# Astronomical Units

On astronomy scale distances are very large, hence comfortable units are: 

| Unit              | Symbol                             | Value SI                            |
| ----------------- | ---------------------------------- | ----------------------------------- |
| parsec            | $${\rm  pc}$$                      | $$3.0857 \times 10^{16 } {\rm; m}$$ |
| megaparsec        | $${\rm     Mpc}$$                  | $$3.0857 \times 10^{22} {\rm ;m}$$  |
| lightyear         | $${\rm ly}$$                       | $$9.4605 \times 10^{15} {\rm ;m}$$  |
| astronomical unit | A.U.                               | $$1.496 \times 10^{11} {\rm ;m}$$   |
| degree            | $$^\circ$$                         | $$17.4532925 {\rm ; mrad}$$         |
| arcsecond         | $$^{\prime\prime}$$ (double prime) | $$0.000277778^\circ$$               |
| arcminute         | $$^\prime$$ (prime)                | $$0.0166667^\circ$$                 |

#### Constants

| Name           | Symbol          | Value                                |
| -------------- | --------------- | ------------------------------------ |
| Earth Radius   | $$R\_{\oplus}$$ | $$6.378 \times 10^6 {\rm ; m}$$      |
| Sun Radius     | $$R\_\odot$$    | $$6.961 \times 10^8 {; m}$$          |
| Earth Mass     | $$M\_\oplus$$   | $$5.975 \times 10^{24}  {\rm ; kg}$$ |
| Sun Mass       | $$M\_\odot$$    | $$1.989 \times 10^{30} {\rm ; kg}$$  |
| Sun Luminosity | $$S\_\odot$$    | $$3.85 \times 10^{26} {\rm ; W}$$    |