On Monday, December 3, 2018 at 7:48:24 AM UTC-7, Chris L Peterson wrote:

In many contexts, "day" is reasonably assumed to mean "solar day", but
I don't see the problem if the context makes it clear what kind of day
is under discussion (and the word "day" in isolation is always
ambiguous in meaning).

Because "day" is
ambiguous.

If you think that "day" is ambiguous, except in the sense of the ambiguity of
"day" versus "night", versus a day and a night together, you've been spending
too long behind your telescope and too long in your ivory tower with other
astronomers.

In technical astronomical jargon, a "sidereal day" is a kind of day. In plain
ordinary English, as used by people who are not astronomers, a sidereal day is
*not* a day, never was, and never will be.

If you are talking to members of the general public, you need to talk to them in their language, not hit them over the head and force them to use yours.

So, instead of saying:

"When we're thinking about the motions of planets, a period can be defined in a couple of different ways. The "rotational period" of an object in the solar system is the amount of time it takes for the object to complete one turn, or rotate on its axis. We call the rotational period of Earth a "day".. Earth's day (or rotational period) is exactly 23.9345 hours (or, 23 hours, 56 minutes, 4.2 seconds)."

they should have put it this way:

When we're thinking about the motions of planets, a period can be defined in a couple of different ways. The "rotational period" of an object in the solar system is the amount of time it takes for the object to complete one turn, or rotate on its axis. Earth's day is 24 hours, but because the Earth moves in orbit around the Sun, from one day to the next, the Sun is in a slightly different direction from the Earth. As a result, Earth's day is actually slightly longer than its rotational period. Earth's rotational period is exactly 23.9345 hours (or, 23 hours, 56 minutes, 4.2 seconds). Astronomers call the rotational period of Earth a "sidereal day".

Now, the term "sidereal day" is gradually introduced as a technical term used by astronomers, but no flat statement is made that a day, as such, is something different than what the reader thought it was before.

John Savard