Key points
- The distances between objects in space are enormous. They are often so large that standard units of measurement, like metres or kilometres, are replaced by alternatives that can cover the huge distances in space.
- Scientists and A scientist that studies the universe and everything outside of the Earth's atmosphere (astronomy). often use units of The distance travelled by light in one year equal to approximately 9.46 trillion kilometres. to talk about the distances to distant objects, such as An enormous ball of gases which produces large amounts of heat and light, due to nuclear fusion reactions in its core. and A collection of millions or billions of stars, together with gas and dust, held together by gravitational attraction. .
- A light year is the distance travelled by light in one year and is approximately equal to 9.46 trillion kilometres or 9,460,000,000,000 km.
Size and distances in space
The solar system
Sometimes metres and kilometres are used to measure objects and distances in space, especially if they are small or within our solar system. A light year would be an inappropriate unit of measurement in these circumstances.
The sizes of Planets orbit stars and can be made of solid or gaseous material. Planets are massive enough to become rounded in shape (due to their own gravity pulling on them)., Moons orbit planets and most planets in the solar system have moons. The only planets that do not have moons are Mercury and Venus – Saturn has 82 known moons., Asteroids are rocky objects which orbit the Sun. Most asteroids are found in the asteroid belt, between the orbits of Mars and Jupiter. and Comets are icy objects that are usually found in the outer solar system. Comets produce long tails if they pass close to the Sun, which can look quite spectacular from Earth. can be measured in metres and kilometres.
It's even possible to measure the distance from a planet to its moon, or from a The inner planets of the solar system, which orbit closest to the Sun and are made of rock and metal. to the The Sun is a star at the centre of our solar system. The Sun is the nearest star to the Earth. It is a yellow dwarf star that gives off different types of energy such as infra-red energy, ultraviolet light, radio waves and light. in metres or kilometres.
For example:
Earth has a diameter of approximately 12,700km.
The distance from Earth to the Moon is approximately 384,400km.
The distance from Earth to the Sun is approximately 150 million km (150,000,000 km).
Beyond the solar system
The distance to the next nearest An enormous ball of gases which produces large amounts of heat and light, due to nuclear fusion reactions in its core. is millions of times larger than the distance between planets in the The sun and the objects orbiting around it. This includes planets, asteroids, comets and more..
The distance between one galaxy and another is millions of times larger than the distance between the stars in a galaxy.
For example:
- The distance from the Sun to The next nearest star, after the Sun., our next nearest star, is approximately 40 quadrillion kilometres (40,208,000,000,000 km).
- The distance from our galaxy the The galaxy our solar system is located within. The Milky Way contains hundreds of billions of stars like our Sun. Earth is located about halfway between the centre of the Milky Way and its outer edge., to Andromeda, the next nearest spiral galaxy, is approximately 24 quintillion kilometres (24,000,000,000,000,000,000 km).
This means that the numbers used to describe and measure distances in space can become too big to understand, compare and write down.
To get around this problem, astronomers and scientists often use alternative units of measurement, such as light years, to measure astronomical distances.
The light year
A light year is a unit used to measure distance, not time. One light year is the distance travelled by light in one year.
Light travels very quickly at around 300,000,000 metres per second (m/s).
There are approximately 31,540,000 seconds in the average year.
We can combine this data with the formula connecting speed, distance and time to work out how far one light year is in metres:
\( Distance = speed \times time \)
\( Distance = 300,000,000 m/s \times 31,540,000 s \)
\( Distance = 9.460 \times 10{^1}{^5} m \) or \( 9,460,000,000,000 km \)
Therefore one light year is equal to approximately 9.46 trillion kilometres (9.46 quadrillion metres).
This unit is used for astronomical measurements, such as the distance to a star, the diameter of a galaxy and the distance between galaxies.
For example:
- The diameter of the Milky Way, is approximately 106,000 light years.
- The distance from the Sun to Proxima Centauri, our next nearest star, is approximately 4.25 light years.
- Barnard's Star is a red dwarf star and is the fourth nearest star to the Sun at approximately 6 light years distance.
- The distance from the Milky Way to Andromeda, the next nearest spiral galaxy, is approximately 2.5 million light years.
Which is the closest galaxy to the Milky Way?
The closest known galaxy to Earth is the Canis Major Dwarf Galaxy, at 25,000 light years from the Sun.
The Sagittarius Dwarf Elliptical Galaxy is the next closest, at 70,000 light years from the Sun.
Other units of measurement
There are other units of distance that are useful to scientists and astronomers when referring to space.
These include:
It is defined as the distance that light travels in free space in one second, and is equal to exactly 299,792,458 metres.: The distance light travels in one minute. Light from the Sun takes approximately 8 minutes and 20 seconds to reach Earth, so the distance between Earth and the Sun is 8.3 light minutes.
The distance from Earth to the Sun equal to approximately 150 million kilometres.: The distance from the Earth to the Sun (approximately 150 million km). Mars is around 1.5 AU from the Sun, and Jupiter is approximately 5.2 AU from the Sun.
A unit of distance used by astronomers. Parsec is short for 'parallax second'. One parsec is approximately 3.26 light years.: One parsec is approximately 3.26 light years. Scientists and astronomers sometimes use parsecs to express the distance to stars or galaxies, instead of light years.
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