I realize that in the above information you stated not to ask "What was that I saw in the sky last night", however I still must ask. Last night I was viewing the night sky (approx. 9:45pm EST)from my home in western PA. In the western sky I noticed a unusual bright "star", so after observing the moon and Jupiter I turned my eyes to the object. What I saw was quite extrorinary, I had thought that this might be Saturn, but instead it was an eclipse of some kind. Two large objects since the object being eclipsed was a large crescent, and the object that was eclipsing it was large in size as well.
My question is simple, "What was it that I saw in the sky last night"?
In general it is very hard to answer these types of questions, this is why we say not to ask them, but in this specific case I have a good idea of what you saw. You were looking at this with a telescope, right? Because from what you are telling me, it seems like you were looking at Venus which goes through phases just like the Moon, but this effect is not visible with the naked eye, only with a (small) telescope.
Only Mercury and Venus show phases as seen from the Earth. This is because they are both "inferior planets", which means that their orbit is within the Earth's orbit. This is because at different times, more or less of the planet is illuminated by the Sun as seen from our point of view. Have a look at this article for more details and to see pictures to compare with what you saw.
An interesting historical fact, is that the phases of Venus were discovered by Galileo with what was at the time among the first telescopes. And that was a major argument for the heliocentric model of the solar system
I will also mention another interesting thing that Venus will do very soon. On June 8th, it will transit, meaning that it will pass in front of the Sun from our point of view. So for people observing the Sun at that moment Venus will appear as a small dark circle that they will be able to see cross the disk of the Sun. This is a fairly rare event. Transits of Venus occur in pairs 8 years apart (so there will be another one in 2012), but then the next pair comes only 120 years later. It is an important event because it is at that moment possible to precisely time the motion of Venus which can be used to determine many distances within the Solar System.
This is probably more information that you were asking for, but I hope this at least answered your question!
This page updated on June 27, 2015