Distances: Crash Course Astronomy #25

CrashCourse
16 Jul 201511:20
EducationalLearning
32 Likes 10 Comments

TLDRThis script explores the vastness of the universe and humanity's quest to understand its scale, starting with the ancient Greeks' realization of the Earth's roundness and Eratosthenes' remarkable calculation of Earth's circumference over 2,000 years ago. It delves into the ingenious methods developed to measure distances in the cosmos, from the Moon to the stars, using parallax and the astronomical unit (AU) as fundamental tools. The evolution of these measurements, from the first accurate estimation of the distance to the Sun to the use of radar pulses and space-based satellites, underscores the blend of curiosity, mathematics, and technology in unraveling the mysteries of our universe.

Takeaways
  • 💡 Ancient Greeks knew the Earth was round and used observations like ships disappearing bottom-first over the horizon to support this.
  • 💥 Eratosthenes calculated the Earth's circumference over 2000 years ago with remarkable accuracy using the angles of shadows in different cities.
  • 📺 The distance to the Moon and the Sun were first estimated by using the Earth's size as a base for further geometric calculations.
  • 📱 Techniques developed by ancient Greeks laid the groundwork for measuring cosmic distances, despite initial inaccuracies.
  • 🔭 The invention of the telescope wasn't necessary for early astronomers like Hipparchus and Ptolemy to make fairly accurate calculations of celestial sizes and distances.
  • 🚀 The concept of the Astronomical Unit (AU) emerged as a crucial measure for understanding the solar system's scale.
  • 🛩 Parallax, the apparent shift of an object when viewed from different positions, is vital for measuring stellar distances using Earth's orbit as a baseline.
  • 📷 The first successful measurement of a star's distance using parallax wasn't achieved until 1838, demonstrating the vastness of space.
  • 🌠 Light years and parsecs are units developed to express the immense distances between celestial bodies more conveniently.
  • 🔍 Modern methods of determining cosmic distances involve a mix of direct measurement and inference from intrinsic brightness, expanding our understanding of the universe's scale.
Q & A

  • What ancient belief did people have about the stars and their placement in the sky?

    -Centuries ago, people believed that the stars were holes in a huge crystal sphere, letting through heavenly light.

  • How did Eratosthenes calculate the circumference of the Earth over 2000 years ago?

    -Eratosthenes calculated the Earth's circumference by using the difference in shadow angles at noon during the summer solstice in Syene and Alexandria. He knew the Sun shone directly down a well in Syene and not in Alexandria. By measuring the angle and knowing the distance between the two cities, he applied geometry to calculate the Earth's circumference as a little over 40,000 km.

  • What is the astronomical unit (AU) and why is it important?

    -The astronomical unit (AU) is a unit of measurement that represents the average distance from the Earth to the Sun, defined to be 149,597,870.7 kilometers. It's fundamental to understanding the solar system and is used as the basic measure in astronomy to calculate distances within our solar system.

  • How was the distance from the Earth to the Sun accurately determined in the 1960s?

    -In the 1960s, astronomers used radio telescopes to bounce radar pulses off of Venus. By knowing the speed of light extremely accurately, they measured the time it takes for the light to get to Venus and back with great precision, which helped to accurately determine the astronomical unit.

  • What is parallax, and how does it help measure the distance to stars?

    -Parallax is the apparent shift in position of an object when viewed from different points. Astronomers use Earth's orbit around the Sun as a large baseline for observing a star at two opposite points in the orbit. The amount of shift helps calculate the star's distance, using trigonometry, based on the known size of Earth's orbit.

  • Why was no parallax observed in stars initially, and what does this signify?

    -Initially, no parallax was observed in stars because they are extremely far away, much farther than the size of Earth's orbit could reveal. This indicated that stars are at vast distances from us, challenging the heliocentric model not because it was wrong, but because it underestimated the vast distances in space.

  • What was the significance of the measurement of 61 Cygni's parallax in 1838?

    -The measurement of 61 Cygni's parallax in 1838 was significant because it was the first star to have its distance successfully measured using parallax. This confirmed that stars are incredibly far away, with 61 Cygni being about 720,000 astronomical units from Earth.

  • What is a light year, and how does it relate to measuring distances in space?

    -A light year is the distance light travels in one year, about 10 trillion kilometers. It's used as a unit of measurement in astronomy to make the vast distances in space more comprehensible. It helps astronomers measure the distance to stars and galaxies beyond the practical reach of parallax.

  • How do astronomers use brightness to determine distances to stars beyond parallax measurement?

    -Astronomers compare the brightness of two stars of the same intrinsic brightness but at different distances. By knowing the distance to the nearer one through parallax, they can determine the distance of the farther one based on how much dimmer it appears, using the inverse square law of light.

  • Why was the discovery of the Earth's size by ancient Greeks crucial for modern astronomy?

    -The discovery of Earth's size was crucial because it laid the foundational knowledge for calculating distances in our solar system and beyond. This knowledge enabled the use of parallax to measure the distance to stars and eventually led to a deeper understanding of the universe's vast scale.

Outlines
00:00
😀 How the Greeks Measured the Distance to the Moon and Stars

This paragraph discusses how the ancient Greeks determined that the Earth was round and used observations during lunar eclipses to calculate the distance to the Moon. It also covers Aristarchus of Samos's methods for finding the distances and sizes of the Moon and Sun.

05:07
🌟 Finding the Distance from the Earth to the Sun Unlocks the Solar System

This paragraph explains the significance of accurately measuring the Earth-Sun distance, known as the astronomical unit (AU), in allowing us to explore the solar system. It recounts various unsuccessful historical attempts before radar measurements in the 1960s finally precisely determined the length of the AU.

10:09
🔭 Measuring Stellar Parallax Leads to Understanding Stars and the Scale of the Universe

This paragraph discusses how knowing the AU enabled astronomers to measure the distances to stars using parallax from the Earth's orbit. It introduces light years as a unit of cosmic distance and describes how stellar brightness and spectroscopy provide information about the stars once their distances are known.

Mindmap
Keywords
💡Parallax
Parallax refers to the apparent shift in position of an object when viewed from different vantage points. It is a key concept in the video, as parallax allows astronomers to measure the distances to stars. Eratosthenes used parallax from different locations on Earth to calculate its circumference. Astronomers later realized Earth's orbit provides a huge baseline for parallax to stars.
💡Astronomical Unit
The astronomical unit (AU) is the average distance between the Earth and Sun. Precisely measuring the AU was critical for astronomers to calculate other distances in the solar system and understand its scale. After many failed attempts over centuries, radar measurements finally determined it as 149,597,870.7 kilometers.
💡Light Year
A light year is the distance light travels in one year - about 10 trillion kilometers. It became an important unit of cosmic distance once nearby star distances were measured using parallax. Light years simplified talking about the vast distances to stars.
💡Spectroscopy
Spectroscopy analyzes the spectrum of light from stars to determine their composition, temperature and other properties. It allowed astronomers to compare intrinsic brightness of stars at different distances to further calibrate cosmic distances.
💡Geometry
Simple geometric principles, combined with accurate measurements, allowed the ancient Greeks to calculate the size of the Earth and distances to the Moon and Sun. Later, Kepler's laws of planetary motion enabled calculating distances across the solar system.
💡Arcsecond
An arcsecond is 1/3600th of a degree. The parallax angle of a star at one parsec is one arcsecond. So the parsec became another important unit for astronomers to express cosmic distances.
💡Luminosity
A star's luminosity or intrinsic brightness indicates how much energy it emits. Once astronomers knew distances to stars from parallax, they could calculate luminosity and other physical properties like size and temperature.
💡Cosmic scale
Understanding the true scale of cosmic distances was a persistent quest over centuries. Measuring first the Earth's size, then the AU, then stellar parallax provided critical rungs on the ladder out to grasping the spatial scale of the entire observable universe.
💡Curiosity
The script argues that curiosity-driven science over millennia, seeking fundamental measures like the size of the Earth and stars' distances, is what ultimately allowed humans to comprehend the scale of the cosmos and launch space exploration.
💡Parallax shift
When nearby objects are viewed along different sight lines, they appear to shift position relative to more distant objects. This effect is called parallax shift. Its magnitude depends on the observer's movement and distance to the object.
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Transcripts

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