# Astronomical units of measure

Astronomical units of measure ¹) are used to meet the specific needs of astronomy and its vast dimensions. It is primarily about masses , distances and radiant power as well as special ways of writing times.

¹) Not to be confused with the astronomical unit , which is one of these units of measurement.

Some typical astronomical measurement systems and their characteristics are also described below.

## introduction

Compared to the (terrestrial) units of measurement we are familiar with , the magnitudes in astronomy are often literally “astronomical” and human perception threatens to fail: The distance from our sun to the nearest star ( Proxima Centauri ) is around 40  trillion kilometers ; our sun weighs around 1.9891 · 10 30  kg. You can compare these values ​​with others, but you risk losing any sense of magnitude .

Therefore, in astronomy one often uses ratios to familiar quantities in order to get an easier comparison.

## Crowds

size Formula symbol * Value
in  kg
Comparative values Source
for value
Solar mass M 1.9891 · 10 30 ≈ 1047.5655 M
≈ 332 946.08 M
( IAU )
Jupiter mass M 1.89881 · 10 27 ≈ 317.8284 M
Earth mass M 5.9736 · 10 24 ≈ 81 M
Moon mass M 7.348 0· 10 22

Note: The product of the mass of a celestial body and the gravitational constant can be determined approx. Five decimal places more precisely than the mass itself. Therefore, the ratio of the masses to each other can also be determined more precisely than the masses.

*For an explanation of the indices see: Astronomical symbol

## Lengths

size Formula symbol Value in km Comparative values Source for value
Solar radius R 6,957 · 10 5 ≈ 9.72 R
≈ 104.26 R
(IAU specifications)
Jupiter radius R Equator: 71492, Pole: 66854 ≈ 11.21 R
Earth radius R Equator: 6378.1 Poles: 6356.8 ≈ 3.67 R
Moon radius R 1738 ( NASA )

The following astronomical units of measurement also do not belong to the International System of Units . In astronomy they are largely indispensable and are therefore also used. The light year is usually not used by astronomers in technical terms.

size Formula symbol Value in meters Comparative values reference Source for value
Astronomical unit AE (international: au) 149 597 870 700 ≈ 499 light seconds
≈ 8.3 light minutes
approx. 390 times the mean distance between earth and moon
roughly the mean distance from earth to sun
Light year Lj 9 460 730 472 580 800 ≈ 63 240 AU
≈ 0.3066 pc
approx. ¼ of the distance to the next star ( Proxima Centauri )
Distance traveled by light in a vacuum during a Julian year .
Parsec pc 30 856 775 777 948 584 ≈ 3.261 563 777 ly From a distance of 1 pc, 1 AU appears at an angle of 1 ". ${\ displaystyle {\ tfrac {1 \, \ mathrm {AE}} {\ tan (1 '')}}}$

## time

According to an astronomical convention of the IAU, one gives

It is not customary to indicate times with ′ and ″ in order to avoid confusion with angular minutes and angular seconds .

To distinguish between solar time (civil time) and sidereal time , the latter is often written with a superscript asterisk, for example
1 h 30 m 00 s = 1 h * 30 m * 14.78 s * .

## power

There is still a convention to illustrate certain services by e.g. B. with the radiant power of the sun, a whole galaxy or the whole universe .

size Formula symbol Value
in watts
Comparative values Definition / reference Source
for value
Sun luminosity L 3.828 x 10 26  W.

## brightness

The apparent brightness (size class or magnitudo m) is generally defined

${\ displaystyle m_ {2} = m_ {1} -2 {,} 5 \ cdot \ lg \ left ({\ frac {I_ {2}} {I_ {1}}} \ right)}$

with I 1,2 radiation intensity (energy per time and area in Js −1 m −2 = Wm −2 ). This is purely a comparison measure, so you need a standard:

size Formula symbol value Comparative values
apparent brightness of the star
Vega in the constellation Lyra
m 0 0 m Polar Star : 2.12 m
Sirius : −1.6 m
Full moon : −12.5 m (average over pg / ag )
Sun : −26.87 m