Moral philosophy is basically about the problem of what to do next–and, more generally, what sort of life to lead, and what sort of world to want.
Morality is a type of Knowledge. This means that moral 'truths' can be discovered by the usual methods of reason, which are essentially the same as those of science (although there are important differences).
Because morality is a type of Knowledge this means that it is also Fallible and can only improve over time through Conjecture and Criticism. As knowledge, morality cannot be sourced from a source of authority (e.g. governments, social-movements, religions).
Morality iterates and improves over time. So, given that, there is only one transcendent morality: protect the means of improving knowledge. In other words maintaining a culture where people are free to conjecture and criticize each other ideas is the most important moral decree.
See also: