Distances in Astronomy

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.

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

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.

Astronomical Units• For example:

oMars is about 1.5 times further away from the Sun as Earth

o Neptune is about 30 times further away from the Sun as Earth

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.

o The Earth is approximately 150,000,000 km from the Sun.

o Therefore, 1 AU = 150,000,000 km

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.

o A light-year is the distance light can travel in one year. A light-year is a measure of distance, not time!

o 1 ly = 9.46 x 1012 km or 6.32 x 104 AU

Light-Years• For example, the nearest star to the

Sun is Proxima Centauri.o Proxima Centauri is 39,900,000,000,000 km

from the Sun. This number way to large to practically use.

o This is equivalent to 271,000 AU. This number is still fairly large.

o 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.

Light-Years

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

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