Distances: Crash Course Astronomy #25

Summary

This script explores the historical quest to measure cosmic distances, starting with the ancient Greeks' determination of Earth's size and the distances to the Moon and Sun. It delves into the astronomical unit's significance, the use of parallax for nearby stars, and brightness comparisons for more distant ones. The script highlights humanity's journey from basic geometry to advanced astrophysics, revealing the scale of the Universe and our place within it.

Takeaways

• 🌌 The ancient Greek perception of stars as holes in a crystal sphere reflects the challenge of understanding the vastness of the universe.
• 📏 The concept of parallax is used to measure distances in space, similar to how our eyes use depth perception to gauge the distance of objects.
• 🌍 Eratosthenes calculated the Earth's circumference using geometry and the angle of the sun's rays at different locations, demonstrating early scientific ingenuity.
• 🌕 Lunar eclipses and the phases of the Moon were used by ancient astronomers to determine the distances and sizes of celestial bodies, including the Earth and the Moon.
• 🔍 Aristarchus of Samos used the Earth's size as a reference to estimate the distances to the Moon and the Sun, showing the progression of astronomical methods.
• 📚 Johannes Kepler and Isaac Newton's work on planetary orbits laid the groundwork for understanding the distances to other planets, highlighting the importance of mathematical principles in astronomy.
• 🌞 The astronomical unit (AU) is a fundamental measure in astronomy, representing the average distance from the Earth to the Sun, and is crucial for understanding the scale of the solar system.
• 📡 In the 1960s, radar pulses bounced off Venus were used to accurately determine the length of an AU, showcasing the advancement of technology in space measurement.
• 👀 The parallax method, using the Earth's orbit as a baseline, was essential for measuring the distances to stars, but initial attempts showed no parallax due to the immense distances involved.
• 🌟 The first successful measurement of a star's parallax was 61 Cygni in 1838, revealing the immense scale of the universe and the limitations of direct observation.
• 🌠 The light year and parsec are units of distance used in astronomy to express the vast distances to stars and galaxies, making these measurements more comprehensible.

Q & A

What is the concept of parallax and how does it relate to measuring distances?

Parallax is the apparent shift in the position of an object when viewed along different lines of sight. It's used in astronomy to measure distances to stars by observing the change in their position relative to more distant objects when the Earth is at opposite points in its orbit around the Sun.

Why did ancient people think the stars were holes in a crystal sphere?

They thought this way because the stars appeared to be fixed points of light in the sky, and the idea of a crystal sphere provided a simple explanation for the distribution of these 'holes' through which divine light shone.

How did Eratosthenes measure the Earth's circumference over 2000 years ago?

Eratosthenes used the angle of the sun's rays at noon in two different cities during the summer solstice and the known distance between them to calculate the Earth's circumference using geometric principles.

What is the astronomical unit and why is it important?

The astronomical unit (AU) is the average distance from the Earth to the Sun, approximately 149,597,870.7 kilometers. It's fundamental in astronomy as it provides a standard measure for distances within our solar system and beyond.

How did ancient astronomers determine the distances to the Moon and the Sun?

They used the Earth's size as a reference and observed phenomena like lunar eclipses and the Moon's phases to apply geometric calculations and determine the relative distances and sizes of the Moon and the Sun.

What is the significance of the transit of Venus in measuring the AU?

The transit of Venus, when the planet crosses the face of the Sun, provides an opportunity to measure the time it takes for the transit to occur from different points on Earth. This timing can be used to calculate the length of an AU using orbital equations.

How did astronomers determine the distance to the first star using parallax?

The first star to have its parallax successfully measured was 61 Cygni in 1838. Astronomers observed its apparent shift in position relative to more distant stars when viewed from opposite points in Earth's orbit and applied trigonometry to calculate its distance.

What is a light year and how is it used in astronomy?

A light year is the distance that light travels in one year, about 9.461×10^12 kilometers. It's used as a unit of distance in astronomy, especially for measuring the vast distances between stars and galaxies.

What is a parsec and how does it relate to parallax?

A parsec is a unit of distance used in astronomy, equal to about 3.26 light years. It's defined as the distance at which a star would show a parallax shift of one arcsecond over the course of a year.

How do astronomers use the brightness of stars to determine their distances?

Astronomers use the inverse square law, which states that the brightness of an object falls off with the square of the distance from the observer. By comparing the apparent brightness of a known distance star with another star, astronomers can estimate the more distant star's brightness and distance.

What role does curiosity play in the advancement of astronomical knowledge?

Curiosity has driven astronomers and scientists to ask questions about the universe, leading to discoveries and methods that have expanded our understanding of the Earth, the solar system, and the cosmos, starting from the simple question of the Earth's size by ancient Greeks.