What is the unit for distance traveled?

Astronomers use many of the same units of measurement as other scientists. They often use meters for length, kilograms for mass, and seconds for time. However, the distances and sizes in the universe can be so big, that astronomers have invented more units to describe distance.

Astronomical Units:

Distances in the solar system are often measured in astronomical units (abbreviated AU). An astronomical unit is the average distance between the Earth and the Sun:

1 AU = 1.496 x 108 km = 93 million miles

Jupiter is about 5.2 AU from the Sun and Pluto is about 39.5 AU from the Sun. The distance from the Sun to the center of the Milky Way is approximately 1.7 x 109 AU.

Light-Years:

To measure the distances between stars, astronomers often use light-years (abbreviated ly). A light-year is the distance that light travels in a vacuum in one year:

1 ly = 9.5 x 1012 km = 63,240 AU

Proxima Centauri is the nearest star to Earth (other than the Sun) and is 4.2 light-years away. This means light from Proxima Centauri takes 4.2 years to travel to Earth.

Parsecs:

Many astronomers prefer to use parsecs (abbreviated pc) to measure distance to stars. This is because its definition is closely related to a method of measuring the distances between stars. A parsec is the distance at which 1 AU subtends an angle of 1 arcsec.

1 pc = 3.09 x 1013 km = 3.26 ly

For even greater distances, astronomers use kiloparsecs and megaparsecs (abbreviated kpc and Mpc).

1 kiloparsec = 1 kpc = 1000 pc = 103 pc
1 megaparsec = 1 Mpc = 1,000,000 pc = 106 pc

Powers of Ten:

The distances and sizes of of the objects astronomers study vary from very small, including atoms and atomic nuclei, to very large including galaxies, clusters of galaxies and the size of the universe. To describe such a huge range, astronomers need a way to avoid confusing terms like "a billion trillion" and "a millionth". Astronomers use a system called powers-of-ten notation, which consolidates all of the zeros that you would normally find attached to very large or small numbers such as 1,000,000,000,000 or 0.0000000001. All of the zeros are put in an exponent, which is written as a superscript, and indicates how many zeros you would need to write out the long form of the number. So for example:

100 = 1
101 = 10
102 = 100
103 = 1000
104 = 10,000
and so on.

In powers-of-ten notation, numbers are written as a figure between one and ten multiplied by a power of ten. So for example, the distance to the Moon of 384,000 km can be re-written as 3.84 x 105 km. Notice that 3.84 is between one and ten. The same number could accurately be rewritten as 38.4 x 104 or 0.384 x 106, but the preferred form is to have the first number be between one and ten.

Very small numbers can also be written using powers-of-ten notation. The exponent is negative for numbers less than one and indicates dividing by that number of tens. So for example:

100 = 1

10-1 = 1/10 = 0.1

10-2 = 1/10 × 1/10 = 0.01

10-3 = 1/10 × 1/10 × 1/10 = 0.001

10-4 = 1/10 × 1/10 × 1/10 × 1/10 = 0.0001

and so on.

Once again, numbers are written as a figure between one and ten multiplied by a power of ten. So for example, a number like 0.00000375 would be expressed as 3.75 x 10-6.

Some familiar numbers written as powers-of-ten:

One hundred (100) 102

One thousand (1000) 103

One million (1,000,000) 106

One billion (1,000,000,000) 109

One trillion (1,000,000,000,000) 1012

One one-hundredth (0.01) 10-2

One one-thousandth (0.001) 10-3

One one-millionth (0.000001) 10-6

One one-billionth (0.000000001) 10-9

One one-trillionth (0.000000000001) 10-12

A few websites offer demonstrations of powers-of-ten and the scale of the universe.

Some examples to try

1. Jupiter is about 5.2 AU away from the Sun. How far is this in km?

2. A star is 4.94 x 1013 km away from the Earth. How long will it take the light from this star to reach the Earth?

3. Another star is 3.5 kpc away from us. How far away is it in km and in ly?

4. Star A is 33 pc away, and Star B is 109 pc away. What is the difference in distance between the two stars in ly?

Answers

1. 7.78 x 108 km

2. 5.2 years

3. 1.08 x 1017 km, 1.14 x 104 ly (or 11,410 ly)

4. 247.76 ly

The unit of distance is a centimetre in the CGS system. In physics, distance is defined as the physical measurement of how far an object has moved. Distance is a scalar quantity.

SI unit of distance is a metre according to the International System of Units. Interestingly, using this as the base unit and some equations, many other derived units or quantities are formed like volume, area, acceleration, and speed.

The metric unit of distance include the C.G.S and M.K.S unit systems. Following is the table which includes the metric units of distance:

Unit	Symbol
Millimetre	mm
Centimetre	cm
Metre	m
Kilometre	km

Below is the table explaining the distance conversion for different units of distance:

	Millimetre (mm)	Centimetre (cm)	Metre (m)	Kilometre (km)
1 millimetre (mm)	1	0.1	0.001	0.000001
1 centimetre (cm)	10	1	0.01	0.00001
1 metre (m)	1000	100	1	0.001
1 kilometre (km)	1000000	100000	1000	1
1 inch (in)	25.4	2.54	0.0254	0.0000254

When we talk about or when we have to measure longer distances they are other units used. Some of them include:

The solar radius is 7 × 1010 cm.
The Astronomical Unit (AU) is the mean distance from the Earth to the Sun, 1.496 × 1013 cm, or about 93 million miles.
A light-year (ly), the distance travelled by light in one year, or 9.46 × 1017 cm. More significantly, one light-year is equal to 9,500,000,000,000 kilometres.
The parsec (pc), the distance at which an object has a parallactic shift of 1 second of arc, or more accurately 3.1 × 1018 cm. Usually, there is 3.26 ly in a pc and 206,265 AU in a pc.

Q1. What is the biggest unit of distance?
Ans: The parsec is the biggest unit of distance that is used to measure the distance between the astronomical objects that are outside the solar system.

Q2. What is the smallest unit of distance?
Ans: The smallest unit of distance is Planck length which is denoted as ℓp and is equal to 1.616255(18)×10-35.

Related articles:

Unit Of Mass

Unit Of Weight

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