It seems that if I don’t have my phone angled just so, it will invisibly rotate my video. The rotation at the beginning of the film is what is used throughout, so if you think you have it horizontal and it turns out your first frame was vertical the entire film is then vertical. And you won’t realize this until you attempt playback.
There are probably a million and one ways to fix the video rotation. Unfortunately none of those are built into the phone. I downloaded the video onto my laptop (running Ubuntu 16.04).
Most of the advice I found was to use a program called Avidemux. I have used it in the past to change audio or video codices or move a video into a different container/wrapper (to mkv for example), and I have found it useful enough. However, it would appear that Avidemux is no longer present in the usual Ubuntu repositories as both apt and the Ubuntu Software Center did not find it.
I poked around and found a pretty simple command line rotation solution using ffmpeg here. I tried the version of the command in the response, but encountered an error:
[aac @ 0x6cf380] The encoder ‘aac’ is experimental but experimental codecs are not enabled, add ‘-strict -2’ if you want to use it.
Since my phone uses aac (apparently) I needed to add the -strict -2 in order to enable an experimental encoder. So metal!
ffmpeg -i Desktop/VID_20161008_144216.mp4 -vf “transpose=2” -strict -2 Desktop/VID_20161008_144216_rotatecc.mp4
It worked great. The response offers some other rotation options, but of course the man page for ffmpeg is extensive.