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| Name | Equation | Description |
| c - Velocity of light | $c = 3 * 10^{8}\frac{m}{s}$ | Velocity of light in vacuum |
| AU - Astronomical Unit | $1 AU = 1,5 * 10^{11}m$ | The mean distance from earth to sun |
| Ly - Lightyear | $1Ly \approx 9,5 * 10^{15}m$ | Distance light travels in one year |
| pc - Parsec | $1 pc \approx 3Ly$ | One parsec is the dinstance, from wich 1 AU looks like an angle of 1 second |
| Name | Formula | where |
| Keplers 3rd Law | \begin{align*}a^3 = \frac{GMP^2}{4\pi^2} \end{align*} | \begin{align*}
a &= \text{Semi-major axis of elliptical orbit}\\
G &=\text{constant}\\
M &=\text{total mass of orbiting bodies}\\
P &=\text{orbital periode}\\
\end{align*} |
| Keplers 3rd law in simplified units | \begin{equation}
a^3 = P^2M
\end{equation} | \begin{align*}
a &= \text{Semi-major axis of elliptical orbit in Units of AU}\\
M &=\text{total mass of bodies in masses of sun}\\
P &=\text{orbital period in years}\\
\end{align*} |
| Keplers 3rd law when mass of planet is much smaller than the star $\frac{M}{M_\odot} \approx 1$ | \begin{equation}
a^3 = P^2
\end{equation} | |
| \begin{equation}
\frac{D2}{D1} = \sin{\alpha} \approx \alpha
\end{equation} | \begin{align*}
\alpha &= \text{viewing angle (for small angles) in rad}\\
D1 &=\text{Distance to object in parsec}\\
D2 &=\text{Extension distance in AU}\\
\end{align*} |