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Astronomical Distances

by Ron Kurtus (15 March 2009)

The distances in Astronomy are so great that using miles or kilometers is insufficient, because the numbers become too large to handle.

When dealing with the great distances within our Solar System, astronomical units (AU), which are multiples of the distance from the Earth to the Sun, are used. However, that unit of measurement is not large enough when considering the distance to other stars or galaxies. In that case, distance is stated in light years, which is how far light travels in a year.

A third unit that is preferred by astronomers is the parsec, which is even greater than a light year.

Questions you may have include:

This lesson will answer those questions.



Astronomical units

The unit of measurement that is convenient for stating the large distances within our Solar System is the astronomical unit (AU). It is defined as the distance from the Earth to the Sun. That distance is approximately 150 million kilometers (150,000,000 km) or 93 million miles (93,000,000 mi).

Some distances within the Solar System in AU are:

The distance between planets depends on their orientation in their orbits. Mars can be between 2.52 AU and 0.53 AU from Earth, depending on their relative positions.

Distances from Sun of Mars and Earth

Distances from Sun of Mars and Earth

As of 2009, the American spacecraft Voyager 1 had traveled over 108 AU from the Sun, the furthest a man-made object has traveled. The distance of 108 AU equates to 16,200,000,000 km or 10,044,000,000 miles.

Light year

Although the astronomical unit is fine for our Solar System, it is not sufficient to designate the greater distances to other stars and galaxies. Instead, the light year is used as a unit of measurement.

A light year is the distance light travels in one year and can be calculated as the speed of light in kilometers/second or miles/second multiplied times the number of seconds in a year.

The speed of light is approximately 300,000 kilometers per second or 186,000 miles per second. One year equals 365 days × 24 hours in a day × 60 minutes in an hour × 60 seconds in a minute, which equals 31,536,000 seconds. Thus, a light year is about:

A light year is also equals 63,241 AU.

Common large distances

Common large distances in space, measured in light years, include:

The time it takes light to travel from the Sun to the Earth (1 AU) is approximately 499 seconds or 8.32 minutes. You could say that 1 AU equals 8.33 light minutes.

Parsec

Astronomers prefer to use the parsec to designate extremely great distances in space, because it relates to the geometric method they commonly use to establish distance. Parsec stands for parallax of one arcsecond. It is 1 AU divided by the tangent of an arcsecond.

Parallax effect determines angle

As an explanation of the geometric method used to measure distance with the parallax effect, consider the situation when you observe a distant object and move side-to-side. The object seems to move or change position. This effect can be used to determine the resulting angle from moving side-to-side as compared to the distant object. That angle, in turn, is used to calculate the distance.

For example, knowing the distance A and the angle a in the illustration below, you can find the distance B using simple trigonometry.

Tangent of angle used to determine length of side

Tangent of angle used to determine length of side

For distant objects in space, the angle becomes very small. An acrsecond is equal to 1/60 of an angular degree. Astronomers can measure angles as small as 0.001 arcsecond.

Parsec is related to AU

In defining the parsec, the distance from the Earth to the Sun (1 AU) is used as the base of the right triangle and the arcsecond is used as the angle. This is because measurements are made in two parts of the Earth's orbit around the Sun, in order to get as great a distance for the base as possible.

Hypothetical star at 1 parsec from Sun

Hypothetical star at 1 parsec from Sun

Thus, 1 parsec = 1 AU / tan(1 arcsecond).

At some given time of the year, an astronomer can line up a distant star or galaxy with a nearby star of known distance from the Earth. Then, six months later, when the Earth is on the other side of its orbit around the Sun, the astronomer will line up the distant star with the known star. But because of the parallax effect, the distant star will seem to have moved and will be an a different angle from the Earth. By measuring that angle and making some calculations, the astronomer can determine the distance to the far-off star or galaxy.

Parsec not popularly used

Although astronomers prefer to use the parsec, other scientists and the general public use the light year to designate large distances. The reason is that it is difficult to visualize a distance in terms of a small angle. Also, one parsec is approximately 3.262 light-years, so there isn't much advantage in using the term.

However, those working in Astronomy must be familiar with the unit of measurement.

Summary

The great distances measured in Astronomy require units much larger than kilometers or miles. For distances within our Solar System, astronomical units (AU) or multiples of the distance from the Earth to the Sun are used. For distances to stars or galaxies, light years are used. A third unit that is preferred by astronomers is the parsec, which is even greater than a light year in distance.


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