# What's the difference between zenith and radial?

When it comes to orbital mechanics, are zenith and radial the same?

Or to put it another way, are nadir and anti-radial the same?

When it comes to orbital mechanics, are zenith and radial the same?

Or to put it another way, are nadir and anti-radial the same?

Sometimes yes and sometimes no!

Mostly this excellent answer to the question In “spacecraft talk” is nadir just a fancy word for “down”? explains the considerations necessary when talking about "up" and "down" for an oblate spheroid like the Earth. In this answer to the question How small a location is possible to identify when any visible star is at its zenith directly above? I link to the Wikipedia article Zenith and show the following diagram:

So when reading texts that include terms like zenith, nadir, and radial, I would say you need to refer to previous paragraphs or chapters within that work to see if they have defined their usage and/or definitions for zenith and nadir. Radial however should mostly be unambiguous.

Radial: Direction pointing outward from the central body's center of mass, through the spacecraft's center of mass.

Zenith: Options:

• That would be the outward normal, designated $\boldsymbol N$ (or sometimes $\boldsymbol n$) in the Frenet frame. – David Hammen Oct 31 '18 at 13:02