1. Distances in Astronomy

2. Which unit would you use to measure your height?
Millimeters
Centimeters
Meters
Kilometers
Talk with a partner about why your choice is the most appropriate unit to use in this scenario.
After the students discuss their choice with a partner, facilitate a short class discussion. Most students should choose meters – meters are the closest unit to the range of the object we are measuring (a person’s height).
Discuss: It would not make sense to measure a person’s height in millimeters, for example, because the number would be unnecessarily large. Using meters provides a measurement that is appropriately scaled and concise.

3. Kilometers
What do you notice about the distances from the solar system activity?
Scientists rarely use kilometers for distances in astronomy. Why do you think that is?
A kilometer would be appropriate to measure the radius of the Earth, the size of an asteroid, the circumference of Mercury, etc.
Measurement of a single body
“What do you notice about the distances from the solar system activity?”
Sample responses from students: All measured in kilometers, gets bigger for each planet, all the numbers are really large
“Scientists rarely use kilometers for distances in astronomy. Why?”
Discuss that the numbers are all very large. Scientists do not like working with very large numbers, as it is more likely to make a mistake (by leaving off a zero, adding an extra zero, etc). To help with large numbers, scientists either use scientific notation or use a larger unit. A kilometer is very small compared to the distance between the Sun and Jupiter.

4. Astronomical Units
Scientists created a new unit called “astronomical units” to describe distances within the solar system. This new unit makes it easier to visualize the solar system.
Earth is the our reference point for all other distances in the solar system. We can compare all other objects to how far away Earth is from the Sun.

5. Astronomical Units For example:
Mars is about 1.5 times further away from the Sun as Earth
Neptune is about 30 times further away from the Sun as Earth
Discuss: Using Earth as a reference point, it becomes easier to visualize how far Neptune is from the Sun.

6. Astronomical Units
This “times further away than Earth” is called an astronomical unit. It is based off the average distance between the Earth and the Sun.
The Earth is approximately 150,000,000 km from the Sun.
Therefore, 1 AU = 150,000,000 km

7. Light-Years
Astronomical units are used for distances within the solar system.
When we start dealing with extremely large distances, like the distance between stars, scientists use an even larger unit called a light-year.
A light-year is the distance light can travel in one year. A light-year is a measure of distance, not time!
1 ly = 9.46 x 1012 km or 6.32 x 104 AU

8. Light-Years
For example, the nearest star to the Sun is Proxima Centauri.
Proxima Centauri is 39,900,000,000,000 km from the Sun. This number way to large to practically use.
This is equivalent to 271,000 AU. This number is still fairly large.
If we use light-years, the distance becomes 4.24 ly. All of these distances are equivalent, but 4.24 ly is a much easier number to use and understand.

9. Light-Years
Scientists use light-years to measure the distances between stars and other bodies in space.

10. Parsecs
There is one more common unit for astronomical distances. It is called the parsec.
You may have heard of it—it is commonly used in science fiction!
Parsecs are useful for measuring the distance between extremely distant bodies in space, such as two galaxies.
1 pc = 3.26 ly
Discuss- Notice that parsecs are not *that* much larger than light-years (compared to the other units we have looked at). Some scientists use parsecs, and some prefer to keep everything in light-years.

11. Astronomic Unit
Kilometers: for distances between planets and orbiting objects Astronomical units: for distances between planets Light years: for distances between stars Parsecs: for distances between galaxies