Astronomical Distance Units
Overview
This lecture introduces astronomical distance units, focusing on the astronomical unit (AU) for solar system measurements and the light year (ly) for interstellar distances.
Everyday Distance Units
- Common distance units like miles, kilometers, or meters are impractical for astronomical distances due to the enormous values involved.
- The Sun is about 93 million miles from Earth, and other planets are much farther, making everyday units cumbersome.
Astronomical Unit (AU)
- The astronomical unit (AU) is a special distance unit used in astronomy, abbreviated as AU.
- 1 AU is defined as the average distance between Earth and the Sun.
- "Average" is used because Earth's distance from the Sun changes during its orbit.
- Mercury is about 0.4 AU from the Sun, while Neptune is about 30 AU away.
- Most solar system objects orbit within 100 AU of the Sun.
Light Year (ly)
- Astronomical units are too small for measuring the distance between stars, as even the closest star is over 60,000 AU away.
- The light year (ly) is used for interstellar and intergalactic distances.
- A light year is the distance light travels in one year.
- The speed of light is constant, which is why "average" is not needed in the definition.
- Light from the Sun takes 8 minutes to reach Earth, so a light year is much larger than an AU.
Key Terms & Definitions
- Astronomical Unit (AU) — The average distance between Earth and the Sun; used for distances within the solar system.
- Light Year (ly) — The distance light travels in a year; used for distances between stars and galaxies.
Action Items / Next Steps
- Review definitions and be prepared to use AU and ly when discussing solar system and interstellar distances.
- Prepare for Law Number Three in the next lecture.