Explain why on some occasions you can see the planets line up in the sky.?

Answer 1

The orbits of all planets are very close to the same plane. From Earth, we see the plane of our orbit as the path that the Sun follows, over a year, against the background of the celestial sphere. This path is called the ecliptic.

The other planets also have their orbital planes and they all appear to be along the ecliptic (or, at least, very close to it).

Therefore, when we see many planets in the sky at the same time, they all appear to be lined up along the ecliptic. (You can't see the line, of course, but you can imagine it).

However, whenever two planets appear very close to each other in the sky, the slight difference between their real orbital planes will be enough to destroy the effect.

The effect is best appreciated when the planets are well separated from each other. The Moon's orbit is also, somewhat, along the ecliptic so that it too can participate in this apparent "lining up".