Distances in Space

To understand distances in space, you should know about common measurement units such as the astronomical unit (au) and the light-year (ly).

For more information, review the lesson measurement units in space.

To visualize distances in space, it's also helpful to have a few real-world examples to imagine how fast you would need to travel to get somewhere. I like to use these ones:

Notes:

Now let's look at some important distances in space, starting here on Earth and working outwards...

Earth & Moon

Earth has a circumference of 40,000 km. To travel all the way around the planet at the equator, this is how long it would take:

The Moon orbits Earth at an average distance of 384,400 km, which is roughly ten times the distance around the Earth's equator. To travel from Earth to the Moon it would take this long:

As noted previously, if the Earth was the size of a basketball the Moon would be the size of a tennis ball. What surprises most people is that the tennis ball would be orbiting the basketball at a distance of 7 metres away.

Earth and Moon to scale

The image above shows the Earth and Moon with both the correct size and distance scale to each other. This is one of the very few examples of celestial objects that can be shown like this—most are too far apart to fit in the same image without the objects shrinking to less than a single pixel. Indeed, it's a very big jump in scale from the Earth/Moon system to the Solar System...

The Solar System

There is some disagreement over the exact size of the Solar System but using the smallest estimate, the Solar System is at least 100,000 times bigger than the Earth/Moon system. The table below shows distances from the Sun to various objects within the Solar System.

Object Distance from Sun Travel Time from Sun
  km au Car Airplane Spacecraft Light speed
Mercury 58,000,000 0.387 66 years 6 years 10 days 3.2 minutes
Venus 108,450,000 0.723 124 years 12 years 18 days 6.0 minutes
Earth 150,000,000 1 171 years 17 years 25 days 8.3 minutes
Mars 228,450,000 1.523 261 years 26 years 38 days 12.6 minutes
Jupiter 780,450,000 5.203 891 years 89 years 4 months 43.2 minutes
Saturn 1,430,700,000 9.538 1,633 years 163 years 8 months 1.3 hours
Uranus 2,972,850,000 19.819 3,934 years 393 years 1 year, 4 months 2.7 hours
Neptune 4,508,700,000 30.058 5,147 years 515 years 2 years, 1 month 4.1 hours
Pluto 5,916,000,000 39.44 6,753 years 673 years 2 years, 8 months 5.5 hours
Voyager 1
(robotic space probe)
18,800,000,000 141 21,461 years 2,146 years 8 years, 6 months 17.4 hours

Notice the big jump from Mars to Jupiter and similar big jumps out to more distant objects. The four inner planets (Mercury, Venus, Earth and Mars) are relatively close to the Sun but the gaps between more distant objects are much larger.

This is a good example of how distances get much bigger as we head outwards from Earth. Here are the planets' orbits relative to each other:

Planet orbits

The Nearest Stars

Apart from the Sun, the nearest stars to us are the three stars in the Alpha Centauri system, about 4.2 light-years away.

The Milky Way Galaxy

Our galaxy, the Milky Way, is about 100,000 light-years wide—over 160 million times the size of the Solar System.

Nearby Galaxies

Andromeda is the nearest similar galaxy to the Milky Way. It's about 2 ½ million light-years away.

The Whole Universe

We don’t really know how big the whole Universe is because we can't see it all. We can see about 13 billion light-years in every direction but when we allow for factors such as the movement of galaxies and the time it takes light to reach us from them, the observable Universe appears to be around 93 billion light-years wide. That’s just the part of the Universe that we can see—there's presumably much more beyond that. It’s possible that the Universe is infinite in size but we don't know.

Summary

I encourage amateur space enthusiasts to practice using units of distance based on the speed of light, for several reasons. They're fairly intuitive and can cover all astronomical distances, and later on as you learn more about the Universe you'll find it helpful to be familiar with light-speed and distances.

Distances based on light speed

Within the Solar System: Light-minutes, light-hours.
Within the Galaxy: Light-years (up to 100,000 ly).
The wider Universe: Millions and billions of light-years.

Remember, everything is bigger than you thought and farther away than you can imagine.


>> Go back to Learning About Space.

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