Some of the changes over time are neither moral progress nor moral drift, because some ethical claims are about worldlines, not world states. For example, killing someone and replacing them with a different person is worse than doing nothing, even if the new person holds similar moral worth to the original, and if the externalities don’t matter (no friends/relatives/etc.).
So even given an unchanging morality, moral attitudes about the present are influenced by (non-moral) facts of the past, and will change in complicated ways over time.
Some of the changes over time are neither moral progress nor moral drift, because some ethical claims are about worldlines, not world states. For example, killing someone and replacing them with a different person is worse than doing nothing, even if the new person holds similar moral worth to the original, and if the externalities don’t matter (no friends/relatives/etc.).
So even given an unchanging morality, moral attitudes about the present are influenced by (non-moral) facts of the past, and will change in complicated ways over time.