Is Moral Nihilism a Morality?

[DLJ]

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I think ALL physical objects (whether experienceable or not) ought to be contained within a single category. Why? Because (while certainly there may be objects that are not experienceable and some that are) this distinction is irrelevant to the topic of ethics. 
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Until we get to Divinity Ethics, methinks.

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1) The sun exists. And it exists in world 1.
2) A biological entity (such as a human being) can look at the sun (ie. have a subjective experience of the sun... feel it's heat... see its position in the sky etc.)
3) A scientist with a telescope can make observations of the sun. These are subjective experiences.
4) From such observations, one may form a theory about what the sun is, what it is made of, why it produces heat ect. (these ideas are a part of world 3)

The thing is, science relies on subjective observations to give us objective knowledge. The sun is there, regardless of our opinions (objective truth). It is comprised mostly of hydrogen and helium, regardless of our opinion (objective truth). But we will never realize any objective truth about the sun without a subjective experience of it (ie. observation).

Some philosophers argue that science is not objective. They say it's a useful fiction. If this is your position, I cannot argue that morality is objective. 

I think science IS objective. I think science gets us closer to the truth. Also, to me, truth isn't subjective. Something either is true or it isn't.

If we can't agree on these things, there is no use for us to proceed into a discussion of ethics. But if we do agree, then this is a fine starting point in our discourse concerning the objectivity of morality.

Technically, “the sun” exists in World 2.

I would not describe science as objective. That would understate its importance.
Instead let me suggest that the scientific method aspires to be and has self-correcting, continuous improvement mechanisms to deliver high value intrinsic quality information: