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.
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.
- The light year (the distance traveled by light in a year) is often used for distance information, a fact that often irritates laypeople because the term “ year ” suggests a period of time.
- The mass of stars , planets or other astronomical objects is often given in multiples of the mass of other objects, e.g. B. in units of solar or earth mass .
- Specifying an angle in rotation bound coordinate systems in the amount of time that the intuitive access to the period of rotation meet, such as the day of the earth , ie 24 hours instead of 360 degrees , for example, rating time = 1 h 23 m 45 s .
|size||Formula symbol *||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· 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.
|size||Formula symbol||Value in km||Comparative values||Source for value|
|Solar radius||R ☉||6,957 · 10 5||≈ 9.72 R ♃
≈ 104.26 R ♁
|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 ".|
According to an astronomical convention of the IAU, one gives
- Time points in the form of 1 h 23 m 45 s , modern also 01:23:45 ,
- However, the duration is 1h 23 m 45s .
|Comparative values||Definition / reference||Source
|Sun luminosity||L ☉||3.828 x 10 26 W.|
The apparent brightness (size class or magnitudo m) is generally defined
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