As Earth orbits the Sun, the apparent position of the Sun against the background stars changes. As a result, we see different parts of the sky at different times of the year. This annual motion of Earth around the Sun is what gives us our “winter sky” and our “summer sky,” respectively.

Not all galaxies are disks, but many are. The reasons have to do with their formation history and a few basic physical laws.

You might have heard that there are different kinds of stars, called stellar types or stellar classes. You might have also heard that these types have cryptic names designated by letters of the alphabet. This post explains what these stellar types are and how they came to be.

In simplest terms, an orbit is the path of a planet around the Sun, or of a satellite around its parent body. The Moon orbiting Earth is an example. But what causes the orbit to have one size and not another? It turns out that all of the properties of an orbit are the result of two physical laws: the law of energy conservation and the law of conservation of angular momentum.

Vectors are mathematical objects. They are too complicated to be described by mere numbers, but like numbers, vectors have a size. However, they also have a direction. As such, they can be used to mathematically describe certain physical items that also have a size and a direction.

Velocity is an example of such a vector quantity; it has both a size or amount (speed) and a direction. What do we mean when we say that a velocity has a direction? A velocity is always forward, back, left, right, up, down or some combination of these. This makes it a perfect candidate for a vector description.

In this article we outline the basic properties of vectors, provide several examples of physical quantities that are described by them and also demonstrate how they are used to describe the world.