This question, perhaps unwittingly (though I suspect not!), touches on a very important question in the philosophy of language.
The Evening Star, a large bright light which often graces the night sky after dusk, was called Hesperus by the ancient Greeks. The god Hesperus was the embodiment of this so-called 'wandering' star.
The god Hesperus had a sibling, a half-brother, Phosphorus. Phosphorus was the name given to another bright star which appeared just before dawn, now known colloquially to us as the Morning Star.
It was only centuries later that the ancient Greeks found out that these two stars were actually the same object. In other words Hesperus and Phosphorus were the same. The Babylonians, whom the Greeks discovered this from had known this for some time.
Nowadays astronomers know that the Morning and Evening stars are not only the same heavenly object, but they are a planet and not a star. They are known to us as the planet Venus.
So what does this have to do with linguistics and the philosophy of language? Well, in the nineteenth century, the German logician, linguist and philosopher, Frege, was studying philosophy of language in relation to the use of Proper names. He used the Evening Star (Abendstern) and Morning Star (Morgenstern) to illustrate a problem. I'll illustrate the problem in relation to the Original Poster's question and some of the answers here.
The Morning and Evening Stars are actually the same heavenly body, as you know. Suppose someone who speaks a different language from you uses a foreign language name which refers to the same object as one that you are referring to. We might very reasonably suppose that, seeing as both words referred to the same thing, they had the same meaning. Even more straightforward perhaps, is that if we have two words which refer to the same object in English, for example, then one sentence which uses one of these words will be equivalent to another that uses the different word. Or, at the very least, we might expect that the truth values for the two sentences might be the same.
However, consider the fact that at the early stages of this question, none of the answers here mentioned that the Evening Star, which we know to be Venus, is actually the same object as the Morning Star (this information has subsequently been edited into the relevant posts).
Now take for example the tautologous statement:
- The Evening Star is the Evening Star.
This statement is obviously true, and seem to carry little informative value. It seems to be substantially different to the assertion that:
- The Evening Star is the Morning Star.
This sentence seems to carry some kind of different information from sentence (1). Indeed it would seem to be informative in a way that sentence (1) most definitely is not.
If we try to substitute the Morning and Evening stars with each other, or even substitute either of these with the name Venus, we might expect the sentences to still have the same truth values though. This is what was argued by philosophers such as John Stuart Mill. However, this will not work, it seems. Consider the following:
- Bob knows that the Evening Star is the Evening Star.
There is no doubt that this is true. However, is would not necessarily have to follow that:
- Bob knows that the Evening Star is the planet Venus.
This may be true and it may not. However supposing that it is, it still by no means follows from this that sentence (5) below is true:
- Bob knows that the Morning Star is the planet Venus.
Now, if to use a name that refers to something is exactly the same as to use a different name that refers to that same thing, then it would be impossible for (4) and (5) to have a different truth value. But, it seems it is indeed entirely possible for a sentence like (4) and a sentence like (5) to have different values. In other words one sentence can be true whilst the other is false and vice verse.
It is because of this kind of problem that Frege argued that names that refer to the same thing can have the same reference but different sense. Since then different philosophers such as Kripke and many others have proposed other solutions to these types of problem.
Nonetheless, the Morning and Evening Stars continue to cause philosophers problems. What do we call the Evening Star in English? Erm, well, there is no doubt that we call the Evening Star the Evening Star. Do we call the evening star Venus? Well, many people will refer to this light in the sky as Venus, but does that mean that Venus is the name for the Evening Star? Maybe and maybe not. Does it mean that the Evening Star is known as the Morning Star, as I cheekily suggested under the Original Poster's question? As they suggest in their reply in the comments - probably not!
Readers who are interested in this problem might want to take a look at this entry in the Stanford Encyclopedia of Philosophy.