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).
- 1 astronomical unit (au) is the average distance from the Sun to the Earth, approximately 150 million km (93 million miles).
- 1 light-year (ly) is the distance that a photon of light can travel through the vacuum of space in one Earth year, approximately 9.5 trillion km (5.9 trillion miles).
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:
- A car traveling on the open road (100 km/h)
- A commercial jet airplane (1000 km/h)
- The fastest robotic spacecraft (250,000 km/h)
- The speed of light (1,079,252,848,800 (1 trillion) km/h)
Notes:
- Robotic spacecraft can go much faster than spaceships carrying people, so it would take a human crew a lot longer to get anywhere.
- The speed of cars, planes and spacecraft are useful when we're talking about things within the Solar System but once you get out into the galaxy and Universe, the speed of light is the only practical one.
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:
- Car: 16.7 days
- Airplane: 1.7 days
- Robotic spacecraft: 9½ minutes
- Speed of light: 0.133 seconds
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:
- Car: 160 days (almost half a year)
- Airplane: 16 days
- Robotic spacecraft: 93 minutes (a spacecraft with human passengers would take about 3 days)
- Speed of light: 1.28 seconds
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.
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:
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.
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.
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