Everything Is Moving, Nothing is Perfect

 When thinking about space travel, I find it helpful to always keep the above two things in mind.

Everything is Moving (EIM):

Every rocket sitting on the launch pad already has velocity because (EIM). All the planets rotate and orbit the sun. Nothing is sitting still (because EIM). Our star and its plane of orbiting planets is rocketing in an elliptical orbit around our galaxy, as are all the other stars in our galaxy. It takes a long time for us to complete an orbit around the Milky Way (about 200-250 million years) but we are always in transit because …

This points out the wonderful utility of the Lagrange points. I used to think of them (especially L4 and L5) as parking spots - specific coordinates in space/time where you can put something and when you come back, they will be there. Finally, it dawned on me that, like all analogies, this wasn’t quite correct. The Lagrange points themselves move in conjunction with the gravitational bodies because - (duh) - EIM. 

 

Nothing is Perfect (NIP):

Although this is the most obvious observation a human can make, I still think it is useful to keep in mind especially regarding space travel. People (I) tend to think of the orbits and rotations of the celestial bodies to be like those fantastic astrolabe machines that ancient Greeks and Mesopotamians (and whoever else) made, that show the orbits and rotations of the planets with the precision of a mechanical clock. All the planets and orbits are perfect circles and nothing wobbles. In reality, of course, very few orbits are circles and lots of bodies are not perfect spheres. Modern computer graphics show circular orbits around centered bodies - but that's not reality. 

Universal orbital mechanics may be perfect, but not “human perfect". My definition of “human perfect” is whole ten-base numbers and Euclidean geometry. The universe is a lot more nuanced than this – the numbers aren’t whole and the shapes aren't geometric. For example, the Earth orbits around the sun in 365.25 days (not 365) which every four years causes a counting problem. Most orbits are not circular, they are some form of an ellipse and they tend to wander. Sometimes they are closer to us, sometimes they are farther away. The Earth is not a “perfect” sphere - it’s an oblate spheroid. The sun is not always at the “center” of our solar system – it orbits a barycenter just above its surface. Same thing with the Earth and Moon. Earth wobbles and Moon is drifting away (very slowly).

To simplify orbital mechanics, we make orbits circular and measure from the center of bodies, but when the smart people who do orbital mechanics for a living, plan missions, they calculate the universe as it is.