U2 astronomy measuring space distances
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U2: Astronomy - Measuring Space Distances . FLIP LESSON. On the Move.

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On the move
On the Move

Nothing in space or our universe stays still. Planets, moons, stars, and galaxies are in constant motion. They are moving due to the expansion of the space in the universe. Space has no known boundaries. The location of Earth in space is relative to other moving objects.

What does this mean
What does this mean?

Each object in space will move through space in proportion to the expansion of space. Think of the Earth and other objects in space as raisins in rising bread dough. As the dough expands or rises, the raisins move away from each other. Such is how objects move through space.

Methods to measure
Methods to Measure

Astronomers use several methods to measure space distances. Lasers, radio and radar waves have helped us measure distances accurately. By using the known distances of closer stars, the distance to further stars can be found. A measurement called the parallax is used for this.


Astronomers use units of measure that express vast distance with small numbers. For shorter measurements in our solar system the astronomical unit (AU) is used. One AU is about 150,000,000 kilometers or 93,000,000 miles. One AU equals the average distance from the Earth to the Sun.

Astronomical unit au
Astronomical Unit (AU)

  • The average distance between the Sun and the Earth.

1 AU is equal to 93 million miles or 150 million kilometers

An AU is a standard of distance used within the solar system

The Planets of Our Solar System

The Inner Planets

Sizes to Scale

Mercury Venus Earth and MoonMars

The planets of our solar system
The Planets of Our Solar System

The Outer Planets



Earth Included for Scale.




Sizes and distances in the solar system


Diameters of Solar System Members:

Sun = 1,392,000 km

Mercury = 4,878 km

Venus = 12,104 km

Earth = 12,756 km

Mars = 6,794 km

Jupiter = 142,984 km

Saturn = 120,536 km

Uranus = 51,118 km

Neptune = 49,530 km

Pluto = 2,304 km

Mean Distance from Sun

57,900,000 km = 0.387 AU

108,200,000 km = 0.723 AU

149,600,000 km = 1.000 AU

227,900,000 km = 1.524 AU

778,300,000 km = 5.203 AU

1,427,000,000 km = 10.07 AU

2,871,000,000 km = 19.19 AU

4,497,000,000 km = 30.06 AU

5,914,000,000 km = 39.53 AU

Earth’s Moon = 3,476 km Mean Distance from Earth = 384,400 km

Fun facts about the planets
Fun Facts About the Planets

  • 18 Mercury’s would fit inside the Earth.

  • Venus is always cloudy.

  • Earth is the only planet we know with life.

  • Mars is red because it is covered with rust.

  • A storm (hurricane) on Jupiter can swallow 2 Earths.

  • Each season on Saturn is more than seven years long.

  • By the time you get to Uranus, you are only halfway to Pluto.

  • Neptune is the windiest planet, with winds blowing

    over 1,000 miles-per-hour.

  • Pluto is a ball of ice and rock and is a dwarf planet.

Light year
Light Year

The light year is the most well-known space distance unit used for measuring distances outside our solar system. It is equal to the distance light will travel in one year. Light travels 300,000 kilometers or 186,000 miles per second. A light year (LY) is over 9,000,000,000,000 kilometers or 5,865,696,000,000 miles long.

Light year ly
Light Year (LY)

  • The distance a ray of light travels in one year

  • The speed of the ray of light would be equal to:

  • 7.2 AU per hour or

  • 300,000 km/sec or

  • 186,000 miles/sec.

A Light Year is a standard unit of measure for interstellar (star) distances beyond the solar system.

Milky Way Galaxy is approx. 100,000 LY across in distance


If the Sun and Earth were 1 foot apart …

Proxima Centauri is 51 miles away!

Nearest star to the sun is foot apart … Proxima or Alpha Centauri C approximately 4.22 LYor 24 trillion miles(6 trillion miles = 1 light year)

Proxima (or Alpha Centauri C) is a red dwarf foot apart …

Alpha Centauri A is a yellow star

Alpha Centuari B is an orange star

Image 1
Image 1 foot apart …

Parsec foot apart …

For measuring distances between galaxies, the parsec is used. A parsec is equal to 3.26 light years.

One parsec equals 206,000 AU’s or about 30,000,000,000 kilometers.

Parsec foot apart …

  • One parsec is equal to 3.26 light years

  • A parsec is a unit of measure used for distances between very far structures in the universe.

  • Example:

    distances between galaxies

Andromeda and milky way
Andromeda and Milky Way foot apart …

  • The Andromeda Galaxy (M31) is slightly less than 800 kpc or

    800,000 parsecs away from Earth.

    Approximately 2.4

    million light years

Image 2
Image 2 foot apart …

Time foot apart …

It is important to consider time. Time is related to distance. The light you view from an object 100 light years away left the object 100 years ago. You are looking back into time 100 years. You see the object only as it once was and where it was years ago.

In the same way, our sun is 8.33 light minutes away. If the sun exploded it would take us 8.33 min. to see the explosion.