of being oblate.
What proof is there that the Earth is slightly oblate?
 Gravity measurements. Gravity is the force of attraction between any two objects.
Increase mass of objects = increase gravity Decrease distances = increase gravity
 If Earth were a perfect sphere, it would be expected to exert an equal force on objects at equal distances from
the center of earth.
The Oblate Sphere
 The difference with a sphere is small
 We cannot see it with the naked eye
Earth still appears like a sphere
4. MOTIONS OF THE EARTH
Earth’s Rotation
• Rotation is the spinning of the Earth on its axis.
• The time for one rotation is 24 hours.
• The speed of rotation 24,855 miles/24 hr or 1,038 miles/hr
Earth’s Revolution
• A revolution occurs as the earth moves around the sun.
• Time for one revolution = 365 1/4 days = 8,766 hours
• The speed of earth’s revolution is about 18 miles per second.
Earth’s Orbit
• The path an object takes as it revolves around another object.
Around and Around We Go
• Every second that you sit in this classroom the Earth's orbital motion carries you about 18 miles through
space.